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PERL(1) USER COMMANDS PERL(1) NNNNAAAAMMMMEEEE perl - Practical Extraction and Report Language SSSSYYYYNNNNOOOOPPPPSSSSIIIISSSS ppppeeeerrrrllll [options] filename args DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN _P_e_r_l is an interpreted language optimized for scanning arbi- trary text files, extracting information from those text files, and printing reports based on that information. It's also a good language for many system management tasks. The language is intended to be practical (easy to use, effi- cient, complete) rather than beautiful (tiny, elegant, minimal). It combines (in the author's opinion, anyway) some of the best features of C, _s_e_d, _a_w_k, and _s_h, so people familiar with those languages should have little difficulty with it. (Language historians will also note some vestiges of _c_s_h, Pascal, and even BASIC-PLUS.) Expression syntax corresponds quite closely to C expression syntax. Unlike most Unix utilities, _p_e_r_l does not arbitrarily limit the size of your data--if you've got the memory, _p_e_r_l can slurp in your whole file as a single string. Recursion is of unlimited depth. And the hash tables used by associative arrays grow as necessary to prevent degraded performance. _P_e_r_l uses sophisticated pattern matching techniques to scan large amounts of data very quickly. Although optimized for scanning text, _p_e_r_l can also deal with binary data, and can make dbm files look like associative arrays (where dbm is available). Setuid _p_e_r_l scripts are safer than C programs through a dataflow tracing mechanism which prevents many stupid security holes. If you have a problem that would ordinarily use _s_e_d or _a_w_k or _s_h, but it exceeds their capa- bilities or must run a little faster, and you don't want to write the silly thing in C, then _p_e_r_l may be for you. There are also translators to turn your _s_e_d and _a_w_k scripts into _p_e_r_l scripts. OK, enough hype. This manual applies only to the MS-DOS version. Unix-only features are mentioned but are not fully documented here. The MS-DOS version of _p_e_r_l attempts to duplicate the Unix version's functionality but is crippled by MS-DOS and by the severe memory limitations MS-DOS imposes. The MS-DOS ver- sion is nevertheless useful for text processing and for lim- ited applications involving subprocess management. Upon startup, _p_e_r_l looks for your script in one of the fol- lowing places: 1. Specified line by line via ----eeee switches on the command line. 2. Contained in the file specified by the first filename on 1 PERL(1) USER COMMANDS PERL(1) the command line. (Note that systems supporting the #! notation invoke interpreters this way.) 3. Passed in implicitly via standard input. This only works if there are no filename arguments--to pass argu- ments to a _s_t_d_i_n script you must explicitly specify a - for the script name. After locating your script, _p_e_r_l compiles it to an internal form. If the script is syntactically correct, it is exe- cuted. OOOOppppttttiiiioooonnnnssss Note: on first reading this section may not make much sense to you. It's here at the front for easy reference. A single-character option may be combined with the following option, if any. This is particularly useful when invoking a script using the #! construct which only allows one argu- ment. Example: #!/usr/bin/perl -spi.bak # same as -s -p -i.bak ... Options include: ----0000_d_i_g_i_t_s specifies the record separator ($/) as an octal number. If there are no digits, the null character is the separator. Other switches may precede or follow the digits. For example, if you have a version of _f_i_n_d which can print filenames terminated by the null char- acter, you can say this: find . -name '*.bak' -print0 | perl -n0e unlink The special value 00 will cause Perl to slurp files in paragraph mode. The value 0777 will cause Perl to slurp files whole since there is no legal character with that value. ----aaaa turns on autosplit mode when used with a ----nnnn or ----pppp. An implicit split command to the @F array is done as the first thing inside the implicit while loop produced by the ----nnnn or ----pppp. perl -ane 'print pop(@F), "\n";' is equivalent to while (<>) { 2 PERL(1) USER COMMANDS PERL(1) @F = split(' '); print pop(@F), "\n"; } ----cccc causes _p_e_r_l to check the syntax of the script and then exit without executing it. ----dddd runs the script under the perl debugger. See the sec- tion on Debugging. ----DDDD_n_u_m_b_e_r sets debugging flags. To watch how it executes your script, use ----DDDD11114444. (This only works if debugging is compiled into your _p_e_r_l.) Another nice value is -D1024, which lists your compiled syntax tree. And -D512 displays compiled regular expressions. ----eeee _c_o_m_m_a_n_d_l_i_n_e may be used to enter one line of script. Multiple ----eeee commands may be given to build up a multi-line script. If ----eeee is given, _p_e_r_l will not look for a script filename in the argument list. On MS-DOS, the ----eeee switch will not work well unless perl is run from an MKS tool, such as the Korn shell. This is due to limi- tations in the standard method of MS-DOS argument pass- ing. ----iiii_e_x_t_e_n_s_i_o_n specifies that files processed by the <> construct are to be edited in-place. It does this by renaming the input file, opening the output file by the same name, and selecting that output file as the default for print statements. The extension, if supplied, is added to the name of the old file to make a backup copy. If no extension is supplied, no backup is made. Saying "perl -p -i.bak -e "s/foo/bar/;" ... " is the same as using the script: #!/usr/bin/perl -pi.bak s/foo/bar/; which is equivalent to 3 PERL(1) USER COMMANDS PERL(1) #!/usr/bin/perl while (<>) { if ($ARGV ne $oldargv) { rename($ARGV, $ARGV . '.bak'); open(ARGVOUT, ">$ARGV"); select(ARGVOUT); $oldargv = $ARGV; } s/foo/bar/; } continue { print; # this prints to original filename } select(STDOUT); except that the ----iiii form doesn't need to compare $ARGV to $oldargv to know when the filename has changed. It does, however, use ARGVOUT for the selected filehandle. Note that _S_T_D_O_U_T is restored as the default output filehandle after the loop. You can use eof to locate the end of each input file, in case you want to append to each file, or reset line numbering (see example under eof). ----IIII_d_i_r_e_c_t_o_r_y may be used in conjunction with ----PPPP to tell the C preprocessor where to look for include files. By default /usr/include and /usr/lib/perl are searched. ----llll_o_c_t_n_u_m enables automatic line-ending processing. It has two effects: first, it automatically chops the line termi- nator when used with ----nnnn or ----pppp ,,,, and second, it assigns $\ to have the value of _o_c_t_n_u_m so that any print state- ments will have that line terminator added back on. If _o_c_t_n_u_m is omitted, sets $\ to the current value of $/. For instance, to trim lines to 80 columns: perl -lpe 'substr($_, 80) = ""' Note that the assignment $\ = $/ is done when the switch is processed, so the input record separator can be different than the output record separator if the ----llll switch is followed by a ----0000 switch: gnufind / -print0 | perl -ln0e 'print "found $_" if -p' This sets $\ to newline and then sets $/ to the null character. ----nnnn causes _p_e_r_l to assume the following loop around your 4 PERL(1) USER COMMANDS PERL(1) script, which makes it iterate over filename arguments somewhat like "sed -n" or _a_w_k: while (<>) { ... # your script goes here } Note that the lines are not printed by default. See ----pppp to have lines printed. Here is an efficient way to delete all files older than a week: find . -mtime +7 -print | perl -nle 'unlink;' This is faster than using the -exec switch of find because you don't have to start a process on every filename found. ----pppp causes _p_e_r_l to assume the following loop around your script, which makes it iterate over filename arguments somewhat like _s_e_d: while (<>) { ... # your script goes here } continue { print; } Note that the lines are printed automatically. To suppress printing use the ----nnnn switch. A ----pppp overrides a ----nnnn switch. ----PPPP causes your script to be run through the C preprocessor before compilation by _p_e_r_l. (Since both comments and cpp directives begin with the # character, you should avoid starting comments with any words recognized by the C preprocessor such as "if", "else" or "define".) ----ssss enables some rudimentary switch parsing for switches on the command line after the script name but before any filename arguments (or before a --). Any switch found there is removed from @ARGV and sets the corresponding variable in the _p_e_r_l script. The following script prints "true" if and only if the script is invoked with a -xyz switch. #!/usr/bin/perl -s if ($xyz) { print "true\n"; } ----SSSS makes _p_e_r_l use the PATH environment variable to search for the script (unless the name of the script starts with a slash). Typically this is used to emulate #! 5 PERL(1) USER COMMANDS PERL(1) startup on machines that don't support #!, in the fol- lowing manner: #!/usr/bin/perl eval "exec /usr/bin/perl -S $0 $*" if $running_under_some_shell; The system ignores the first line and feeds the script to /bin/sh, which proceeds to try to execute the _p_e_r_l script as a shell script. The shell executes the second line as a normal shell command, and thus starts up the _p_e_r_l interpreter. On some systems $0 doesn't always contain the full pathname, so the ----SSSS tells _p_e_r_l to search for the script if necessary. After _p_e_r_l locates the script, it parses the lines and ignores them because the variable $running_under_some_shell is never true. A better construct than $* would be ${1+"$@"}, which handles embedded spaces and such in the filenames, but doesn't work if the script is being interpreted by csh. In order to start up sh rather than csh, some systems may have to replace the #! line with a line containing just a colon, which will be pol- itely ignored by perl. Other systems can't control that, and need a totally devious construct that will work under any of csh, sh or perl, such as the follow- ing: eval '(exit $?0)' && eval 'exec /usr/bin/perl -S $0 ${1+"$@"}' & eval 'exec /usr/bin/perl -S $0 $argv:q' if 0; ----uuuu causes _p_e_r_l to dump core after compiling your script. This switch is not supported on MS-DOS. ----UUUU allows _p_e_r_l to do unsafe operations. Currently the only "unsafe" operation is the unlinking of directories while running as superuser. ----vvvv prints the version and patchlevel of your _p_e_r_l execut- able. ----wwww prints warnings about identifiers that are mentioned only once, and scalar variables that are used before being set. Also warns about redefined subroutines, and references to undefined filehandles or filehandles opened readonly that you are attempting to write on. Also warns you if you use == on values that don't look like numbers, and if your subroutines recurse more than 100 deep. ----xxxx_d_i_r_e_c_t_o_r_y 6 PERL(1) USER COMMANDS PERL(1) tells _p_e_r_l that the script is embedded in a message. Leading garbage will be discarded until the first line that starts with #! and contains the string "perl". Any meaningful switches on that line will be applied (but only one group of switches, as with normal #! pro- cessing). If a directory name is specified, Perl will switch to that directory before running the script. The ----xxxx switch only controls the the disposal of leading garbage. The script must be terminated with __END__ if there is trailing garbage to be ignored (the script can process any or all of the trailing garbage via the DATA filehandle if desired). DDDDaaaattttaaaa TTTTyyyyppppeeeessss aaaannnndddd OOOObbbbjjjjeeeeccccttttssss _P_e_r_l has three data types: scalars, arrays of scalars, and associative arrays of scalars. Normal arrays are indexed by number, and associative arrays by string. The interpretation of operations and values in perl some- times depends on the requirements of the context around the operation or value. There are three major contexts: string, numeric and array. Certain operations return array values in contexts wanting an array, and scalar values otherwise. (If this is true of an operation it will be mentioned in the documentation for that operation.) Operations which return scalars don't care whether the context is looking for a string or a number, but scalar variables and values are interpreted as strings or numbers as appropriate to the con- text. A scalar is interpreted as TRUE in the boolean sense if it is not the null string or 0. Booleans returned by operators are 1 for true and 0 or '' (the null string) for false. There are actually two varieties of null string: defined and undefined. Undefined null strings are returned when there is no real value for something, such as when there was an error, or at end of file, or when you refer to an uninitial- ized variable or element of an array. An undefined null string may become defined the first time you access it, but prior to that you can use the defined() operator to deter- mine whether the value is defined or not. References to scalar variables always begin with '$', even when referring to a scalar that is part of an array. Thus: $days # a simple scalar variable $days[28] # 29th element of array @days $days{'Feb'} # one value from an associative array $#days # last index of array @days but entire arrays or array slices are denoted by '@': 7 PERL(1) USER COMMANDS PERL(1) @days # ($days[0], $days[1],... $days[n]) @days[3,4,5] # same as @days[3..5] @days{'a','c'} # same as ($days{'a'},$days{'c'}) and entire associative arrays are denoted by '%': %days # (key1, val1, key2, val2 ...) Any of these eight constructs may serve as an lvalue, that is, may be assigned to. (It also turns out that an assign- ment is itself an lvalue in certain contexts--see examples under s, tr and chop.) Assignment to a scalar evaluates the righthand side in a scalar context, while assignment to an array or array slice evaluates the righthand side in an array context. You may find the length of array @days by evaluating "$#days", as in _c_s_h. (Actually, it's not the length of the array, it's the subscript of the last element, since there is (ordinarily) a 0th element.) Assigning to $#days changes the length of the array. Shortening an array by this method does not actually destroy any values. Lengthening an array that was previously shortened recovers the values that were in those elements. You can also gain some measure of effi- ciency by preextending an array that is going to get big. (You can also extend an array by assigning to an element that is off the end of the array. This differs from assign- ing to $#whatever in that intervening values are set to null rather than recovered.) You can truncate an array down to nothing by assigning the null list () to it. The following are exactly equivalent @whatever = (); $#whatever = $[ - 1; If you evaluate an array in a scalar context, it returns the length of the array. The following is always true: @whatever == $#whatever - $[ + 1; Multi-dimensional arrays are not directly supported, but see the discussion of the $; variable later for a means of emu- lating multiple subscripts with an associative array. You could also write a subroutine to turn multiple subscripts into a single subscript. Every data type has its own namespace. You can, without fear of conflict, use the same name for a scalar variable, an array, an associative array, a filehandle, a subroutine name, and/or a label. Since variable and array references 8 PERL(1) USER COMMANDS PERL(1) always start with '$', '@', or '%', the "reserved" words aren't in fact reserved with respect to variable names. (They ARE reserved with respect to labels and filehandles, however, which don't have an initial special character. Hint: you could say open(LOG,'logfile') rather than open(log,'logfile'). Using uppercase filehandles also improves readability and protects you from conflict with future reserved words.) Case IS significant--"FOO", "Foo" and "foo" are all different names. Names which start with a letter may also contain digits and underscores. Names which do not start with a letter are limited to one character, e.g. "$%" or "$$". (Most of the one character names have a predefined significance to _p_e_r_l. More later.) Numeric literals are specified in any of the usual floating point or integer formats: 12345 12345.67 .23E-10 0xffff # hex 0377 # octal String literals are delimited by either single or double quotes. They work much like shell quotes: double-quoted string literals are subject to backslash and variable sub- stitution; single-quoted strings are not (except for \' and \\). The usual backslash rules apply for making characters such as newline, tab, etc., as well as some more exotic forms: \t tab \n newline \r return \f form feed \b backspace \a alarm (bell) \e escape \033 octal char \x1b hex char \c[ control char \l lowercase next char \u uppercase next char \L lowercase till \E \U uppercase till \E \E end case modification You can also embed newlines directly in your strings, i.e. they can end on a different line than they begin. This is nice, but if you forget your trailing quote, the error will not be reported until _p_e_r_l finds another line containing the quote character, which may be much further on in the script. 9 PERL(1) USER COMMANDS PERL(1) Variable substitution inside strings is limited to scalar variables, normal array values, and array slices. (In other words, identifiers beginning with $ or @, followed by an optional bracketed expression as a subscript.) The follow- ing code segment prints out "The price is $100." $Price = '$100'; # not interpreted print "The price is $Price.\n";# interpreted Note that you can put curly brackets around the identifier to delimit it from following alphanumerics. Also note that a single quoted string must be separated from a preceding word by a space, since single quote is a valid character in an identifier (see Packages). Two special literals are __LINE__ and __FILE__, which represent the current line number and filename at that point in your program. They may only be used as separate tokens; they will not be interpolated into strings. In addition, the token __END__ may be used to indicate the logical end of the script before the actual end of file. Any following text is ignored (but may be read via the DATA filehandle). The two control characters ^D and ^Z are synonyms for __END__. A word that doesn't have any other interpretation in the grammar will be treated as if it had single quotes around it. For this purpose, a word consists only of alphanumeric characters and underline, and must start with an alphabetic character. As with filehandles and labels, a bare word that consists entirely of lowercase letters risks conflict with future reserved words, and if you use the ----wwww switch, Perl will warn you about any such words. Array values are interpolated into double-quoted strings by joining all the elements of the array with the delimiter specified in the $" variable, space by default. (Since in versions of perl prior to 3.0 the @ character was not a metacharacter in double-quoted strings, the interpolation of @array, $array[EXPR], @array[LIST], $array{EXPR}, or @array{LIST} only happens if array is referenced elsewhere in the program or is predefined.) The following are equivalent: $temp = join($",@ARGV); system "echo $temp"; system "echo @ARGV"; Within search patterns (which also undergo double-quotish substitution) there is a bad ambiguity: Is /$foo[bar]/ to be interpreted as /${foo}[bar]/ (where [bar] is a character 10 PERL(1) USER COMMANDS PERL(1) class for the regular expression) or as /${foo[bar]}/ (where [bar] is the subscript to array @foo)? If @foo doesn't oth- erwise exist, then it's obviously a character class. If @foo exists, perl takes a good guess about [bar], and is almost always right. If it does guess wrong, or if you're just plain paranoid, you can force the correct interpreta- tion with curly brackets as above. A line-oriented form of quoting is based on the shell here- is syntax. Following a << you specify a string to terminate the quoted material, and all lines following the current line down to the terminating string are the value of the item. The terminating string may be either an identifier (a word), or some quoted text. If quoted, the type of quotes you use determines the treatment of the text, just as in regular quoting. An unquoted identifier works like double quotes. There must be no space between the << and the iden- tifier. (If you put a space it will be treated as a null identifier, which is valid, and matches the first blank line--see Merry Christmas example below.) The terminating string must appear by itself (unquoted and with no surround- ing whitespace) on the terminating line. print <<EOF; # same as above The price is $Price. EOF print <<"EOF"; # same as above The price is $Price. EOF print << x 10; # null identifier is delimiter Merry Christmas! print <<`EOC`; # execute commands echo hi there echo lo there EOC print <<foo, <<bar; # you can stack them I said foo. foo I said bar. bar Array literals are denoted by separating individual values by commas, and enclosing the list in parentheses: (LIST) In a context not requiring an array value, the value of the array literal is the value of the final element, as in the C 11 PERL(1) USER COMMANDS PERL(1) comma operator. For example, @foo = ('cc', '-E', $bar); assigns the entire array value to array foo, but $foo = ('cc', '-E', $bar); assigns the value of variable bar to variable foo. Note that the value of an actual array in a scalar context is the length of the array; the following assigns to $foo the value 3: @foo = ('cc', '-E', $bar); $foo = @foo; # $foo gets 3 You may have an optional comma before the closing parenthesis of an array literal, so that you can say: @foo = ( 1, 2, 3, ); When a LIST is evaluated, each element of the list is evaluated in an array context, and the resulting array value is interpolated into LIST just as if each individual element were a member of LIST. Thus arrays lose their identity in a LIST--the list (@foo,@bar,&SomeSub) contains all the elements of @foo followed by all the ele- ments of @bar, followed by all the elements returned by the subroutine named SomeSub. A list value may also be subscripted like a normal array. Examples: $time = (stat($file))[8]; # stat returns array value $digit = ('a','b','c','d','e','f')[$digit-10]; return (pop(@foo),pop(@foo))[0]; Array lists may be assigned to if and only if each element of the list is an lvalue: ($a, $b, $c) = (1, 2, 3); ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00); 12 PERL(1) USER COMMANDS PERL(1) The final element may be an array or an associative array: ($a, $b, @rest) = split; local($a, $b, %rest) = @_; You can actually put an array anywhere in the list, but the first array in the list will soak up all the values, and anything after it will get a null value. This may be useful in a local(). An associative array literal contains pairs of values to be interpreted as a key and a value: # same as map assignment above %map = ('red',0x00f,'blue',0x0f0,'green',0xf00); Array assignment in a scalar context returns the number of elements produced by the expression on the right side of the assignment: $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2 There are several other pseudo-literals that you should know about. If a string is enclosed by backticks (grave accents), it first undergoes variable substitution just like a double quoted string. It is then interpreted as a com- mand, and the output of that command is the value of the pseudo-literal, like in a shell. In a scalar context, a single string consisting of all the output is returned. In an array context, an array of values is returned, one for each line of output. (You can set $/ to use a different line terminator.) The command is executed each time the pseudo-literal is evaluated. The status value of the com- mand is returned in $? (see Predefined Names for the interpretation of $?). Unlike in _c_s_h, no translation is done on the return data--newlines remain newlines. Unlike in any of the shells, single quotes do not hide variable names in the command from interpretation. To pass a $ through to the shell you need to hide it with a backslash. Evaluating a filehandle in angle brackets yields the next line from that file (newline included, so it's never false until EOF, at which time an undefined value is returned). Ordinarily you must assign that value to a variable, but there is one situation where an automatic assignment hap- pens. If (and only if) the input symbol is the only thing inside the conditional of a _w_h_i_l_e loop, the value is automatically assigned to the variable "$_". (This may seem like an odd thing to you, but you'll use the construct in almost every _p_e_r_l script you write.) Anyway, the following lines are equivalent to each other: 13 PERL(1) USER COMMANDS PERL(1) while ($_ = <STDIN>) { print; } while (<STDIN>) { print; } for (;<STDIN>;) { print; } print while $_ = <STDIN>; print while <STDIN>; The filehandles _S_T_D_I_N, _S_T_D_O_U_T and _S_T_D_E_R_R are predefined. (The filehandles _s_t_d_i_n, _s_t_d_o_u_t and _s_t_d_e_r_r will also work except in packages, where they would be interpreted as local identifiers rather than global.) Additional filehandles may be created with the _o_p_e_n function. If a <FILEHANDLE> is used in a context that is looking for an array, an array consisting of all the input lines is returned, one line per array element. It's easy to make a LARGE data space this way, so use with care. The null filehandle <> is special and can be used to emulate the behavior of _s_e_d and _a_w_k. Input from <> comes either from standard input, or from each file listed on the command line. Here's how it works: the first time <> is evaluated, the ARGV array is checked, and if it is null, $ARGV[0] is set to '-', which when opened gives you standard input. The ARGV array is then processed as a list of filenames. The loop while (<>) { ... # code for each line } is equivalent to unshift(@ARGV, '-') if $#ARGV < $[; while ($ARGV = shift) { open(ARGV, $ARGV); while (<ARGV>) { ... # code for each line } } except that it isn't as cumbersome to say. It really does shift array ARGV and put the current filename into variable ARGV. It also uses filehandle ARGV internally. You can modify @ARGV before the first <> as long as you leave the first filename at the beginning of the array. Line numbers ($.) continue as if the input was one big happy file. (But see example under eof for how to reset line numbers on each file.) 14 PERL(1) USER COMMANDS PERL(1) If you want to set @ARGV to your own list of files, go right ahead. If you want to pass switches into your script, you can put a loop on the front like this: while ($_ = $ARGV[0], /^-/) { shift; last if /^--$/; /^-D(.*)/ && ($debug = $1); /^-v/ && $verbose++; ... # other switches } while (<>) { ... # code for each line } The <> symbol will return FALSE only once. If you call it again after this it will assume you are processing another @ARGV list, and if you haven't set @ARGV, will input from _S_T_D_I_N. If the string inside the angle brackets is a reference to a scalar variable (e.g. <$foo>), then that variable contains the name of the filehandle to input from. If the string inside angle brackets is not a filehandle, it is interpreted as a filename pattern to be globbed, and either an array of filenames or the next filename in the list is returned, depending on context. One level of $ interpretation is done first, but you can't say <$foo> because that's an indirect filehandle as explained in the previous paragraph. You could insert curly brackets to force interpretation as a filename glob: <${foo}>. Example: while (<*.c>) { chmod 0644, $_; } is equivalent to open(foo, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); while (<foo>) { chop; chmod 0644, $_; } In fact, it's currently implemented that way. (Which means it will not work on filenames with spaces in them unless you have /bin/csh on your machine.) Of course, the shortest way to do the above is: chmod 0644, <*.c>; 15 PERL(1) USER COMMANDS PERL(1) SSSSyyyynnnnttttaaaaxxxx A _p_e_r_l script consists of a sequence of declarations and commands. The only things that need to be declared in _p_e_r_l are report formats and subroutines. See the sections below for more information on those declarations. All uninitial- ized user-created objects are assumed to start with a null or 0 value until they are defined by some explicit operation such as assignment. The sequence of commands is executed just once, unlike in _s_e_d and _a_w_k scripts, where the sequence of commands is executed for each input line. While this means that you must explicitly loop over the lines of your input file (or files), it also means you have much more con- trol over which files and which lines you look at. (Actu- ally, I'm lying--it is possible to do an implicit loop with either the ----nnnn or ----pppp switch.) A declaration can be put anywhere a command can, but has no effect on the execution of the primary sequence of commands--declarations all take effect at compile time. Typically all the declarations are put at the beginning or the end of the script. _P_e_r_l is, for the most part, a free-form language. (The only exception to this is format declarations, for fairly obvious reasons.) Comments are indicated by the # character, and extend to the end of the line. If you attempt to use /* */ C comments, it will be interpreted either as division or pattern matching, depending on the context. So don't do that. CCCCoooommmmppppoooouuuunnnndddd ssssttttaaaatttteeeemmmmeeeennnnttttssss In _p_e_r_l, a sequence of commands may be treated as one com- mand by enclosing it in curly brackets. We will call this a BLOCK. The following compound commands may be used to control flow: if (EXPR) BLOCK if (EXPR) BLOCK else BLOCK if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK LABEL while (EXPR) BLOCK LABEL while (EXPR) BLOCK continue BLOCK LABEL for (EXPR; EXPR; EXPR) BLOCK LABEL foreach VAR (ARRAY) BLOCK LABEL BLOCK continue BLOCK Note that, unlike C and Pascal, these are defined in terms of BLOCKs, not statements. This means that the curly brack- ets are _r_e_q_u_i_r_e_d--no dangling statements allowed. If you want to write conditionals without curly brackets there are 16 PERL(1) USER COMMANDS PERL(1) several other ways to do it. The following all do the same thing: if (!open(foo)) { die "Can't open $foo: $!"; } die "Can't open $foo: $!" unless open(foo); open(foo) || die "Can't open $foo: $!"; # foo or bust! open(foo) ? 'hi mom' : die "Can't open $foo: $!"; # a bit exotic, that last one The _i_f statement is straightforward. Since BLOCKs are always bounded by curly brackets, there is never any ambi- guity about which _i_f an _e_l_s_e goes with. If you use _u_n_l_e_s_s in place of _i_f, the sense of the test is reversed. The _w_h_i_l_e statement executes the block as long as the expression is true (does not evaluate to the null string or 0). The LABEL is optional, and if present, consists of an identifier followed by a colon. The LABEL identifies the loop for the loop control statements _n_e_x_t, _l_a_s_t, and _r_e_d_o (see below). If there is a _c_o_n_t_i_n_u_e BLOCK, it is always executed just before the conditional is about to be evaluated again, similarly to the third part of a _f_o_r loop in C. Thus it can be used to increment a loop variable, even when the loop has been continued via the _n_e_x_t statement (similar to the C "continue" statement). If the word _w_h_i_l_e is replaced by the word _u_n_t_i_l, the sense of the test is reversed, but the conditional is still tested before the first iteration. In either the _i_f or the _w_h_i_l_e statement, you may replace "(EXPR)" with a BLOCK, and the conditional is true if the value of the last command in that block is true. The _f_o_r loop works exactly like the corresponding _w_h_i_l_e loop: for ($i = 1; $i < 10; $i++) { ... } is the same as $i = 1; while ($i < 10) { ... } continue { $i++; } 17 PERL(1) USER COMMANDS PERL(1) The foreach loop iterates over a normal array value and sets the variable VAR to be each element of the array in turn. The variable is implicitly local to the loop, and regains its former value upon exiting the loop. The "foreach" key- word is actually identical to the "for" keyword, so you can use "foreach" for readability or "for" for brevity. If VAR is omitted, $_ is set to each value. If ARRAY is an actual array (as opposed to an expression returning an array value), you can modify each element of the array by modify- ing VAR inside the loop. Examples: for (@ary) { s/foo/bar/; } foreach $elem (@elements) { $elem *= 2; } for ((10,9,8,7,6,5,4,3,2,1,'BOOM')) { print $_, "\n"; sleep(1); } for (1..15) { print "Merry Christmas\n"; } foreach $item (split(/:[\\\n:]*/, $ENV{'TERMCAP'})) { print "Item: $item\n"; } The BLOCK by itself (labeled or not) is equivalent to a loop that executes once. Thus you can use any of the loop con- trol statements in it to leave or restart the block. The _c_o_n_t_i_n_u_e block is optional. This construct is particularly nice for doing case structures. foo: { if (/^abc/) { $abc = 1; last foo; } if (/^def/) { $def = 1; last foo; } if (/^xyz/) { $xyz = 1; last foo; } $nothing = 1; } There is no official switch statement in perl, because there are already several ways to write the equivalent. In addi- tion to the above, you could write foo: { $abc = 1, last foo if /^abc/; $def = 1, last foo if /^def/; $xyz = 1, last foo if /^xyz/; $nothing = 1; } 18 PERL(1) USER COMMANDS PERL(1) or foo: { /^abc/ && do { $abc = 1; last foo; }; /^def/ && do { $def = 1; last foo; }; /^xyz/ && do { $xyz = 1; last foo; }; $nothing = 1; } or foo: { /^abc/ && ($abc = 1, last foo); /^def/ && ($def = 1, last foo); /^xyz/ && ($xyz = 1, last foo); $nothing = 1; } or even if (/^abc/) { $abc = 1; } elsif (/^def/) { $def = 1; } elsif (/^xyz/) { $xyz = 1; } else {$nothing = 1;} As it happens, these are all optimized internally to a switch structure, so perl jumps directly to the desired statement, and you needn't worry about perl executing a lot of unnecessary statements when you have a string of 50 elsifs, as long as you are testing the same simple scalar variable using ==, eq, or pattern matching as above. (If you're curious as to whether the optimizer has done this for a particular case statement, you can use the -D1024 switch to list the syntax tree before execution.) SSSSiiiimmmmpppplllleeee ssssttttaaaatttteeeemmmmeeeennnnttttssss The only kind of simple statement is an expression evaluated for its side effects. Every expression (simple statement) must be terminated with a semicolon. Note that this is like C, but unlike Pascal (and _a_w_k). Any simple statement may optionally be followed by a single modifier, just before the terminating semicolon. The possi- ble modifiers are: 19 PERL(1) USER COMMANDS PERL(1) if EXPR unless EXPR while EXPR until EXPR The _i_f and _u_n_l_e_s_s modifiers have the expected semantics. The _w_h_i_l_e and _u_n_t_i_l modifiers also have the expected seman- tics (conditional evaluated first), except when applied to a do-BLOCK or a do-SUBROUTINE command, in which case the block executes once before the conditional is evaluated. This is so that you can write loops like: do { $_ = <STDIN>; ... } until $_ eq ".\n"; (See the _d_o operator below. Note also that the loop control commands described later will NOT work in this construct, since modifiers don't take loop labels. Sorry.) EEEExxxxpppprrrreeeessssssssiiiioooonnnnssss Since _p_e_r_l expressions work almost exactly like C expres- sions, only the differences will be mentioned here. Here's what _p_e_r_l has that C doesn't: ** The exponentiation operator. **= The exponentiation assignment operator. () The null list, used to initialize an array to null. . Concatenation of two strings. .= The concatenation assignment operator. eq String equality (== is numeric equality). For a mnemonic just think of "eq" as a string. (If you are used to the _a_w_k behavior of using == for either string or numeric equality based on the current form of the comparands, beware! You must be explicit here.) ne String inequality (!= is numeric inequality). lt String less than. gt String greater than. 20 PERL(1) USER COMMANDS PERL(1) le String less than or equal. ge String greater than or equal. cmp String comparison, returning -1, 0, or 1. <=> Numeric comparison, returning -1, 0, or 1. =~ Certain operations search or modify the string "$_" by default. This operator makes that kind of opera- tion work on some other string. The right argument is a search pattern, substitution, or translation. The left argument is what is supposed to be searched, substituted, or translated instead of the default "$_". The return value indicates the suc- cess of the operation. (If the right argument is an expression other than a search pattern, substitu- tion, or translation, it is interpreted as a search pattern at run time. This is less efficient than an explicit search, since the pattern must be compiled every time the expression is evaluated.) The pre- cedence of this operator is lower than unary minus and autoincrement/decrement, but higher than every- thing else. !~ Just like =~ except the return value is negated. x The repetition operator. Returns a string consist- ing of the left operand repeated the number of times specified by the right operand. In an array con- text, if the left operand is a list in parens, it repeats the list. print '-' x 80; # print row of dashes print '-' x80; # illegal, x80 is identifier print "\t" x ($tab/8), ' ' x ($tab%8); # tab over @ones = (1) x 80; # an array of 80 1's @ones = (5) x @ones; # set all elements to 5 x= The repetition assignment operator. Only works on scalars. .. The range operator, which is really two different operators depending on the context. In an array context, returns an array of values counting (by ones) from the left value to the right value. This is useful for writing "for (1..10)" loops and for doing slice operations on arrays. 21 PERL(1) USER COMMANDS PERL(1) In a scalar context, .. returns a boolean value. The operator is bistable, like a flip-flop.. Each .. operator maintains its own boolean state. It is false as long as its left operand is false. Once the left operand is true, the range operator stays true until the right operand is true, AFTER which the range operator becomes false again. (It doesn't become false till the next time the range operator is evaluated. It can become false on the same evaluation it became true, but it still returns true once.) The right operand is not evaluated while the operator is in the "false" state, and the left operand is not evaluated while the operator is in the "true" state. The scalar .. operator is pri- marily intended for doing line number ranges after the fashion of _s_e_d or _a_w_k. The precedence is a lit- tle lower than || and &&. The value returned is either the null string for false, or a sequence number (beginning with 1) for true. The sequence number is reset for each range encountered. The final sequence number in a range has the string 'E0' appended to it, which doesn't affect its numeric value, but gives you something to search for if you want to exclude the endpoint. You can exclude the beginning point by waiting for the sequence number to be greater than 1. If either operand of scalar .. is static, that operand is implicitly compared to the $. variable, the current line number. Examples: As a scalar operator: if (101 .. 200) { print; } # print 2nd hundred lines next line if (1 .. /^$/); # skip header lines s/^/> / if (/^$/ .. eof()); # quote body As an array operator: for (101 .. 200) { print; } # print $_ 100 times @foo = @foo[$[ .. $#foo]; # an expensive no-op @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items -x A file test. This unary operator takes one argu- ment, either a filename or a filehandle, and tests the associated file to see if something is true about it. If the argument is omitted, tests $_, except for -t, which tests _S_T_D_I_N. It returns 1 for true and '' for false, or the undefined value if the file doesn't exist. Precedence is higher than logi- cal and relational operators, but lower than arith- metic operators. The operator may be any of: 22 PERL(1) USER COMMANDS PERL(1) -r File is readable by effective uid. -w File is writable by effective uid. -x File is executable by effective uid. -o File is owned by effective uid. -R File is readable by real uid. -W File is writable by real uid. -X File is executable by real uid. -O File is owned by real uid. -e File exists. -z File has zero size. -s File has non-zero size (returns size). -f File is a plain file. -d File is a directory. -l File is a symbolic link. -p File is a named pipe (FIFO). -S File is a socket. -b File is a block special file. -c File is a character special file. -u File has setuid bit set. -g File has setgid bit set. -k File has sticky bit set. -t Filehandle is opened to a tty. -T File is a text file. -B File is a binary file (opposite of -T). -M Age of file in days when script started. -A Same for access time. -C Same for inode change time. The interpretation of the file permission operators -r, -R, -w, -W, -x and -X is based solely on the mode of the file and the uids and gids of the user. There may be other reasons you can't actually read, write or execute the file. (Note that serveral of the above don't mean much under MS-DOS.) Example: while (<>) { chop; next unless -f $_; # ignore specials ... } Note that -s/a/b/ does not do a negated substitu- tion. Saying -exp($foo) still works as expected, however--only single letters following a minus are interpreted as file tests. The -T and -B switches work as follows. The first block or so of the file is examined for odd charac- ters such as strange control codes or metacharac- ters. If too many odd characters (>10%) are found, 23 PERL(1) USER COMMANDS PERL(1) it's a -B file, otherwise it's a -T file. Also, any file containing null in the first block is con- sidered a binary file. If -T or -B is used on a filehandle, the current stdio buffer is examined rather than the first block. Both -T and -B return TRUE on a null file, or a file at EOF when testing a filehandle. If any of the file tests (or either stat operator) are given the special filehandle consisting of a solitary underline, then the stat structure of the previous file test (or stat operator) is used, saving a system call. (This doesn't work with -t, and you need to remember that lstat and -l will leave values in the stat structure for the symbolic link, not the real file.) Example: print "Can do.\n" if -r $a || -w _ || -x _; stat($filename); print "Readable\n" if -r _; print "Writable\n" if -w _; print "Executable\n" if -x _; print "Setuid\n" if -u _; print "Setgid\n" if -g _; print "Sticky\n" if -k _; print "Text\n" if -T _; print "Binary\n" if -B _; Here is what C has that _p_e_r_l doesn't: unary & Address-of operator. unary * Dereference-address operator. (TYPE) Type casting operator. Like C, _p_e_r_l does a certain amount of expression evaluation at compile time, whenever it determines that all of the arguments to an operator are static and have no side effects. In particular, string concatenation happens at compile time between literals that don't do variable substi- tution. Backslash interpretation also happens at compile time. You can say 'Now is the time for all' . "\n" . 'good men to come to.' and this all reduces to one string internally. The autoincrement operator has a little extra built-in magic to it. If you increment a variable that is numeric, or that 24 PERL(1) USER COMMANDS PERL(1) has ever been used in a numeric context, you get a normal increment. If, however, the variable has only been used in string contexts since it was set, and has a value that is not null and matches the pattern /^[a-zA-Z]*[0-9]*$/, the increment is done as a string, preserving each character within its range, with carry: print ++($foo = '99'); # prints '100' print ++($foo = 'a0'); # prints 'a1' print ++($foo = 'Az'); # prints 'Ba' print ++($foo = 'zz'); # prints 'aaa' The autodecrement is not magical. The range operator (in an array context) makes use of the magical autoincrement algorithm if the minimum and maximum are strings. You can say @alphabet = ('A' .. 'Z'); to get all the letters of the alphabet, or $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15]; to get a hexadecimal digit, or @z2 = ('01' .. '31'); print @z2[$mday]; to get dates with leading zeros. (If the final value speci- fied is not in the sequence that the magical increment would produce, the sequence goes until the next value would be longer than the final value specified.) The || and && operators differ from C's in that, rather than returning 0 or 1, they return the last value evaluated. Thus, a portable way to find out the home directory might be: $home = $ENV{'HOME'} || $ENV{'LOGDIR'} || (getpwuid($<))[7] || die "You're homeless!\n"; Along with the literals and variables mentioned earlier, the operations in the following section can serve as terms in an expression. Some of these operations take a LIST as an argument. Such a list can consist of any combination of scalar arguments or array values; the array values will be included in the list as if each individual element were interpolated at that point in the list, forming a longer single-dimensional array value. Elements of the LIST should be separated by commas. If an operation is listed both with and without parentheses around its arguments, it means you 25 PERL(1) USER COMMANDS PERL(1) can either use it as a unary operator or as a function call. To use it as a function call, the next token on the same line must be a left parenthesis. (There may be intervening white space.) Such a function then has highest precedence, as you would expect from a function. If any token other than a left parenthesis follows, then it is a unary opera- tor, with a precedence depending only on whether it is a LIST operator or not. LIST operators have lowest pre- cedence. All other unary operators have a precedence greater than relational operators but less than arithmetic operators. See the section on Precedence. /PATTERN/ See m/PATTERN/. ?PATTERN? This is just like the /pattern/ search, except that it matches only once between calls to the _r_e_s_e_t operator. This is a useful optimization when you only want to see the first occurrence of something in each file of a set of files, for instance. Only ?? patterns local to the current package are reset. accept(NEWSOCKET,GENERICSOCKET) Does the same thing that the accept system call does. Sockets are not yet supported on MS-DOS. alarm(SECONDS) alarm SECONDS Not supported on MS-DOS. atan2(Y,X) Returns the arctangent of Y/X in the range -PI to PI. bind(SOCKET,NAME) Does the same thing that the bind system call does. Returns true if it succeeded, false otherwise. NAME should be a packed address of the proper type for the socket. See example in section on Interprocess Communication. binmode(FILEHANDLE) binmode FILEHANDLE Arranges for the file to be read in "binary" mode in operating systems that distinguish between binary and text files. Files that are not read in binary mode have CR LF sequences translated to LF on input and LF translated to CR LF on output. Binmode has no effect under Unix. If FILEHANDLE is an 26 PERL(1) USER COMMANDS PERL(1) expression, the value is taken as the name of the filehandle. caller(EXPR) caller Returns the context of the current subroutine call: ($package,$filename,$line) = caller; With EXPR, returns some extra information that the debugger uses to print a stack trace. The value of EXPR indicates how many call frames to go back before the current one. chdir(EXPR) chdir EXPR Changes the working directory to EXPR, if possible. If EXPR is omitted, changes to home directory. Returns 1 upon success, 0 otherwise. See example under _d_i_e. On MS-DOS, the path and/or the directory can be specified. chdir "c:/junk/subdir"; chdir "e:"; chdir "abc/xyz"; The first of these changes the default drive and directory. The second changes to the default direc- tory on drive E:. The third changes the directory on the current drive. As with all filenames, for- ward slashes are recommended inside _p_e_r_l. (See the section on MS-DOS.) chmod(LIST) chmod LIST Changes the permissions of a list of files. The first element of the list must be the numerical mode. Returns the number of files successfully changed. $cnt = chmod 0755, 'foo', 'bar'; chmod 0755, @executables; 27 PERL(1) USER COMMANDS PERL(1) chop(LIST) chop(VARIABLE) chop VARIABLE chop Chops off the last character of a string and returns the character chopped. It's used primarily to remove the newline from the end of an input record, but is much more efficient than s/\n// because it neither scans nor copies the string. If VARIABLE is omitted, chops $_. Example: while (<>) { chop; # avoid \n on last field @array = split(/:/); ... } You can actually chop anything that's an lvalue, including an assignment: chop($cwd = `pwd`); chop($answer = <STDIN>); If you chop a list, each element is chopped. Only the value of the last chop is returned. chown(LIST) chown LIST Changes the owner (and group) of a list of files. The first two elements of the list must be the NUMERICAL uid and gid, in that order. Returns the number of files successfully changed. $cnt = chown $uid, $gid, 'foo', 'bar'; chown $uid, $gid, @filenames; 28 PERL(1) USER COMMANDS PERL(1) Here's an example that looks up non-numeric uids in the passwd file: print "User: "; $user = <STDIN>; chop($user); print "Files: " $pattern = <STDIN>; chop($pattern); open(pass, '/etc/passwd') || die "Can't open passwd: $!\n"; while (<pass>) { ($login,$pass,$uid,$gid) = split(/:/); $uid{$login} = $uid; $gid{$login} = $gid; } @ary = <${pattern}>; # get filenames if ($uid{$user} eq '') { die "$user not in passwd file"; } else { chown $uid{$user}, $gid{$user}, @ary; } chroot(FILENAME) chroot FILENAME Not supported on MS-DOS. close(FILEHANDLE) close FILEHANDLE Closes the file or pipe associated with the file handle. You don't have to close FILEHANDLE if you are immediately going to do another open on it, since open will close it for you. (See _o_p_e_n.) How- ever, an explicit close on an input file resets the line counter ($.), while the implicit close done by _o_p_e_n does not. Also, closing a pipe will wait for the process executing on the pipe to complete, in case you want to look at the output of the pipe afterwards. Closing a pipe explicitly also puts the status value of the command into $?. Example: open(OUTPUT, '|sort >foo'); # pipe to sort ... # print stuff to output close OUTPUT; # wait for sort to finish open(INPUT, 'foo'); # get sort's results FILEHANDLE may be an expression whose value gives the real filehandle name. 29 PERL(1) USER COMMANDS PERL(1) closedir(DIRHANDLE) closedir DIRHANDLE Closes a directory opened by opendir(). connect(SOCKET,NAME) Does the same thing that the connect system call does. Sockets are not yet supported on MS-DOS. cos(EXPR) cos EXPR Returns the cosine of EXPR (expressed in radians). If EXPR is omitted takes cosine of $_. crypt(PLAINTEXT,SALT) Encrypts a string exactly like the crypt() function in the C library. Sockets are not yet supported on MS-DOS. dbmclose(ASSOC_ARRAY) dbmclose ASSOC_ARRAY Breaks the binding between a dbm file and an associ- ative array. The values remaining in the associa- tive array are meaningless unless you happen to want to know what was in the cache for the dbm file. This function is only useful if you have ndbm. dbmopen(ASSOC,DBNAME,MODE) This binds a dbm or ndbm file to an associative array. ASSOC is the name of the associative array. (Unlike normal open, the first argument is NOT a filehandle, even though it looks like one). DBNAME is the name of the database (without the .dir or .pag extension). If the database does not exist, it is created with protection specified by MODE (as modified by the umask). If your system only sup- ports the older dbm functions, you may only have one dbmopen in your program. If your system has neither dbm nor ndbm, calling dbmopen produces a fatal error. (MS-DOS users: the MS-DOS port uses Gnu-dbm, which supports multiple files.) Values assigned to the associative array prior to the dbmopen are lost. A certain number of values from the dbm file are cached in memory. By default this number is 64, but you can increase it by preal- locating that number of garbage entries in the asso- ciative array before the dbmopen. You can flush the cache if necessary with the reset command. 30 PERL(1) USER COMMANDS PERL(1) If you don't have write access to the dbm file, you can only read associative array variables, not set them. If you want to test whether you can write, either use file tests or try setting a dummy array entry inside an eval, which will trap the error. Note that functions such as keys() and values() may return huge array values when used on large dbm files. You may prefer to use the each() function to iterate over large dbm files. Example: # print out history file offsets dbmopen(HIST,'/usr/lib/news/history',0666); while (($key,$val) = each %HIST) { print $key, ' = ', unpack('L',$val), "\n"; } dbmclose(HIST); defined(EXPR) defined EXPR Returns a boolean value saying whether the lvalue EXPR has a real value or not. Many operations return the undefined value under exceptional condi- tions, such as end of file, uninitialized variable, system error and such. This function allows you to distinguish between an undefined null string and a defined null string with operations that might return a real null string, in particular referencing elements of an array. You may also check to see if arrays or subroutines exist. Use on predefined variables is not guaranteed to produce intuitive results. Examples: print if defined $switch{'D'}; print "$val\n" while defined($val = pop(@ary)); die "Can't readlink $sym: $!" unless defined($value = readlink $sym); eval '@foo = ()' if defined(@foo); die "No XYZ package defined" unless defined %_XYZ; sub foo { defined &bar ? &bar(@_) : die "No bar"; } See also undef. delete $ASSOC{KEY} Deletes the specified value from the specified asso- ciative array. Returns the deleted value, or the undefined value if nothing was deleted. Deleting from $ENV{} modifies the environment. Deleting from an array bound to a dbm file deletes the entry from the dbm file. 31 PERL(1) USER COMMANDS PERL(1) The following deletes all the values of an associa- tive array: foreach $key (keys %ARRAY) { delete $ARRAY{$key}; } (But it would be faster to use the _r_e_s_e_t command. Saying undef %ARRAY is faster yet.) die(LIST) die LIST Outside of an eval, prints the value of LIST to _S_T_D_E_R_R and exits with the current value of $! (errno). If $! is 0, exits with the value of ($? >> 8) (`command` status). If ($? >> 8) is 0, exits with 255. Inside an eval, the error message is stuffed into $@ and the eval is terminated with the undefined value. Equivalent examples: die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; chdir '/usr/spool/news' || die "Can't cd to spool: $!\n" If the value of EXPR does not end in a newline, the current script line number and input line number (if any) are also printed, and a newline is supplied. Hint: sometimes appending ", stopped" to your mes- sage will cause it to make better sense when the string "at foo line 123" is appended. Suppose you are running script "canasta". die "/etc/games is no good"; die "/etc/games is no good, stopped"; produce, respectively /etc/games is no good at canasta line 123. /etc/games is no good, stopped at canasta line 123. See also _e_x_i_t. do BLOCK Returns the value of the last command in the sequence of commands indicated by BLOCK. When modi- fied by a loop modifier, executes the BLOCK once before testing the loop condition. (On other 32 PERL(1) USER COMMANDS PERL(1) statements the loop modifiers test the conditional first.) do SUBROUTINE (LIST) Executes a SUBROUTINE declared by a _s_u_b declaration, and returns the value of the last expression evaluated in SUBROUTINE. If there is no subroutine by that name, produces a fatal error. (You may use the "defined" operator to determine if a subroutine exists.) If you pass arrays as part of LIST you may wish to pass the length of the array in front of each array. (See the section on subroutines later on.) SUBROUTINE may be a scalar variable, in which case the variable contains the name of the subrou- tine to execute. The parentheses are required to avoid confusion with the "do EXPR" form. As an alternate form, you may call a subroutine by prefixing the name with an ampersand: &foo(@args). If you aren't passing any arguments, you don't have to use parentheses. If you omit the parentheses, no @_ array is passed to the subroutine. The & form is also used to specify subroutines to the defined and undef operators. do EXPR Uses the value of EXPR as a filename and executes the contents of the file as a _p_e_r_l script. Its pri- mary use is to include subroutines from a _p_e_r_l sub- routine library. do 'stat.pl'; is just like eval `cat stat.pl`; except that it's more efficient, more concise, keeps track of the current filename for error messages, and searches all the ----IIII libraries if the file isn't in the current directory (see also the @INC array in Predefined Names). It's the same, however, in that it does reparse the file every time you call it, so if you are going to use the file inside a loop you might prefer to use -P and #include, at the expense of a little more startup time. (The main problem with #include is that cpp doesn't grok # comments--a workaround is to use ";#" for standalone comments.) Note that the following are NOT equivalent: do $foo; # eval a file do $foo(); # call a subroutine 33 PERL(1) USER COMMANDS PERL(1) Note that inclusion of library routines is better done with the "require" operator. dump LABEL This causes an immediate core dump. Not supported on MS-DOS. Example: #!/usr/bin/perl require 'getopt.pl'; require 'stat.pl'; %days = ( 'Sun',1, 'Mon',2, 'Tue',3, 'Wed',4, 'Thu',5, 'Fri',6, 'Sat',7); dump QUICKSTART if $ARGV[0] eq '-d'; QUICKSTART: do Getopt('f'); each(ASSOC_ARRAY) each ASSOC_ARRAY Returns a 2 element array consisting of the key and value for the next value of an associative array, so that you can iterate over it. Entries are returned in an apparently random order. When the array is entirely read, a null array is returned (which when assigned produces a FALSE (0) value). The next call to each() after that will start iterating again. The iterator can be reset only by reading all the elements from the array. You must not modify the array while iterating over it. There is a single iterator for each associative array, shared by all each(), keys() and values() function calls in the program. The following prints out your environment like the printenv program, only in a different order: while (($key,$value) = each %ENV) { print "$key=$value\n"; } See also keys() and values(). 34 PERL(1) USER COMMANDS PERL(1) eof(FILEHANDLE) eof() eof Returns 1 if the next read on FILEHANDLE will return end of file, or if FILEHANDLE is not open. FILEHAN- DLE may be an expression whose value gives the real filehandle name. (Note that this function actually reads a character and then ungetc's it, so it is not very useful in an interactive context.) An eof without an argument returns the eof status for the last file read. Empty parentheses () may be used to indicate the pseudo file formed of the files listed on the command line, i.e. eof() is reasonable to use inside a while (<>) loop to detect the end of only the last file. Use eof(ARGV) or eof without the parentheses to test EACH file in a while (<>) loop. Examples: # insert dashes just before last line of last file while (<>) { if (eof()) { print "--------------\n"; } print; } # reset line numbering on each input file while (<>) { print "$.\t$_"; if (eof) { # Not eof(). close(ARGV); } } eval(EXPR) eval EXPR EXPR is parsed and executed as if it were a little _p_e_r_l program. It is executed in the context of the current _p_e_r_l program, so that any variable settings, subroutine or format definitions remain afterwards. The value returned is the value of the last expres- sion evaluated, just as with subroutines. If there is a syntax error or runtime error, or a die state- ment is executed, an undefined value is returned by eval, and $@ is set to the error message. If there was no error, $@ is guaranteed to be a null string. If EXPR is omitted, evaluates $_. The final semi- colon, if any, may be omitted from the expression. 35 PERL(1) USER COMMANDS PERL(1) Note that, since eval traps otherwise-fatal errors, it is useful for determining whether a particular feature (such as dbmopen or symlink) is implemented. It is also Perl's exception trapping mechanism, where the die operator is used to raise exceptions. exec(LIST) exec LIST If there is more than one argument in LIST, or if LIST is an array with more than one value, calls execvp() with the arguments in LIST. If there is only one scalar argument, the argument is checked for shell metacharacters. If there are any, the entire argument is passed to "/bin/sh -c" for pars- ing. If there are none, the argument is split into words and passed directly to execvp(), which is more efficient. Note: exec (and system) do not flush your output buffer, so you may need to set $| to avoid lost output. Examples: exec '/bin/echo', 'Your arguments are: ', @ARGV; exec "sort $outfile | uniq"; If you don't really want to execute the first argu- ment, but want to lie to the program you are execut- ing about its own name, you can specify the program you actually want to run by assigning that to a variable and putting the name of the variable in front of the LIST without a comma. (This always forces interpretation of the LIST as a multi-valued list, even if there is only a single scalar in the list.) Example: $shell = '/bin/csh'; exec $shell '-sh'; # pretend it's a login shell The MS-DOS implementation of _e_x_e_c has problems. Unlike _s_y_s_t_e_m and pipes it is not MKS toolkit compa- tible. As of this writing (September 1991) it will not even allow for temporary file cleanup. The use of _e_x_e_c on MS-DOS is strongly discouraged. exit(EXPR) exit EXPR Evaluates EXPR and exits immediately with that value. Example: $ans = <STDIN>; exit 0 if $ans =~ /^[Xx]/; 36 PERL(1) USER COMMANDS PERL(1) See also _d_i_e. If EXPR is omitted, exits with 0 status. exp(EXPR) exp EXPR Returns _e to the power of EXPR. If EXPR is omitted, gives exp($_). fcntl(FILEHANDLE,FUNCTION,SCALAR) Implements the fcntl(2) function. M Not supported on MS-DOS. fileno(FILEHANDLE) fileno FILEHANDLE Returns the file descriptor for a filehandle. Use- ful for constructing bitmaps for select(). If FILEHANDLE is an expression, the value is taken as the name of the filehandle. flock(FILEHANDLE,OPERATION) Calls flock(2) on FILEHANDLE. See manual page for flock(2) for definition of OPERATION. Returns true for success, false on failure. Will produce a fatal error if used on a machine that doesn't implement flock(2). Here's a mailbox appender for BSD sys- tems. $LOCK_SH = 1; $LOCK_EX = 2; $LOCK_NB = 4; $LOCK_UN = 8; sub lock { flock(MBOX,$LOCK_EX); # and, in case someone appended # while we were waiting... seek(MBOX, 0, 2); } sub unlock { flock(MBOX,$LOCK_UN); } open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}") || die "Can't open mailbox: $!"; do lock(); print MBOX $msg,"\n\n"; do unlock(); 37 PERL(1) USER COMMANDS PERL(1) fork Does a fork() call. Not supported on MS-DOS. getc(FILEHANDLE) getc FILEHANDLE getc Returns the next character from the input file attached to FILEHANDLE, or a null string at EOF. If FILEHANDLE is omitted, reads from STDIN. getlogin Returns the current login from /etc/utmp, if any. (Not supported on MS-DOS.) getpeername(SOCKET) Returns the packed sockaddr address of other end of the SOCKET connection. (Sockets are not yet sup- ported on MS-DOS.) getpgrp(PID) getpgrp PID Returns the current process group for the specified PID, 0 for the current process. Not supported on MS-DOS. getppid On Unix, returns the process id of the parent pro- cess. Not supported on MS-DOS. getpriority(WHICH,WHO) Returns the current priority for a process, a pro- cess group, or a user. Not supported on MS-DOS. getpwnam(NAME) getgrnam(NAME) gethostbyname(NAME) getnetbyname(NAME) getprotobyname(NAME) getpwuid(UID) getgrgid(GID) getservbyname(NAME,PROTO) gethostbyaddr(ADDR,ADDRTYPE) 38 PERL(1) USER COMMANDS PERL(1) getnetbyaddr(ADDR,ADDRTYPE) getprotobynumber(NUMBER) getservbyport(PORT,PROTO) getpwent getgrent gethostent getnetent getprotoent getservent setpwent setgrent sethostent(STAYOPEN) setnetent(STAYOPEN) setprotoent(STAYOPEN) setservent(STAYOPEN) endpwent endgrent endhostent endnetent endprotoent endservent These routines perform the same functions as their counterparts in the system library. None are sup- ported on MS-DOS. getsockname(SOCKET) Returns the packed sockaddr address of this end of the SOCKET connection. Sockets are not yet supported on MS-DOS. getsockopt(SOCKET,LEVEL,OPTNAME) Returns the socket option requested, or undefined if 39 PERL(1) USER COMMANDS PERL(1) there is an error. Sockets are not yet supported on MS-DOS. gmtime(EXPR) gmtime EXPR Converts a time as returned by the time function to a 9-element array with the time analyzed for the Greenwich timezone. Typically used as follows: ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = gmtime(time); All array elements are numeric, and come straight out of a struct tm. In particular this means that $mon has the range 0..11 and $wday has the range 0..6. If EXPR is omitted, does gmtime(time). goto LABEL Finds the statement labeled with LABEL and resumes execution there. Currently you may only go to statements in the main body of the program that are not nested inside a do {} construct. This statement is not implemented very efficiently, and is here only to make the _s_e_d-to-_p_e_r_l translator easier. I may change its semantics at any time, consistent with support for translated _s_e_d scripts. Use it at your own risk. Better yet, don't use it at all. grep(EXPR,LIST) Evaluates EXPR for each element of LIST (locally setting $_ to each element) and returns the array value consisting of those elements for which the expression evaluated to true. In a scalar context, returns the number of times the expression was true. @foo = grep(!/^#/, @bar); # weed out comments Note that, since $_ is a reference into the array value, it can be used to modify the elements of the array. While this is useful and supported, it can cause bizarre results if the LIST is not a named array. hex(EXPR) hex EXPR Returns the decimal value of EXPR interpreted as an hex string. (To interpret strings that might start with 0 or 0x see oct().) If EXPR is omitted, uses $_. 40 PERL(1) USER COMMANDS PERL(1) index(STR,SUBSTR,POSITION) index(STR,SUBSTR) Returns the position of the first occurrence of SUBSTR in STR at or after POSITION. If POSITION is omitted, starts searching from the beginning of the string. The return value is based at 0, or whatever you've set the $[ variable to. If the substring is not found, returns one less than the base, ordi- narily -1. int(EXPR) int EXPR Returns the integer portion of EXPR. If EXPR is omitted, uses $_. ioctl(FILEHANDLE,FUNCTION,SCALAR) Implements the ioctl(2) function. You'll probably have to say require "ioctl.ph"; # probably /usr/local/lib/perl/ioctl.ph first to get the correct function definitions. If ioctl.ph doesn't exist or doesn't have the correct definitions you'll have to roll your own, based on your C header files such as <sys/ioctl.h>. (There is a perl script called h2ph that comes with the Unix perl kit which may help you in this.) SCALAR will be read and/or written depending on the FUNCTION--a pointer to the string value of SCALAR will be passed as the third argument of the actual ioctl call. (If SCALAR has no string value but does have a numeric value, that value will be passed rather than a pointer to the string value. To guarantee this to be true, add a 0 to the scalar before using it.) The pack() and unpack() functions are useful for manipulating the values of structures used by ioctl(). The following example sets the erase character to DEL. require 'ioctl.ph'; $sgttyb_t = "ccccs"; # 4 chars and a short if (ioctl(STDIN,$TIOCGETP,$sgttyb)) { @ary = unpack($sgttyb_t,$sgttyb); $ary[2] = 127; $sgttyb = pack($sgttyb_t,@ary); ioctl(STDIN,$TIOCSETP,$sgttyb) || die "Can't ioctl: $!"; } The return value of ioctl (and fcntl) is as follows: 41 PERL(1) USER COMMANDS PERL(1) if OS returns: perl returns: -1 undefined value 0 string "0 but true" anything else that number Thus perl returns true on success and false on failure, yet you can still easily determine the actual value returned by the operating system: ($retval = ioctl(...)) || ($retval = -1); printf "System returned %d\n", $retval; join(EXPR,LIST) join(EXPR,ARRAY) Joins the separate strings of LIST or ARRAY into a single string with fields separated by the value of EXPR, and returns the string. Example: $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); See _s_p_l_i_t. keys(ASSOC_ARRAY) keys ASSOC_ARRAY Returns a normal array consisting of all the keys of the named associative array. The keys are returned in an apparently random order, but it is the same order as either the values() or each() function pro- duces (given that the associative array has not been modified). Here is yet another way to print your environment: @keys = keys %ENV; @values = values %ENV; while ($#keys >= 0) { print pop(@keys), '=', pop(@values), "\n"; } or how about sorted by key: foreach $key (sort(keys %ENV)) { print $key, '=', $ENV{$key}, "\n"; } 42 PERL(1) USER COMMANDS PERL(1) kill(LIST) kill LIST Sends a signal to a list of processes. The first element of the list must be the signal to send. Returns the number of processes successfully sig- naled. $cnt = kill 1, $child1, $child2; kill 9, @goners; If the signal is negative, kills process groups instead of processes. (On System V, a negative _p_r_o_- _c_e_s_s number will also kill process groups, but that's not portable.) You may use a signal name in quotes. last LABEL last The _l_a_s_t command is like the _b_r_e_a_k statement in C (as used in loops); it immediately exits the loop in question. If the LABEL is omitted, the command refers to the innermost enclosing loop. The _c_o_n_- _t_i_n_u_e block, if any, is not executed: line: while (<STDIN>) { last line if /^$/; # exit when done with header ... } length(EXPR) length EXPR Returns the length in characters of the value of EXPR. If EXPR is omitted, returns length of $_. link(OLDFILE,NEWFILE) Creates a new filename linked to the old filename. Returns 1 for success, 0 otherwise. listen(SOCKET,QUEUESIZE) Does the same thing that the listen system call does. Sockets are not yet supported on MS-DOS. local(LIST) Declares the listed variables to be local to the enclosing block, subroutine, eval or "do". All the listed elements must be legal lvalues. This opera- tor works by saving the current values of those variables in LIST on a hidden stack and restoring them upon exiting the block, subroutine or eval. 43 PERL(1) USER COMMANDS PERL(1) This means that called subroutines can also refer- ence the local variable, but not the global one. The LIST may be assigned to if desired, which allows you to initialize your local variables. (If no ini- tializer is given for a particular variable, it is created with an undefined value.) Commonly this is used to name the parameters to a subroutine. Exam- ples: sub RANGEVAL { local($min, $max, $thunk) = @_; local($result) = ''; local($i); # Presumably $thunk makes reference to $i for ($i = $min; $i < $max; $i++) { $result .= eval $thunk; } $result; } if ($sw eq '-v') { # init local array with global array local(@ARGV) = @ARGV; unshift(@ARGV,'echo'); system @ARGV; } # @ARGV restored # temporarily add to digits associative array if ($base12) { # (NOTE: not claiming this is efficient!) local(%digits) = (%digits,'t',10,'e',11); do parse_num(); } Note that local() is a run-time command, and so gets executed every time through a loop, using up more stack storage each time until it's all released at once when the loop is exited. localtime(EXPR) localtime EXPR Converts a time as returned by the time function to a 9-element array with the time analyzed for the local timezone. Typically used as follows: 44 PERL(1) USER COMMANDS PERL(1) ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime(time); All array elements are numeric, and come straight out of a struct tm. In particular this means that $mon has the range 0..11 and $wday has the range 0..6. If EXPR is omitted, does localtime(time). log(EXPR) log EXPR Returns logarithm (base _e) of EXPR. If EXPR is omitted, returns log of $_. lstat(FILEHANDLE) lstat FILEHANDLE lstat(EXPR) lstat SCALARVARIABLE Used to stat symbolic links on Unix. On MS-DOS, same as _s_t_a_t. m/PATTERN/gio /PATTERN/gio Searches a string for a pattern match, and returns true (1) or false (''). If no string is specified via the =~ or !~ operator, the $_ string is searched. (The string specified with =~ need not be an lvalue--it may be the result of an expression evaluation, but remember the =~ binds rather tightly.) See also the section on regular expres- sions. If / is the delimiter then the initial 'm' is optional. With the 'm' you can use any pair of non-alphanumeric characters as delimiters. This is particularly useful for matching Unix path names that contain '/'. If the final delimiter is fol- lowed by the optional letter 'i', the matching is done in a case-insensitive manner. PATTERN may con- tain references to scalar variables, which will be interpolated (and the pattern recompiled) every time the pattern search is evaluated. (Note that $) and $| may not be interpolated because they look like end-of-string tests.) If you want such a pattern to be compiled only once, add an "o" after the trailing delimiter. This avoids expensive run-time recompi- lations, and is useful when the value you are inter- polating won't change over the life of the script. 45 PERL(1) USER COMMANDS PERL(1) If the PATTERN evaluates to a null string, the most recent successful regular expression is used instead. If used in a context that requires an array value, a pattern match returns an array consisting of the subexpressions matched by the parentheses in the pattern, i.e. ($1, $2, $3...). It does NOT actually set $1, $2, etc. in this case, nor does it set $+, $`, $& or $'. If the match fails, a null array is returned. If the match succeeds, but there were no parentheses, an array value of (1) is returned. Examples: open(tty, '/dev/tty'); <tty> =~ /^y/i && do foo(); # do foo if desired if (/Version: *([0-9.]*)/) { $version = $1; } next if m#^/usr/spool/uucp#; # poor man's grep $arg = shift; while (<>) { print if /$arg/o; # compile only once } if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) This last example splits $foo into the first two words and the remainder of the line, and assigns those three fields to $F1, $F2 and $Etc. The condi- tional is true if any variables were assigned, i.e. if the pattern matched. The "g" modifier specifies global pattern matching--that is, matching as many times as possi- ble within the string. How it behaves depends on the context. In an array context, it returns a list of all the substrings matched by all the parentheses in the regular expression. If there are no parentheses, it returns a list of all the matched strings, as if there were parentheses around the whole pattern. In a scalar context, it iterates through the string, returning TRUE each time it matches, and FALSE when it eventually runs out of matches. (In other words, it remembers where it left off last time and restarts the search at that point.) It presumes that you have not modified the string since the last match. Modifying the string between matches may result in undefined behavior. 46 PERL(1) USER COMMANDS PERL(1) (You can actually get away with in-place modifica- tions via substr() that do not change the length of the entire string. In general, however, you should be using s///g for such modifications.) Examples: # array context ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); # scalar context $/ = 1; $* = 1; while ($paragraph = <>) { while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { $sentences++; } } print "$sentences\n"; mkdir(FILENAME,MODE) Creates the directory specified by FILENAME, with permissions specified by MODE (as modified by umask). If it succeeds it returns 1, otherwise it returns 0 and sets $! (errno). msgctl(ID,CMD,ARG) msgget(KEY,FLAGS) msgsnd(ID,MSG,FLAGS) msgrcv(ID,VAR,SIZE,TYPE,FLAGS) These System V inter-process communications func- tions are not available on MS-DOS. next LABEL next The _n_e_x_t command is like the _c_o_n_t_i_n_u_e statement in C; it starts the next iteration of the loop: line: while (<STDIN>) { next line if /^#/; # discard comments ... } Note that if there were a _c_o_n_t_i_n_u_e block on the above, it would get executed even on discarded lines. If the LABEL is omitted, the command refers to the innermost enclosing loop. oct(EXPR) 47 PERL(1) USER COMMANDS PERL(1) oct EXPR Returns the decimal value of EXPR interpreted as an octal string. (If EXPR happens to start off with 0x, interprets it as a hex string instead.) The following will handle decimal, octal and hex in the standard notation: $val = oct($val) if $val =~ /^0/; If EXPR is omitted, uses $_. open(FILEHANDLE,EXPR) open(FILEHANDLE) open FILEHANDLE Opens the file whose filename is given by EXPR, and associates it with FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name of the real filehandle wanted. If EXPR is omitted, the scalar variable of the same name as the FILEHANDLE contains the filename. If the filename begins with "<" or nothing, the file is opened for input. If the filename begins with ">", the file is opened for output. If the filename begins with ">>", the file is opened for appending. (You can put a '+' in front of the '>' or '<' to indicate that you want both read and write access to the file.) If the filename begins with "|", the filename is inter- preted as a command to which output is to be piped, and if the filename ends with a "|", the filename is interpreted as command which pipes input to us. (You may not have a command that pipes both in and out.) Opening '-' opens _S_T_D_I_N and opening '>-' opens _S_T_D_O_U_T. Open returns non-zero upon success, the undefined value otherwise. If the open involved a pipe, the return value happens to be the pid of the subprocess. Examples: $article = 100; open article || die "Can't find article $article: $!\n"; while (<article>) {... open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved) open(article, "caesar <$article |"); # decrypt article open(extract, "|sort >/tmp/Tmp$$"); # $$ is our process# 48 PERL(1) USER COMMANDS PERL(1) # process argument list of files along with any includes foreach $file (@ARGV) { do process($file, 'fh00'); # no pun intended } sub process { local($filename, $input) = @_; $input++; # this is a string increment unless (open($input, $filename)) { print STDERR "Can't open $filename: $!\n"; return; } while (<$input>) { # note use of indirection if (/^#include "(.*)"/) { do process($1, $input); next; } ... # whatever } } You may also, in the Bourne shell tradition, specify an EXPR beginning with ">&", in which case the rest of the string is interpreted as the name of a filehandle (or file descriptor, if numeric) which is to be duped and opened. You may use & after >, >>, <, +>, +>> and +<. The mode you specify should match the mode of the original filehandle. Here is a script that saves, redirects, and restores _S_T_D_O_U_T and _S_T_D_E_R_R: 49 PERL(1) USER COMMANDS PERL(1) #!/usr/bin/perl open(SAVEOUT, ">&STDOUT"); open(SAVEERR, ">&STDERR"); open(STDOUT, ">foo.out") || die "Can't redirect stdout"; open(STDERR, ">&STDOUT") || die "Can't dup stdout"; select(STDERR); $| = 1; # make unbuffered select(STDOUT); $| = 1; # make unbuffered print STDOUT "stdout 1\n"; # this works for print STDERR "stderr 1\n"; # subprocesses too close(STDOUT); close(STDERR); open(STDOUT, ">&SAVEOUT"); open(STDERR, ">&SAVEERR"); print STDOUT "stdout 2\n"; print STDERR "stderr 2\n"; Explicitly closing any piped filehandle causes the parent process to wait for the child to finish, and returns the status value in $?. Note: on any opera- tion which may do a fork, unflushed buffers remain unflushed in both processes, which means you may need to set $| to avoid duplicate output. On MS-DOS, "pipes" are actually files, and the sub- process is run to completion. For output pipes, the subprocess is run when _p_e_r_l exits or when the pipe is explicitly closed. For input pipes, the subpro- gram is run to completion when the _o_p_e_n is done. On MS-DOS, it is recommended that forward slashes be used to delimit path components in file names. (See the section on MS-DOS.) opendir(DIRHANDLE,EXPR) Opens a directory named EXPR for processing by read- dir(), telldir(), seekdir(), rewinddir() and closedir(). Returns true if successful. DIRHANDLEs have their own namespace separate from FILEHANDLEs. ord(EXPR) ord EXPR Returns the numeric ascii value of the first charac- ter of EXPR. If EXPR is omitted, uses $_. 50 PERL(1) USER COMMANDS PERL(1) pack(TEMPLATE,LIST) Takes an array or list of values and packs it into a binary structure, returning the string containing the structure. The TEMPLATE is a sequence of char- acters that give the order and type of values, as follows: A An ascii string, will be space padded. a An ascii string, will be null padded. c A signed char value. C An unsigned char value. s A signed short value. S An unsigned short value. i A signed integer value. I An unsigned integer value. l A signed long value. L An unsigned long value. n A short in "network" order. N A long in "network" order. f A single-precision float in the native format. d A double-precision float in the native format. p A pointer to a string. x A null byte. X Back up a byte. @ Null fill to absolute position. u A uuencoded string. b A bit string (ascending bit order, like vec()). B A bit string (descending bit order). h A hex string (low nybble first). H A hex string (high nybble first). Each letter may optionally be followed by a number which gives a repeat count. With all types except "a", "A", "b", "B", "h" and "H", the pack function will gobble up that many values from the LIST. A * for the repeat count means to use however many items are left. The "a" and "A" types gobble just one value, but pack it as a string of length count, pad- ding with nulls or spaces as necessary. (When unpacking, "A" strips trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B" fields pack a string that many bits long. The "h" and "H" fields pack a string that many nybbles long. Real numbers (floats and doubles) are in the native machine format only; due to the multiplicity of floating formats around, and the lack of a standard "network" representation, no facility for inter- change has been made. This means that packed float- ing point data written on one machine may not be readable on another - even if both use IEEE floating point arithmetic (as the endian-ness of the memory representation is not part of the IEEE spec). Note 51 PERL(1) USER COMMANDS PERL(1) that perl uses doubles internally for all numeric calculation, and converting from double -> float -> double will lose precision (i.e. unpack("f", pack("f", $foo)) will not in general equal $foo). Examples: $foo = pack("cccc",65,66,67,68); # foo eq "ABCD" $foo = pack("c4",65,66,67,68); # same thing $foo = pack("ccxxcc",65,66,67,68); # foo eq "AB\0\0CD" $foo = pack("s2",1,2); # "\1\0\2\0" on little-endian # "\0\1\0\2" on big-endian $foo = pack("a4","abcd","x","y","z"); # "abcd" $foo = pack("aaaa","abcd","x","y","z"); # "axyz" $foo = pack("a14","abcdefg"); # "abcdefg\0\0\0\0\0\0\0" $foo = pack("i9pl", gmtime); # a real struct tm (on my system anyway) sub bintodec { unpack("N", pack("B32", substr("0" x 32 . shift, -32))); } The same template may generally also be used in the unpack function. pipe(READHANDLE,WRITEHANDLE) Opens a pair of connected pipes like the correspond- ing system call. Not supported on MS-DOS. pop(ARRAY) pop ARRAY Pops and returns the last value of the array, shor- tening the array by 1. Has the same effect as $tmp = $ARRAY[$#ARRAY--]; If there are no elements in the array, returns the undefined value. 52 PERL(1) USER COMMANDS PERL(1) print(FILEHANDLE LIST) print(LIST) print FILEHANDLE LIST print LIST print Prints a string or a comma-separated list of strings. Returns non-zero if successful. FILEHAN- DLE may be a scalar variable name, in which case the variable contains the name of the filehandle, thus introducing one level of indirection. (NOTE: If FILEHANDLE is a variable and the next token is a term, it may be misinterpreted as an operator unless you interpose a + or put parens around the argu- ments.) If FILEHANDLE is omitted, prints by default to standard output (or to the last selected output channel--see select()). If LIST is also omitted, prints $_ to _S_T_D_O_U_T. To set the default output channel to something other than _S_T_D_O_U_T use the select operation. Note that, because print takes a LIST, anything in the LIST is evaluated in an array context, and any subroutine that you call will have one or more of its expressions evaluated in an array context. Also be careful not to follow the print keyword with a left parenthesis unless you want the corresponding right parenthesis to terminate the arguments to the print--interpose a + or put parens around all the arguments. printf(FILEHANDLE LIST) printf(LIST) printf FILEHANDLE LIST printf LIST Equivalent to a "print FILEHANDLE sprintf(LIST)". push(ARRAY,LIST) Treats ARRAY (@ is optional) as a stack, and pushes the values of LIST onto the end of ARRAY. The length of ARRAY increases by the length of LIST. Has the same effect as for $value (LIST) { $ARRAY[++$#ARRAY] = $value; } but is more efficient. 53 PERL(1) USER COMMANDS PERL(1) q/STRING/ qq/STRING/ qx/STRING/ These are not really functions, but simply syntactic sugar to let you avoid putting too many backslashes into quoted strings. The q operator is a general- ized single quote, and the qq operator a generalized double quote. The qx operator is a generalized backquote. Any non-alphanumeric delimiter can be used in place of /, including newline. If the del- imiter is an opening bracket or parenthesis, the final delimiter will be the corresponding closing bracket or parenthesis. (Embedded occurrences of the closing bracket need to be backslashed as usual.) Examples: $foo = q!I said, "You said, 'She said it.'"!; $bar = q('This is it.'); $today = qx{ date }; $_ .= qq *** The previous line contains the naughty word "$&".\n if /(ibm|apple|awk)/; # :-) rand(EXPR) rand EXPR rand Returns a random fractional number between 0 and the value of EXPR. (EXPR should be positive.) If EXPR is omitted, returns a value between 0 and 1. See also srand(). read(FILEHANDLE,SCALAR,LENGTH,OFFSET) read(FILEHANDLE,SCALAR,LENGTH) Attempts to read LENGTH bytes of data into variable SCALAR from the specified FILEHANDLE. Returns the number of bytes actually read, or undef if there was an error. SCALAR will be grown or shrunk to the length actually read. An OFFSET may be specified to place the read data at some other place than the beginning of the string. This call is actually implemented in terms of stdio's fread call. To get a true read system call, see sysread. readdir(DIRHANDLE) readdir DIRHANDLE Returns the next directory entry for a directory 54 PERL(1) USER COMMANDS PERL(1) opened by opendir(). If used in an array context, returns all the rest of the entries in the direc- tory. If there are no more entries, returns an undefined value in a scalar context or a null list in an array context. readlink(EXPR) readlink EXPR Returns the value of a symbolic link. Not supported on MS-DOS. recv(SOCKET,SCALAR,LEN,FLAGS) Receives a message on a socket. SOCKET filehandle. Returns the address of the sender, or the undefined value if there's an error. SCALAR will be grown or shrunk to the length actu- ally read. Takes the same flags as the system call of the same name. Sockets are not yet supported on MS-DOS. redo LABEL redo The _r_e_d_o command restarts the loop block without evaluating the conditional again. The _c_o_n_t_i_n_u_e block, if any, is not executed. If the LABEL is omitted, the command refers to the innermost enclos- ing loop. This command is normally used by programs that want to lie to themselves about what was just input: # a simpleminded Pascal comment stripper # (warning: assumes no { or } in strings) line: while (<STDIN>) { while (s|({.*}.*){.*}|$1 |) {} s|{.*}| |; if (s|{.*| |) { $front = $_; while (<STDIN>) { if (/}/) { # end of comment? s|^|$front{|; redo line; } } } print; } rename(OLDNAME,NEWNAME) Changes the name of a file. Returns 1 for success, 55 PERL(1) USER COMMANDS PERL(1) 0 otherwise. Will not work across filesystem boun- daries. require(EXPR) require EXPR require Includes the library file specified by EXPR, or by $_ if EXPR is not supplied. Has semantics similar to the following subroutine: sub require { local($filename) = @_; return 1 if $INC{$filename}; local($realfilename,$result); ITER: { foreach $prefix (@INC) { $realfilename = "$prefix/$filename"; if (-f $realfilename) { $result = do $realfilename; last ITER; } } die "Can't find $filename in \@INC"; } die $@ if $@; die "$filename did not return true value" unless $result; $INC{$filename} = $realfilename; $result; } Note that the file will not be included twice under the same specified name. reset(EXPR) reset EXPR reset Generally used in a _c_o_n_t_i_n_u_e block at the end of a loop to clear variables and reset ?? searches so that they work again. The expression is interpreted as a list of single characters (hyphens allowed for ranges). All variables and arrays beginning with one of those letters are reset to their pristine state. If the expression is omitted, one-match searches (?pattern?) are reset to match again. Only resets variables or searches in the current package. Always returns 1. Examples: reset 'X'; # reset all X variables reset 'a-z'; # reset lower case variables reset; # just reset ?? searches 56 PERL(1) USER COMMANDS PERL(1) Note: resetting "A-Z" is not recommended since you'll wipe out your ARGV and ENV arrays. The use of reset on dbm associative arrays does not change the dbm file. (It does, however, flush any entries cached by perl, which may be useful if you are sharing the dbm file. Then again, maybe not.) return LIST Returns from a subroutine with the value specified. (Note that a subroutine can automatically return the value of the last expression evaluated. That's the preferred method--use of an explicit _r_e_t_u_r_n is a bit slower.) reverse(LIST) reverse LIST In an array context, returns an array value consist- ing of the elements of LIST in the opposite order. In a scalar context, returns a string value consist- ing of the bytes of the first element of LIST in the opposite order. rewinddir(DIRHANDLE) rewinddir DIRHANDLE Sets the current position to the beginning of the directory for the readdir() routine on DIRHANDLE. rindex(STR,SUBSTR,POSITION) rindex(STR,SUBSTR) Works just like index except that it returns the position of the LAST occurrence of SUBSTR in STR. If POSITION is specified, returns the last occurrence at or before that position. rmdir(FILENAME) rmdir FILENAME Deletes the directory specified by FILENAME if it is empty. If it succeeds it returns 1, otherwise it returns 0 and sets $! (errno). If FILENAME is omit- ted, uses $_. s/PATTERN/REPLACEMENT/gieo Searches a string for a pattern, and if found, replaces that pattern with the replacement text and returns the number of substitutions made. Otherwise it returns false (0). The "g" is optional, and if present, indicates that all occurrences of the 57 PERL(1) USER COMMANDS PERL(1) pattern are to be replaced. The "i" is also optional, and if present, indicates that matching is to be done in a case-insensitive manner. The "e" is likewise optional, and if present, indicates that the replacement string is to be evaluated as an expression rather than just as a double-quoted string. Any non-alphanumeric delimiter may replace the slashes; if single quotes are used, no interpre- tation is done on the replacement string (the e modifier overrides this, however); if backquotes are used, the replacement string is a command to execute whose output will be used as the actual replacement text. If no string is specified via the =~ or !~ operator, the $_ string is searched and modified. (The string specified with =~ must be a scalar vari- able, an array element, or an assignment to one of those, i.e. an lvalue.) If the pattern contains a $ that looks like a variable rather than an end-of- string test, the variable will be interpolated into the pattern at run-time. If you only want the pat- tern compiled once the first time the variable is interpolated, add an "o" at the end. If the PATTERN evaluates to a null string, the most recent success- ful regular expression is used instead. See also the section on regular expressions. Examples: s/\bgreen\b/mauve/g; # don't change wintergreen $path =~ s|/usr/bin|/usr/local/bin|; s/Login: $foo/Login: $bar/; # run-time pattern ($foo = $bar) =~ s/bar/foo/; $_ = 'abc123xyz'; s/\d+/$&*2/e; # yields 'abc246xyz' s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields (Note the use of $ instead of \ in the last example. See section on regular expressions.) scalar(EXPR) Forces EXPR to be interpreted in a scalar context and returns the value of EXPR. seek(FILEHANDLE,POSITION,WHENCE) Randomly positions the file pointer for FILEHANDLE, just like the fseek() call of stdio. FILEHANDLE may be an expression whose value gives the name of the 58 PERL(1) USER COMMANDS PERL(1) filehandle. Returns 1 upon success, 0 otherwise. seekdir(DIRHANDLE,POS) Sets the current position for the readdir() routine on DIRHANDLE. POS must be a value returned by telldir(). Has the same caveats about possible directory compaction as the corresponding system library routine. select(FILEHANDLE) select Returns the currently selected filehandle. Sets the current default filehandle for output, if FILEHANDLE is supplied. This has two effects: first, a _w_r_i_t_e or a _p_r_i_n_t without a filehandle will default to this FILEHANDLE. Second, references to variables related to output will refer to this output channel. For example, if you have to set the top of form format for more than one output channel, you might do the following: select(REPORT1); $^ = 'report1_top'; select(REPORT2); $^ = 'report2_top'; FILEHANDLE may be an expression whose value gives the name of the actual filehandle. Thus: $oldfh = select(STDERR); $| = 1; select($oldfh); select(RBITS,WBITS,EBITS,TIMEOUT) This calls the Unix select system call, which is not implemented on MS-DOS. The _s_e_l_e_c_t system call is not implemented on MS-DOS. semctl(ID,SEMNUM,CMD,ARG) semget(KEY,NSEMS,SIZE,FLAGS) semop(KEY,OPSTRING) These System V inter-process communications func- tions are not available on MS-DOS. setpgrp(PID,PGRP) Sets the current process group for the specified PID, 0 for the current process. Not supported on MS-DOS. 59 PERL(1) USER COMMANDS PERL(1) setpriority(WHICH,WHO,PRIORITY) Sets the current priority for a process, a process group, or a user. Not supported on MS-DOS. setsockopt(SOCKET,LEVEL,OPTNAME,OPTVAL) Sets the socket option requested. Sockets are not yet supported on MS-DOS. shift(ARRAY) shift ARRAY shift Shifts the first value of the array off and returns it, shortening the array by 1 and moving everything down. If there are no elements in the array, returns the undefined value. If ARRAY is omitted, shifts the @ARGV array in the main program, and the @_ array in subroutines. (This is determined lexi- cally.) See also unshift(), push() and pop(). Shift() and unshift() do the same thing to the left end of an array that push() and pop() do to the right end. shmctl(ID,CMD,ARG) shmget(KEY,SIZE,FLAGS) shmread(ID,VAR,POS,SIZE) shmwrite(ID,STRING,POS,SIZE) These System V inter-process communications func- tions are not available on MS-DOS. sin(EXPR) sin EXPR Returns the sine of EXPR (expressed in radians). If EXPR is omitted, returns sine of $_. sleep(EXPR) sleep EXPR sleep Causes the script to sleep for EXPR seconds, or for- ever if no EXPR. May be interrupted by sending the process a SIGALARM. Pretty worthless on MS-DOS, as it just wastes the number of seconds requested. socket(SOCKET,DOMAIN,TYPE,PROTOCOL) Opens a socket of the specified kind and attaches it to filehandle SOCKET. call of the same name. You may need to run h2ph on sys/socket.h to get the 60 PERL(1) USER COMMANDS PERL(1) proper values handy in a perl library file. Return true if successful. See the example in the section on Interprocess Communication. Sockets are not yet supported on MS-DOS. socketpair(SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL) Creates an unnamed pair of sockets in the specified domain, of the specified type. Sockets are not yet supported on MS-DOS. sort(SUBROUTINE LIST) sort(LIST) sort SUBROUTINE LIST sort LIST Sorts the LIST and returns the sorted array value. Nonexistent values of arrays are stripped out. If SUBROUTINE is omitted, sorts in standard string com- parison order. If SUBROUTINE is specified, gives the name of a subroutine that returns an integer less than, equal to, or greater than 0, depending on how the elements of the array are to be ordered. (The <=> and cmp operators are extremely useful in such routines.) In the interests of efficiency the normal calling code for subroutines is bypassed, with the following effects: the subroutine may not be a recursive subroutine, and the two elements to be compared are passed into the subroutine not via @_ but as $a and $b (see example below). They are passed by reference so don't modify $a and $b. SUB- ROUTINE may be a scalar variable name, in which case the value provides the name of the subroutine to use. Examples: sub byage { $age{$a} <=> $age{$b}; # presuming integers } @sortedclass = sort byage @class; sub reverse { $b cmp $a; } @harry = ('dog','cat','x','Cain','Abel'); @george = ('gone','chased','yz','Punished','Axed'); print sort @harry; # prints AbelCaincatdogx print sort reverse @harry; # prints xdogcatCainAbel print sort @george, 'to', @harry; # prints AbelAxedCainPunishedcatchaseddoggonetoxyz 61 PERL(1) USER COMMANDS PERL(1) splice(ARRAY,OFFSET,LENGTH,LIST) splice(ARRAY,OFFSET,LENGTH) splice(ARRAY,OFFSET) Removes the elements designated by OFFSET and LENGTH from an array, and replaces them with the elements of LIST, if any. Returns the elements removed from the array. The array grows or shrinks as necessary. If LENGTH is omitted, removes everything from OFFSET onward. The following equivalencies hold (assuming $[ == 0): push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y) pop(@a) splice(@a,-1) shift(@a) splice(@a,0,1) unshift(@a,$x,$y) splice(@a,0,0,$x,$y) $a[$x] = $y splice(@a,$x,1,$y); Example, assuming array lengths are passed before arrays: sub aeq { # compare two array values local(@a) = splice(@_,0,shift); local(@b) = splice(@_,0,shift); return 0 unless @a == @b; # same len? while (@a) { return 0 if pop(@a) ne pop(@b); } return 1; } if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... } split(/PATTERN/,EXPR,LIMIT) split(/PATTERN/,EXPR) split(/PATTERN/) split Splits a string into an array of strings, and returns it. (If not in an array context, returns the number of fields found and splits into the @_ array. (In an array context, you can force the split into @_ by using ?? as the pattern delimiters, but it still returns the array value.)) If EXPR is omitted, splits the $_ string. If PATTERN is also omitted, splits on whitespace (/[ \t\n]+/). Any- thing matching PATTERN is taken to be a delimiter separating the fields. (Note that the delimiter may be longer than one character.) If LIMIT is speci- fied, splits into no more than that many fields (though it may split into fewer). If LIMIT is 62 PERL(1) USER COMMANDS PERL(1) unspecified, trailing null fields are stripped (which potential users of pop() would do well to remember). A pattern matching the null string (not to be confused with a null pattern //, which is just one member of the set of patterns matching a null string) will split the value of EXPR into separate characters at each point it matches that way. For example: print join(':', split(/ */, 'hi there')); produces the output 'h:i:t:h:e:r:e'. The LIMIT parameter can be used to partially split a line ($login, $passwd, $remainder) = split(/:/, $_, 3); (When assigning to a list, if LIMIT is omitted, perl supplies a LIMIT one larger than the number of vari- ables in the list, to avoid unnecessary work. For the list above LIMIT would have been 4 by default. In time critical applications it behooves you not to split into more fields than you really need.) If the PATTERN contains parentheses, additional array elements are created from each matching sub- string in the delimiter. split(/([,-])/,"1-10,20"); produces the array value (1,'-',10,',',20) The pattern /PATTERN/ may be replaced with an expression to specify patterns that vary at runtime. (To do runtime compilation only once, use /$variable/o.) As a special case, specifying a space (' ') will split on white space just as split with no arguments does, but leading white space does NOT produce a null first field. Thus, split(' ') can be used to emulate _a_w_k's default behavior, whereas split(/ /) will give you as many null ini- tial fields as there are leading spaces. Example: 63 PERL(1) USER COMMANDS PERL(1) open(passwd, '/etc/passwd'); while (<passwd>) { ($login, $passwd, $uid, $gid, $gcos, $home, $shell) = split(/:/); ... } (Note that $shell above will still have a newline on it. See chop().) See also _j_o_i_n. sprintf(FORMAT,LIST) Returns a string formatted by the usual printf con- ventions. The * character is not supported. sqrt(EXPR) sqrt EXPR Return the square root of EXPR. If EXPR is omitted, returns square root of $_. srand(EXPR) srand EXPR Sets the random number seed for the _r_a_n_d operator. If EXPR is omitted, does srand(time). stat(FILEHANDLE) stat FILEHANDLE stat(EXPR) stat SCALARVARIABLE Returns a 13-element array giving the statistics for a file, either the file opened via FILEHANDLE, or named by EXPR. Typically used as follows: ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size, $atime,$mtime,$ctime,$blksize,$blocks) = stat($filename); If stat is passed the special filehandle consisting of an underline, no stat is done, but the current contents of the stat structure from the last stat or filetest are returned. (Note that on MS-DOS several of these values are spurious.) Example: if (-x $file && (($d) = stat(_)) && $d < 0) { print "$file is executable NFS file\n"; } (This only works on machines for which the device 64 PERL(1) USER COMMANDS PERL(1) number is negative under NFS.) study(SCALAR) study SCALAR study Takes extra time to study SCALAR ($_ if unspecified) in anticipation of doing many pattern matches on the string before it is next modified. This may or may not save time, depending on the nature and number of patterns you are searching on, and on the distribu- tion of character frequencies in the string to be searched--you probably want to compare runtimes with and without it to see which runs faster. Those loops which scan for many short constant strings (including the constant parts of more complex pat- terns) will benefit most. You may have only one study active at a time--if you study a different scalar the first is "unstudied". (The way study works is this: a linked list of every character in the string to be searched is made, so we know, for example, where all the 'k' characters are. From each search string, the rarest character is selected, based on some static frequency tables con- structed from some C programs and English text. Only those places that contain this "rarest" charac- ter are examined.) For example, here is a loop which inserts index pro- ducing entries before any line containing a certain pattern: while (<>) { study; print ".IX foo\n" if /\bfoo\b/; print ".IX bar\n" if /\bbar\b/; print ".IX blurfl\n" if /\bblurfl\b/; ... print; } In searching for /\bfoo\b/, only those locations in $_ that contain 'f' will be looked at, because 'f' is rarer than 'o'. In general, this is a big win except in pathological cases. The only question is whether it saves you more time than it took to build the linked list in the first place. Note that if you have to look for strings that you don't know till runtime, you can build an entire loop as a string and eval that to avoid recompiling all your patterns all the time. Together with 65 PERL(1) USER COMMANDS PERL(1) undefining $/ to input entire files as one record, this can be very fast, often faster than specialized programs like fgrep. The following scans a list of files (@files) for a list of words (@words), and prints out the names of those files that contain a match: $search = 'while (<>) { study;'; foreach $word (@words) { $search .= "++\$seen{\$ARGV} if /\b$word\b/;\n"; } $search .= "}"; @ARGV = @files; undef $/; eval $search; # this screams $/ = "\n"; # put back to normal input delim foreach $file (sort keys(%seen)) { print $file, "\n"; } substr(EXPR,OFFSET,LEN) substr(EXPR,OFFSET) Extracts a substring out of EXPR and returns it. First character is at offset 0, or whatever you've set $[ to. If OFFSET is negative, starts that far from the end of the string. If LEN is omitted, returns everything to the end of the string. You can use the substr() function as an lvalue, in which case EXPR must be an lvalue. If you assign some- thing shorter than LEN, the string will shrink, and if you assign something longer than LEN, the string will grow to accommodate it. To keep the string the same length you may need to pad or chop your value using sprintf(). symlink(OLDFILE,NEWFILE) Creates a new filename symbolically linked to the old filename. (Note that MS-DOS does not support symbolic links.) syscall(LIST) syscall LIST Calls the system call specified as the first element of the list, passing the remaining elements as argu- ments to the system call. If unimplemented, pro- duces a fatal error. The arguments are interpreted as follows: if a given argument is numeric, the argument is passed as an int. If not, the pointer to the string value is passed. You are responsible 66 PERL(1) USER COMMANDS PERL(1) to make sure a string is pre-extended long enough to receive any result that might be written into a string. If your integer arguments are not literals and have never been interpreted in a numeric con- text, you may need to add 0 to them to force them to look like numbers. require 'syscall.ph'; # may need to run h2ph syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9); sysread(FILEHANDLE,SCALAR,LENGTH,OFFSET) sysread(FILEHANDLE,SCALAR,LENGTH) Attempts to read LENGTH bytes of data into variable SCALAR from the specified FILEHANDLE, using the sys- tem call read(2). It bypasses stdio, so mixing this with other kinds of reads may cause confusion. Returns the number of bytes actually read, or undef if there was an error. SCALAR will be grown or shrunk to the length actually read. An OFFSET may be specified to place the read data at some other place than the beginning of the string. system(LIST) system LIST Does exactly the same thing as "exec LIST" except that a fork is done first, and the parent process waits for the child process to complete. Note that argument processing varies depending on the number of arguments. The return value is the exit status of the program as returned by the wait() call. To get the actual exit value divide by 256. See also _e_x_e_c. On MS-DOS, this is implemented using the _s_p_a_w_n sys- tem call. syswrite(FILEHANDLE,SCALAR,LENGTH,OFFSET) syswrite(FILEHANDLE,SCALAR,LENGTH) Attempts to write LENGTH bytes of data from variable SCALAR to the specified FILEHANDLE, using the system call write(2). It bypasses stdio, so mixing this with prints may cause confusion. Returns the number of bytes actually written, or undef if there was an error. An OFFSET may be specified to place the read data at some other place than the beginning of the string. 67 PERL(1) USER COMMANDS PERL(1) tell(FILEHANDLE) tell FILEHANDLE tell Returns the current file position for FILEHANDLE. FILEHANDLE may be an expression whose value gives the name of the actual filehandle. If FILEHANDLE is omitted, assumes the file last read. telldir(DIRHANDLE) telldir DIRHANDLE Returns the current position of the readdir() rou- tines on DIRHANDLE. Value may be given to seekdir() to access a particular location in a directory. Has the same caveats about possible directory compaction as the corresponding system library routine. time Returns the number of non-leap seconds since 00:00:00 UTC, January 1, 1970. Suitable for feeding to gmtime() and localtime(). times Returns a four-element array giving the user and system times, in seconds, for this process and the children of this process. Not implmented on MS-DOS. tr/SEARCHLIST/REPLACEMENTLIST/cds y/SEARCHLIST/REPLACEMENTLIST/cds Translates all occurrences of the characters found in the search list with the corresponding character in the replacement list. It returns the number of characters replaced or deleted. If no string is specified via the =~ or !~ operator, the $_ string is translated. (The string specified with =~ must be a scalar variable, an array element, or an assignment to one of those, i.e. an lvalue.) For _s_e_d devotees, _y is provided as a synonym for _t_r. If the c modifier is specified, the SEARCHLIST char- acter set is complemented. If the d modifier is specified, any characters specified by SEARCHLIST that are not found in REPLACEMENTLIST are deleted. (Note that this is slightly more flexible than the behavior of some _t_r programs, which delete anything they find in the SEARCHLIST, period.) If the s modifier is specified, sequences of characters that were translated to the same character are squashed down to 1 instance of the character. If the d modifier was used, the REPLACEMENTLIST is always interpreted exactly as specified. Otherwise, 68 PERL(1) USER COMMANDS PERL(1) if the REPLACEMENTLIST is shorter than the SEAR- CHLIST, the final character is replicated till it is long enough. If the REPLACEMENTLIST is null, the SEARCHLIST is replicated. This latter is useful for counting characters in a class, or for squashing character sequences in a class. Examples: $ARGV[1] =~ y/A-Z/a-z/; # canonicalize to lower case $cnt = tr/*/*/; # count the stars in $_ $cnt = tr/0-9//; # count the digits in $_ tr/a-zA-Z//s; # bookkeeper -> bokeper ($HOST = $host) =~ tr/a-z/A-Z/; y/a-zA-Z/ /cs; # change non-alphas to single space tr/\200-\377/\0-\177/; # delete 8th bit truncate(FILEHANDLE,LENGTH) truncate(EXPR,LENGTH) Truncates the file opened on FILEHANDLE, or named by EXPR, to the specified length. Produces a fatal error if truncate isn't implemented on your system. umask(EXPR) umask EXPR umask Sets the umask for the process and returns the old one. Not supported on MS-DOS. undef(EXPR) undef EXPR undef Undefines the value of EXPR, which must be an lvalue. Use only on a scalar value, an entire array, or a subroutine name (using &). (Undef will probably not do what you expect on most predefined variables or dbm array values.) Always returns the undefined value. You can omit the EXPR, in which case nothing is undefined, but you still get an undefined value that you could, for instance, return from a subroutine. Examples: 69 PERL(1) USER COMMANDS PERL(1) undef $foo; undef $bar{'blurfl'}; undef @ary; undef %assoc; undef &mysub; return (wantarray ? () : undef) if $they_blew_it; unlink(LIST) unlink LIST Deletes a list of files. Returns the number of files successfully deleted. (MS-DOS users should note that open files cannot be deleted.) $cnt = unlink 'a', 'b', 'c'; unlink @goners; unlink <*.bak>; Note: unlink will not delete directories unless you are superuser and the ----UUUU flag is supplied to _p_e_r_l. Even if these conditions are met, be warned that unlinking a directory can inflict damage on your filesystem. Use rmdir instead. unpack(TEMPLATE,EXPR) Unpack does the reverse of pack: it takes a string representing a structure and expands it out into an array value, returning the array value. (In a scalar context, it merely returns the first value produced.) The TEMPLATE has the same format as in the pack function. Here's a subroutine that does substring: sub substr { local($what,$where,$howmuch) = @_; unpack("x$where a$howmuch", $what); } and then there's sub ord { unpack("c",$_[0]); } In addition, you may prefix a field with a %<number> to indicate that you want a <number>-bit checksum of the items instead of the items themselves. Default is a 16-bit checksum. For example, the following computes the same number as the System V sum pro- gram: 70 PERL(1) USER COMMANDS PERL(1) while (<>) { $checksum += unpack("%16C*", $_); } $checksum %= 65536; unshift(ARRAY,LIST) Does the opposite of a _s_h_i_f_t. Or the opposite of a _p_u_s_h, depending on how you look at it. Prepends list to the front of the array, and returns the number of elements in the new array. unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/; utime(LIST) utime LIST Changes the access and modification times on each file of a list of files. The first two elements of the list must be the NUMERICAL access and modifica- tion times, in that order. (On MS-DOS, the access time field is ignored.) Returns the number of files successfully changed. The inode modification time of each file is set to the current time. Example of a "touch" command: #!/usr/bin/perl $now = time; utime $now, $now, @ARGV; values(ASSOC_ARRAY) values ASSOC_ARRAY Returns a normal array consisting of all the values of the named associative array. The values are returned in an apparently random order, but it is the same order as either the keys() or each() func- tion would produce on the same array. See also keys() and each(). vec(EXPR,OFFSET,BITS) Treats a string as a vector of unsigned integers, and returns the value of the bitfield specified. May also be assigned to. BITS must be a power of two from 1 to 32. Vectors created with vec() can also be manipulated with the logical operators |, & and ^, which will assume a bit vector operation is desired when both operands are strings. This interpretation is not 71 PERL(1) USER COMMANDS PERL(1) enabled unless there is at least one vec() in your program, to protect older programs. To transform a bit vector into a string or array of 0's and 1's, use these: $bits = unpack("b*", $vector); @bits = split(//, unpack("b*", $vector)); If you know the exact length in bits, it can be used in place of the *. wait Waits for a child process to terminate and returns the pid of the deceased process, or -1 if there are no child processes. The status is returned in $?. waitpid(PID,FLAGS) Waits for a particular child process to terminate and returns the pid of the deceased process, or -1 if there is no such child process. Not supported on MS-DOS. wantarray Returns true if the context of the currently execut- ing subroutine is looking for an array value. Returns false if the context is looking for a scalar. return wantarray ? () : undef; warn(LIST) warn LIST Produces a message on STDERR just like "die", but doesn't exit. write(FILEHANDLE) write(EXPR) write Writes a formatted record (possibly multi-line) to the specified file, using the format associated with that file. By default the format for a file is the one having the same name is the filehandle, but the format for the current output channel (see _s_e_l_e_c_t) may be set explicitly by assigning the name of the format to the $~ variable. Top of form processing is handled automatically: if there is insufficient room on the current page for the formatted record, the page is advanced by 72 PERL(1) USER COMMANDS PERL(1) writing a form feed, a special top-of-page format is used to format the new page header, and then the record is written. By default the top-of-page for- mat is "top", but it may be set to the format of your choice by assigning the name to the $^ vari- able. The number of lines remaining on the current page is in variable $-, which can be set to 0 to force a new page. If FILEHANDLE is unspecified, output goes to the current default output channel, which starts out as _S_T_D_O_U_T but may be changed by the _s_e_l_e_c_t operator. If the FILEHANDLE is an EXPR, then the expression is evaluated and the resulting string is used to look up the name of the FILEHANDLE at run time. For more on formats, see the section on formats later on. Note that write is NOT the opposite of read. PPPPrrrreeeecccceeeeddddeeeennnncccceeee _P_e_r_l operators have the following associativity and pre- cedence: nonassoc print printf exec system sort reverse chmod chown kill unlink utime die return left , right = += -= *= etc. right ?: nonassoc .. left || left && left | ^ left & nonassoc == != <=> eq ne cmp nonassoc < > <= >= lt gt le ge nonassoc chdir exit eval reset sleep rand umask nonassoc -r -w -x etc. left << >> left + - . left * / % x left =~ !~ right ! ~ and unary minus right ** nonassoc ++ -- left '(' As mentioned earlier, if any list operator (print, etc.) or any unary operator (chdir, etc.) is followed by a left parenthesis as the next token on the same line, the operator and arguments within parentheses are taken to be of highest precedence, just like a normal function call. Examples: 73 PERL(1) USER COMMANDS PERL(1) chdir $foo || die; # (chdir $foo) || die chdir($foo) || die; # (chdir $foo) || die chdir ($foo) || die; # (chdir $foo) || die chdir +($foo) || die; # (chdir $foo) || die but, because * is higher precedence than ||: chdir $foo * 20; # chdir ($foo * 20) chdir($foo) * 20; # (chdir $foo) * 20 chdir ($foo) * 20; # (chdir $foo) * 20 chdir +($foo) * 20; # chdir ($foo * 20) rand 10 * 20; # rand (10 * 20) rand(10) * 20; # (rand 10) * 20 rand (10) * 20; # (rand 10) * 20 rand +(10) * 20; # rand (10 * 20) In the absence of parentheses, the precedence of list opera- tors such as print, sort or chmod is either very high or very low depending on whether you look at the left side of operator or the right side of it. For example, in @ary = (1, 3, sort 4, 2); print @ary; # prints 1324 the commas on the right of the sort are evaluated before the sort, but the commas on the left are evaluated after. In other words, list operators tend to gobble up all the argu- ments that follow them, and then act like a simple term with regard to the preceding expression. Note that you have to be careful with parens: # These evaluate exit before doing the print: print($foo, exit); # Obviously not what you want. print $foo, exit; # Nor is this. # These do the print before evaluating exit: (print $foo), exit; # This is what you want. print($foo), exit; # Or this. print ($foo), exit; # Or even this. Also note that print ($foo & 255) + 1, "\n"; probably doesn't do what you expect at first glance. SSSSuuuubbbbrrrroooouuuuttttiiiinnnneeeessss A subroutine may be declared as follows: sub NAME BLOCK 74 PERL(1) USER COMMANDS PERL(1) Any arguments passed to the routine come in as array @_, that is ($_[0], $_[1], ...). The array @_ is a local array, but its values are references to the actual scalar parame- ters. The return value of the subroutine is the value of the last expression evaluated, and can be either an array value or a scalar value. Alternately, a return statement may be used to specify the returned value and exit the sub- routine. To create local variables see the _l_o_c_a_l operator. A subroutine is called using the _d_o operator or the & opera- tor. Example: sub MAX { local($max) = pop(@_); foreach $foo (@_) { $max = $foo if $max < $foo; } $max; } ... $bestday = &MAX($mon,$tue,$wed,$thu,$fri); Example: # get a line, combining continuation lines # that start with whitespace sub get_line { $thisline = $lookahead; line: while ($lookahead = <STDIN>) { if ($lookahead =~ /^[ \t]/) { $thisline .= $lookahead; } else { last line; } } $thisline; } $lookahead = <STDIN>; # get first line while ($_ = do get_line()) { ... } 75 PERL(1) USER COMMANDS PERL(1) Use array assignment to a local list to name your formal arguments: sub maybeset { local($key, $value) = @_; $foo{$key} = $value unless $foo{$key}; } This also has the effect of turning call-by-reference into call-by-value, since the assignment copies the values. Subroutines may be called recursively. If a subroutine is called using the & form, the argument list is optional. If omitted, no @_ array is set up for the subroutine; the @_ array at the time of the call is visible to subroutine instead. do foo(1,2,3); # pass three arguments &foo(1,2,3); # the same do foo(); # pass a null list &foo(); # the same &foo; # pass no arguments--more efficient PPPPaaaassssssssiiiinnnngggg BBBByyyy RRRReeeeffffeeeerrrreeeennnncccceeee Sometimes you don't want to pass the value of an array to a subroutine but rather the name of it, so that the subroutine can modify the global copy of it rather than working with a local copy. In perl you can refer to all the objects of a particular name by prefixing the name with a star: *foo. When evaluated, it produces a scalar value that represents all the objects of that name, including any filehandle, for- mat or subroutine. When assigned to within a local() opera- tion, it causes the name mentioned to refer to whatever * value was assigned to it. Example: sub doubleary { local(*someary) = @_; foreach $elem (@someary) { $elem *= 2; } } do doubleary(*foo); do doubleary(*bar); Assignment to *name is currently recommended only inside a local(). You can actually assign to *name anywhere, but the previous referent of *name may be stranded forever. This may or may not bother you. Note that scalars are already passed by reference, so you 76 PERL(1) USER COMMANDS PERL(1) can modify scalar arguments without using this mechanism by referring explicitly to the $_[nnn] in question. You can modify all the elements of an array by passing all the ele- ments as scalars, but you have to use the * mechanism to push, pop or change the size of an array. The * mechanism will probably be more efficient in any case. Since a *name value contains unprintable binary data, if it is used as an argument in a print, or as a %s argument in a printf or sprintf, it then has the value '*name', just so it prints out pretty. Even if you don't want to modify an array, this mechanism is useful for passing multiple arrays in a single LIST, since normally the LIST mechanism will merge all the array values so that you can't extract out the individual arrays. RRRReeeegggguuuullllaaaarrrr EEEExxxxpppprrrreeeessssssssiiiioooonnnnssss The patterns used in pattern matching are regular expres- sions such as those supplied in the Version 8 regexp rou- tines. (In fact, the routines are derived from Henry Spencer's freely redistributable reimplementation of the V8 routines.) In addition, \w matches an alphanumeric charac- ter (including "_") and \W a nonalphanumeric. Word boun- daries may be matched by \b, and non-boundaries by \B. A whitespace character is matched by \s, non-whitespace by \S. A numeric character is matched by \d, non-numeric by \D. You may use \w, \s and \d within character classes. Also, \n, \r, \f, \t and \NNN have their normal interpretations. Within character classes \b represents backspace rather than a word boundary. Alternatives may be separated by |. The bracketing construct ( ... ) may also be used, in which case \<digit> matches the digit'th substring. (Outside of the pattern, always use $ instead of \ in front of the digit. The scope of $<digit> (and $`, $& and $') extends to the end of the enclosing BLOCK or eval string, or to the next pat- tern match with subexpressions. The \<digit> notation some- times works outside the current pattern, but should not be relied upon.) You may have as many parentheses as you wish. If you have more than 9 substrings, the variables $10, $11, ... refer to the corresponding substring. Within the pat- tern, \10, \11, etc. refer back to substrings if there have been at least that many left parens before the backrefer- ence. Otherwise (for backward compatibilty) \10 is the same as \010, a backspace, and \11 the same as \011, a tab. And so on. (\1 through \9 are always backreferences.) $+ returns whatever the last bracket match matched. $& returns the entire matched string. ($0 used to return the same thing, but not any more.) $` returns everything before the matched string. $' returns everything after the matched 77 PERL(1) USER COMMANDS PERL(1) string. Examples: s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words if (/Time: (..):(..):(..)/) { $hours = $1; $minutes = $2; $seconds = $3; } By default, the ^ character is only guaranteed to match at the beginning of the string, the $ character only at the end (or before the newline at the end) and _p_e_r_l does certain optimizations with the assumption that the string contains only one line. The behavior of ^ and $ on embedded newlines will be inconsistent. You may, however, wish to treat a string as a multi-line buffer, such that the ^ will match after any newline within the string, and $ will match before any newline. At the cost of a little more overhead, you can do this by setting the variable $* to 1. Setting it back to 0 makes _p_e_r_l revert to its old behavior. To facilitate multi-line substitutions, the . character never matches a newline (even when $* is 0). In particular, the following leaves a newline on the $_ string: $_ = <STDIN>; s/.*(some_string).*/$1/; If the newline is unwanted, try one of s/.*(some_string).*\n/$1/; s/.*(some_string)[^\000]*/$1/; s/.*(some_string)(.|\n)*/$1/; chop; s/.*(some_string).*/$1/; /(some_string)/ && ($_ = $1); Any item of a regular expression may be followed with digits in curly brackets of the form {n,m}, where n gives the minimum number of times to match the item and m gives the maximum. The form {n} is equivalent to {n,n} and matches exactly n times. The form {n,} matches n or more times. (If a curly bracket occurs in any other context, it is treated as a regular character.) The * modifier is equivalent to {0,}, the + modifier to {1,} and the ? modif- ier to {0,1}. There is no limit to the size of n or m, but large numbers will chew up more memory. You will note that all backslashed metacharacters in _p_e_r_l are alphanumeric, such as \b, \w, \n. Unlike some other regular expression languages, there are no backslashed sym- bols that aren't alphanumeric. So anything that looks like 78 PERL(1) USER COMMANDS PERL(1) \\, $, $, \<, \>, \{, or \} is always interpreted as a literal character, not a metacharacter. This makes it sim- ple to quote a string that you want to use for a pattern but that you are afraid might contain metacharacters. Simply quote all the non-alphanumeric characters: $pattern =~ s/(\W)/\\$1/g; FFFFoooorrrrmmmmaaaattttssss Output record formats for use with the _w_r_i_t_e operator may declared as follows: format NAME = FORMLIST . If name is omitted, format "STDOUT" is defined. FORMLIST consists of a sequence of lines, each of which may be of one of three types: 1. A comment. 2. A "picture" line giving the format for one output line. 3. An argument line supplying values to plug into a picture line. Picture lines are printed exactly as they look, except for certain fields that substitute values into the line. Each picture field starts with either @ or ^. The @ field (not to be confused with the array marker @) is the normal case; ^ fields are used to do rudimentary multi-line text block filling. The length of the field is supplied by padding out the field with multiple <, >, or | characters to specify, respectively, left justification, right justification, or centering. As an alternate form of right justification, you may also use # characters (with an optional .) to specify a numeric field. (Use of ^ instead of @ causes the field to be blanked if undefined.) If any of the values supplied for these fields contains a newline, only the text up to the newline is printed. The special field @* can be used for printing multi-line values. It should appear by itself on a line. The values are specified on the following line, in the same order as the picture fields. The values should be separated by commas. Picture fields that begin with ^ rather than @ are treated specially. The value supplied must be a scalar variable 79 PERL(1) USER COMMANDS PERL(1) name which contains a text string. _P_e_r_l puts as much text as it can into the field, and then chops off the front of the string so that the next time the variable is referenced, more of the text can be printed. Normally you would use a sequence of fields in a vertical stack to print out a block of text. If you like, you can end the final field with ..., which will appear in the output if the text was too long to appear in its entirety. You can change which characters are legal to break on by changing the variable $: to a list of the desired characters. Since use of ^ fields can produce variable length records if the text to be formatted is short, you can suppress blank lines by putting the tilde (~) character anywhere in the line. (Normally you should put it in the front if possible, for visibility.) The tilde will be translated to a space upon output. If you put a second tilde contiguous to the first, the line will be repeated until all the fields on the line are exhausted. (If you use a field of the @ variety, the expression you supply had better not give the same value every time forever!) Examples: # a report on the /etc/passwd file format STDOUT_TOP = Passwd File Name Login Office Uid Gid Home ------------------------------------------------------------------ . format STDOUT = @<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<< $name, $login, $office,$uid,$gid, $home . 80 PERL(1) USER COMMANDS PERL(1) # a report from a bug report form format STDOUT_TOP = Bug Reports @<<<<<<<<<<<<<<<<<<<<<<< @||| @>>>>>>>>>>>>>>>>>>>>>>> $system, $%, $date ------------------------------------------------------------------ . format STDOUT = Subject: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< $subject Index: @<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< $index, $description Priority: @<<<<<<<<<< Date: @<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< $priority, $date, $description From: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< $from, $description Assigned to: @<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< $programmer, $description ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< $description ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< $description ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< $description ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< $description ~ ^<<<<<<<<<<<<<<<<<<<<<<<... $description . It is possible to intermix prints with writes on the same output channel, but you'll have to handle $- (lines left on the page) yourself. If you are printing lots of fields that are usually blank, you should consider using the reset operator between records. Not only is it more efficient, but it can prevent the bug of adding another field and forgetting to zero it. IIIInnnntttteeeerrrrpppprrrroooocccceeeessssssss CCCCoooommmmmmmmuuuunnnniiiiccccaaaattttiiiioooonnnn The IPC facilities of perl are built on the Berkeley socket mechanism. If you don't have sockets, you can ignore this section. (Sockets are not yet supported on MS-DOS.) PPPPrrrreeeeddddeeeeffffiiiinnnneeeedddd NNNNaaaammmmeeeessss The following names have special meaning to _p_e_r_l. I could have used alphabetic symbols for some of these, but I didn't want to take the chance that someone would say reset "a-zA-Z" and wipe them all out. You'll just have to suffer along with these silly symbols. Most of them have 81 PERL(1) USER COMMANDS PERL(1) reasonable mnemonics, or analogues in one of the shells. $_ The default input and pattern-searching space. The following pairs are equivalent: while (<>) {... # only equivalent in while! while ($_ = <>) {... /^Subject:/ $_ =~ /^Subject:/ y/a-z/A-Z/ $_ =~ y/a-z/A-Z/ chop chop($_) (Mnemonic: underline is understood in certain opera- tions.) $. The current input line number of the last filehandle that was read. Readonly. Remember that only an explicit close on the filehandle resets the line number. Since <> never does an explicit close, line numbers increase across ARGV files (but see examples under eof). (Mnemonic: many programs use . to mean the current line number.) $/ The input record separator, newline by default. Works like _a_w_k's RS variable, including treating blank lines as delimiters if set to the null string. You may set it to a multicharacter string to match a multi-character delimiter. (Mnemonic: / is used to delimit line boundaries when quoting poetry.) $, The output field separator for the print operator. Ordinarily the print operator simply prints out the comma separated fields you specify. In order to get behavior more like _a_w_k, set this variable as you would set _a_w_k's OFS variable to specify what is printed between fields. (Mnemonic: what is printed when there is a , in your print statement.) $" This is like $, except that it applies to array values interpolated into a double-quoted string (or similar interpreted string). Default is a space. (Mnemonic: obvious, I think.) $\ The output record separator for the print operator. Ordinarily the print operator simply prints out the comma separated fields you specify, with no trailing newline or record separator assumed. In order to 82 PERL(1) USER COMMANDS PERL(1) get behavior more like _a_w_k, set this variable as you would set _a_w_k's ORS variable to specify what is printed at the end of the print. (Mnemonic: you set $\ instead of adding \n at the end of the print. Also, it's just like /, but it's what you get "back" from _p_e_r_l.) $# The output format for printed numbers. This vari- able is a half-hearted attempt to emulate _a_w_k's OFMT variable. There are times, however, when _a_w_k and _p_e_r_l have differing notions of what is in fact numeric. Also, the initial value is %.20g rather than %.6g, so you need to set $# explicitly to get _a_w_k's value. (Mnemonic: # is the number sign.) $% The current page number of the currently selected output channel. (Mnemonic: % is page number in nroff.) $= The current page length (printable lines) of the currently selected output channel. Default is 60. (Mnemonic: = has horizontal lines.) $- The number of lines left on the page of the currently selected output channel. (Mnemonic: lines_on_page - lines_printed.) $~ The name of the current report format for the currently selected output channel. Default is name of the filehandle. (Mnemonic: brother to $^.) $^ The name of the current top-of-page format for the currently selected output channel. Default is name of the filehandle with "_TOP" appended. (Mnemonic: points to top of page.) $| If set to nonzero, forces a flush after every write or print on the currently selected output channel. Default is 0. Note that _S_T_D_O_U_T will typically be line buffered if output is to the terminal and block buffered otherwise. Setting this variable is useful primarily when you are outputting to a pipe, such as when you are running a _p_e_r_l script under rsh and want to see the output as it's happening. (Mnemonic: when you want your pipes to be piping hot.) $$ The process number of the _p_e_r_l running this script. (Mnemonic: same as shells.) On MS-DOS, this is _p_e_r_l's PSP (program segment prefix) segment. 83 PERL(1) USER COMMANDS PERL(1) $? The status returned by the last pipe close, backtick (``) command or _s_y_s_t_e_m operator. Note that this is the status word returned by the wait() system call, so the exit value of the subprocess is actually ($? >> 8). $? & 255 gives which signal, if any, the process died from, and whether there was a core dump. MS-DOS users should note that the MS-DOS return values are fudged internally so that $? gives Unix-like results. (Mnemonic: similar to sh and ksh.) $& The string matched by the last pattern match (not counting any matches hidden within a BLOCK or eval enclosed by the current BLOCK). (Mnemonic: like & in some editors.) $` The string preceding whatever was matched by the last pattern match (not counting any matches hidden within a BLOCK or eval enclosed by the current BLOCK). (Mnemonic: ` often precedes a quoted string.) $' The string following whatever was matched by the last pattern match (not counting any matches hidden within a BLOCK or eval enclosed by the current BLOCK). (Mnemonic: ' often follows a quoted string.) Example: $_ = 'abcdefghi'; /def/; print "$`:$&:$'\n"; # prints abc:def:ghi $+ The last bracket matched by the last search pattern. This is useful if you don't know which of a set of alternative patterns matched. For example: /Version: (.*)|Revision: (.*)/ && ($rev = $+); (Mnemonic: be positive and forward looking.) $* Set to 1 to do multiline matching within a string, 0 to tell _p_e_r_l that it can assume that strings contain a single line, for the purpose of optimizing pattern matches. Pattern matches on strings containing mul- tiple newlines can produce confusing results when $* is 0. Default is 0. (Mnemonic: * matches multiple things.) Note that this variable only influences the interpretation of ^ and $. A literal newline can be searched for even when $* == 0. 84 PERL(1) USER COMMANDS PERL(1) $0 Contains the name of the file containing the _p_e_r_l script being executed. Assigning to $0 modifies the argument area that the ps(1) program sees. (Mnemonic: same as sh and ksh.) $<digit> Contains the subpattern from the corresponding set of parentheses in the last pattern matched, not counting patterns matched in nested blocks that have been exited already. (Mnemonic: like \digit.) $[ The index of the first element in an array, and of the first character in a substring. Default is 0, but you could set it to 1 to make _p_e_r_l behave more like _a_w_k (or Fortran) when subscripting and when evaluating the index() and substr() functions. (Mnemonic: [ begins subscripts.) $] The string printed out when you say "perl -v". It can be used to determine at the beginning of a script whether the perl interpreter executing the script is in the right range of versions. If used in a numeric context, returns the version + patchlevel / 1000. Example: # see if getc is available ($version,$patchlevel) = $] =~ /(\d+\.\d+).*\nPatch level: (\d+)/; print STDERR "(No filename completion available.)\n" if $version * 1000 + $patchlevel < 2016; or, used numerically, warn "No checksumming!\n" if $] < 3.019; (Mnemonic: Is this version of perl in the right bracket?) $; The subscript separator for multi-dimensional array emulation. If you refer to an associative array element as $foo{$a,$b,$c} it really means $foo{join($;, $a, $b, $c)} But don't put @foo{$a,$b,$c} # a slice--note the @ which means 85 PERL(1) USER COMMANDS PERL(1) ($foo{$a},$foo{$b},$foo{$c}) Default is "\034", the same as SUBSEP in _a_w_k. Note that if your keys contain binary data there might not be any safe value for $;. (Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon. Yeah, I know, it's pretty lame, but $, is already taken for something more important.) $! If used in a numeric context, yields the current value of errno, with all the usual caveats. (This means that you shouldn't depend on the value of $! to be anything in particular unless you've gotten a specific error return indicating a system error.) If used in a string context, yields the correspond- ing system error string. You can assign to $! in order to set errno if, for instance, you want $! to return the string for error n, or you want to set the exit value for the die operator. (Mnemonic: What just went bang?) $@ The perl syntax error message from the last eval command. If null, the last eval parsed and executed correctly (although the operations you invoked may have failed in the normal fashion). (Mnemonic: Where was the syntax error "at"?) $< The real uid of this process. (Mnemonic: it's the uid you came FROM, if you're running setuid.) $> The effective uid of this process. Example: $< = $>; # set real uid to the effective uid ($<,$>) = ($>,$<); # swap real and effective uid (Mnemonic: it's the uid you went TO, if you're run- ning setuid.) Note: $< and $> can only be swapped on machines supporting setreuid(). $( The real gid of this process. If you are on a machine that supports membership in multiple groups simultaneously, gives a space separated list of groups you are in. The first number is the one returned by getgid(), and the subsequent ones by getgroups(), one of which may be the same as the first number. (Mnemonic: parentheses are used to GROUP things. The real gid is the group you LEFT, if you're running setgid.) $) The effective gid of this process. If you are on a machine that supports membership in multiple groups simultaneously, gives a space separated list of 86 PERL(1) USER COMMANDS PERL(1) groups you are in. The first number is the one returned by getegid(), and the subsequent ones by getgroups(), one of which may be the same as the first number. (Mnemonic: parentheses are used to GROUP things. The effective gid is the group that's RIGHT for you, if you're running setgid.) Note: $<, $>, $( and $) can only be set on machines that support the corresponding set[re][ug]id() rou- tine. $( and $) can only be swapped on machines supporting setregid(). $: The current set of characters after which a string may be broken to fill continuation fields (starting with ^) in a format. Default is " \n-", to break on whitespace or hyphens. (Mnemonic: a "colon" in poe- try is a part of a line.) $^D The current value of the debugging flags. (Mnemonic: value of ----DDDD switch.) $^F The maximum system file descriptor, ordinarily 2. System file descriptors are passed to subprocesses, while higher file descriptors are not. During an open, system file descriptors are preserved even if the open fails. Ordinary file descriptors are closed before the open is attempted. $^I The current value of the inplace-edit extension. Use undef to disable inplace editing. (Mnemonic: value of ----iiii switch.) $^P The internal flag that the debugger clears so that it doesn't debug itself. You could conceivable dis- able debugging yourself by clearing it. $^T The time at which the script began running, in seconds since the epoch. The values returned by the ----MMMM ,,,, ----AAAA and ----CCCC filetests are based on this value. $^W The current value of the warning switch. (Mnemonic: related to the ----wwww switch.) $^X The name that Perl itself was executed as, from argv[0]. $ARGV contains the name of the current file when reading from <>. @ARGV The array ARGV contains the command line arguments intended for the script. Note that $#ARGV is the generally number of arguments minus one, since 87 PERL(1) USER COMMANDS PERL(1) $ARGV[0] is the first argument, NOT the command name. See $0 for the command name. @INC The array INC contains the list of places to look for _p_e_r_l scripts to be evaluated by the "do EXPR" command or the "require" command. It initially con- sists of the arguments to any ----IIII command line switches, followed by the default _p_e_r_l library, probably "/usr/local/lib/perl", followed by ".", to represent the current directory. %INC The associative array INC contains entries for each filename that has been included via "do" or "require". The key is the filename you specified, and the value is the location of the file actually found. The "require" command uses this array to determine whether a given file has already been included. $ENV{expr} The associative array ENV contains your current environment. Setting a value in ENV changes the environment for child processes. $SIG{expr} The associative array SIG is used to set signal handlers for various signals. Example: sub handler { # 1st argument is signal name local($sig) = @_; print "Caught a SIG$sig--shutting down\n"; close(LOG); exit(0); } $SIG{'INT'} = 'handler'; $SIG{'QUIT'} = 'handler'; ... $SIG{'INT'} = 'DEFAULT'; # restore default action $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT The SIG array only contains values for the signals actually set within the perl script. PPPPaaaacccckkkkaaaaggggeeeessss Perl provides a mechanism for alternate namespaces to pro- tect packages from stomping on each others variables. By default, a perl script starts compiling into the package known as "main". By use of the _p_a_c_k_a_g_e declaration, you can switch namespaces. The scope of the package declaration is from the declaration itself to the end of the enclosing 88 PERL(1) USER COMMANDS PERL(1) block (the same scope as the local() operator). Typically it would be the first declaration in a file to be included by the "require" operator. You can switch into a package in more than one place; it merely influences which symbol table is used by the compiler for the rest of that block. You can refer to variables and filehandles in other packages by pre- fixing the identifier with the package name and a single quote. If the package name is null, the "main" package as assumed. Only identifiers starting with letters are stored in the packages symbol table. All other symbols are kept in pack- age "main". In addition, the identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV, INC and SIG are forced to be in package "main", even when used for other purposes than their built-in one. Note also that, if you have a package called "m", "s" or "y", the you can't use the qualified form of an identifier since it will be interpreted instead as a pattern match, a substitution or a translation. Eval'ed strings are compiled in the package in which the eval was compiled in. (Assignments to $SIG{}, however, assume the signal handler specified is in the main package. Qualify the signal handler name if you wish to have a signal handler in a package.) For an example, examine perldb.pl in the perl library. It initially switches to the DB package so that the debugger doesn't interfere with variables in the script you are trying to debug. At various points, however, it temporarily switches back to the main package to evaluate various expressions in the context of the main package. The symbol table for a package happens to be stored in the associative array of that name prepended with an underscore. The value in each entry of the associative array is what you are referring to when you use the *name notation. In fact, the following have the same effect (in package main, any- way), though the first is more efficient because it does the symbol table lookups at compile time: local(*foo) = *bar; local($_main{'foo'}) = $_main{'bar'}; You can use this to print out all the variables in a pack- age, for instance. Here is dumpvar.pl from the perl library: 89 PERL(1) USER COMMANDS PERL(1) package dumpvar; sub main'dumpvar { ($package) = @_; local(*stab) = eval("*_$package"); while (($key,$val) = each(%stab)) { { local(*entry) = $val; if (defined $entry) { print "\$$key = '$entry'\n"; } if (defined @entry) { print "\@$key = (\n"; foreach $num ($[ .. $#entry) { print " $num\t'",$entry[$num],"'\n"; } print ")\n"; } if ($key ne "_$package" && defined %entry) { print "\%$key = (\n"; foreach $key (sort keys(%entry)) { print " $key\t'",$entry{$key},"'\n"; } print ")\n"; } } } } Note that, even though the subroutine is compiled in package dumpvar, the name of the subroutine is qualified so that its name is inserted into package "main". SSSSttttyyyylllleeee Each programmer will, of course, have his or her own prefer- ences in regards to formatting, but there are some general guidelines that will make your programs easier to read. 1. Just because you CAN do something a particular way doesn't mean that you SHOULD do it that way. _P_e_r_l is designed to give you several ways to do anything, so consider picking the most readable one. For instance open(FOO,$foo) || die "Can't open $foo: $!"; is better than die "Can't open $foo: $!" unless open(FOO,$foo); because the second way hides the main point of the statement in a modifier. On the other hand 90 PERL(1) USER COMMANDS PERL(1) print "Starting analysis\n" if $verbose; is better than $verbose && print "Starting analysis\n"; since the main point isn't whether the user typed -v or not. Similarly, just because an operator lets you assume default arguments doesn't mean that you have to make use of the defaults. The defaults are there for lazy sys- tems programmers writing one-shot programs. If you want your program to be readable, consider supplying the argument. Along the same lines, just because you _c_a_n omit parentheses in many places doesn't mean that you ought to: return print reverse sort num values array; return print(reverse(sort num (values(%array)))); When in doubt, parenthesize. At the very least it will let some poor schmuck bounce on the % key in vi. Even if you aren't in doubt, consider the mental welfare of the person who has to maintain the code after you, and who will probably put parens in the wrong place. 2. Don't go through silly contortions to exit a loop at the top or the bottom, when _p_e_r_l provides the "last" opera- tor so you can exit in the middle. Just outdent it a little to make it more visible: line: for (;;) { statements; last line if $foo; next line if /^#/; statements; } 3. Don't be afraid to use loop labels--they're there to enhance readability as well as to allow multi-level loop breaks. See last example. 4. For portability, when using features that may not be implemented on every machine, test the construct in an eval to see if it fails. If you know what version or patchlevel a particular feature was implemented, you can 91 PERL(1) USER COMMANDS PERL(1) test $] to see if it will be there. 5. Choose mnemonic identifiers. 6. Be consistent. DDDDeeeebbbbuuuuggggggggiiiinnnngggg If you invoke _p_e_r_l with a ----dddd switch, your script will be run under a debugging monitor. It will halt before the first executable statement and ask you for a command, such as: h Prints out a help message. T Stack trace. s Single step. Executes until it reaches the beginning of another statement. n Next. Executes over subroutine calls, until it reaches the beginning of the next statement. f Finish. Executes statements until it has fin- ished the current subroutine. c Continue. Executes until the next breakpoint is reached. c line Continue to the specified line. Inserts a one- time-only breakpoint at the specified line. <CR> Repeat last n or s. l min+incr List incr+1 lines starting at min. If min is omitted, starts where last listing left off. If incr is omitted, previous value of incr is used. l min-max List lines in the indicated range. l line List just the indicated line. l List next window. - List previous window. w line List window around line. l subname List subroutine. If it's a long subroutine it just lists the beginning. Use "l" to list more. 92 PERL(1) USER COMMANDS PERL(1) /pattern/ Regular expression search forward for pattern; the final / is optional. ?pattern? Regular expression search backward for pattern; the final ? is optional. L List lines that have breakpoints or actions. S Lists the names of all subroutines. t Toggle trace mode on or off. b line condition Set a breakpoint. If line is omitted, sets a breakpoint on the line that is about to be exe- cuted. If a condition is specified, it is evaluated each time the statement is reached and a breakpoint is taken only if the condition is true. Breakpoints may only be set on lines that begin an executable statement. b subname condition Set breakpoint at first executable line of sub- routine. d line Delete breakpoint. If line is omitted, deletes the breakpoint on the line that is about to be executed. D Delete all breakpoints. a line command Set an action for line. A multi-line command may be entered by backslashing the newlines. A Delete all line actions. < command Set an action to happen before every debugger prompt. A multi-line command may be entered by backslashing the newlines. > command Set an action to happen after the prompt when you've just given a command to return to execut- ing the script. A multi-line command may be entered by backslashing the newlines. V package List all variables in package. Default is main package. ! number Redo a debugging command. If number is omitted, redoes the previous command. 93 PERL(1) USER COMMANDS PERL(1) ! -number Redo the command that was that many commands ago. H -number Display last n commands. Only commands longer than one character are listed. If number is omitted, lists them all. q or ^D Quit. command Execute command as a perl statement. A missing semicolon will be supplied. p expr Same as "print DB'OUT expr". The DB'OUT filehandle is opened to /dev/tty, regardless of where STDOUT may be redirected to. If you want to modify the debugger, copy perldb.pl from the perl library to your current directory and modify it as necessary. (You'll also have to put -I. on your command line.) You can do some customization by setting up a .perldb file which contains initialization code. For instance, you could make aliases like these: $DB'alias{'len'} = 's/^len(.*)/p length($1)/'; $DB'alias{'stop'} = 's/^stop (at|in)/b/'; $DB'alias{'.'} = 's/^\./p "\$DB\'sub(\$DB\'line):\t",\$DB\'line[\$DB\'line]/'; SSSSeeeettttuuuuiiiidddd SSSSccccrrrriiiippppttttssss Setuid processes are not supported on MS-DOS. _P_e_r_l is designed to make it easy to write secure setuid and setgid scripts. Unlike shells, which are based on multiple substi- tution passes on each line of the script, _p_e_r_l uses a more conventional evaluation scheme with fewer hidden "gotchas". Additionally, since the language has more built-in func- tionality, it has to rely less upon external (and possibly untrustworthy) programs to accomplish its purposes. In an unpatched 4.2 or 4.3bsd kernel, setuid scripts are intrinsically insecure, but this kernel feature can be dis- abled. If it is, _p_e_r_l can emulate the setuid and setgid mechanism when it notices the otherwise useless setuid/gid bits on perl scripts. If the kernel feature isn't disabled, _p_e_r_l will complain loudly that your setuid script is insecure. You'll need to either disable the kernel setuid script feature, or put a C wrapper around the script. When _p_e_r_l is executing a setuid script, it takes special precautions to prevent you from falling into any obvious traps. (In some ways, a perl script is more secure than the 94 PERL(1) USER COMMANDS PERL(1) corresponding C program.) Any command line argument, environment variable, or input is marked as "tainted", and may not be used, directly or indirectly, in any command that invokes a subshell, or in any command that modifies files, directories or processes. Any variable that is set within an expression that has previously referenced a tainted value also becomes tainted (even if it is logically impossible for the tainted value to influence the variable). For example: $foo = shift; # $foo is tainted $bar = $foo,'bar'; # $bar is also tainted $xxx = <>; # Tainted $path = $ENV{'PATH'}; # Tainted, but see below $abc = 'abc'; # Not tainted system "echo $foo"; # Insecure system "/bin/echo", $foo; # Secure (doesn't use sh) system "echo $bar"; # Insecure system "echo $abc"; # Insecure until PATH set $ENV{'PATH'} = '/bin:/usr/bin'; $ENV{'IFS'} = '' if $ENV{'IFS'} ne ''; $path = $ENV{'PATH'}; # Not tainted system "echo $abc"; # Is secure now! open(FOO,"$foo"); # OK open(FOO,">$foo"); # Not OK open(FOO,"echo $foo|"); # Not OK, but... open(FOO,"-|") || exec 'echo', $foo; # OK $zzz = `echo $foo`; # Insecure, zzz tainted unlink $abc,$foo; # Insecure umask $foo; # Insecure exec "echo $foo"; # Insecure exec "echo", $foo; # Secure (doesn't use sh) exec "sh", '-c', $foo; # Considered secure, alas The taintedness is associated with each scalar value, so some elements of an array can be tainted, and others not. If you try to do something insecure, you will get a fatal error saying something like "Insecure dependency" or "Insecure PATH". Note that you can still write an insecure system call or exec, but only by explicitly doing something like the last example above. You can also bypass the taint- ing mechanism by referencing subpatterns--_p_e_r_l presumes that if you reference a substring using $1, $2, etc, you knew what you were doing when you wrote the pattern: 95 PERL(1) USER COMMANDS PERL(1) $ARGV[0] =~ /^-P(\w+)$/; $printer = $1; # Not tainted This is fairly secure since \w+ doesn't match shell meta- characters. Use of .+ would have been insecure, but _p_e_r_l doesn't check for that, so you must be careful with your patterns. This is the ONLY mechanism for untainting user supplied filenames if you want to do file operations on them (unless you make $> equal to $<). It's also possible to get into trouble with other operations that don't care whether they use tainted values. Make judi- cious use of the file tests in dealing with any user- supplied filenames. When possible, do opens and such after setting $> = $<. _P_e_r_l doesn't prevent you from opening tainted filenames for reading, so be careful what you print out. The tainting mechanism is intended to prevent stupid mistakes, not to remove the need for thought. MMMMSSSS----DDDDOOOOSSSS CCCCOOOONNNNSSSSIIIIDDDDEEEERRRRAAAATTTTIIIIOOOONNNNSSSS This section describes the MS-DOS version of _p_e_r_l. MS-DOS _p_e_r_l has been tested with MS-DOS versions 3.2, 3.3, 4.01, and 5.0, and should work with 2.0 or higher. 640 K-bytes are needed to get anything done. MS-DOS 5.0 running in high memory is recommended. CCCCoooommmmmmmmaaaannnndddd----LLLLiiiinnnneeee AAAArrrrgggguuuummmmeeeennnnttttssss iiiinnnn MMMMSSSS----DDDDOOOOSSSS MS-DOS _p_e_r_l recognizes the MKS calling conventions. The MKS Korn shell will expand the command-line wildcards and pass _p_e_r_l arguments in a manner similar to Unix. Large numbers of arguments can be passed this way, and ----eeee commands can be enclosed in single quotes just as in Unix. This works even if $MKSARGS (see the section on the environment below) is not set, but not if MKS support is compiled out. When invoked from a non-MKS tool with MKS tooling present, _p_e_r_l will use the MKS glob program (see $GLOB in the section on the MS-DOS environment, below) to expand the wildcards. When run on a system that doesn't have MKS tools, _p_e_r_l will attempt to expand wildcards it sees in the argument list. It is not possible to handle ----eeee arguments properly due to limitations in the standard MS-DOS calling conventions. Unix and MKS do globbing differently than MS-DOS. Under Unix and MKS, "abc*" matches abc.c, Under DOS, though, the * won't match "." The MKS tools use the former interpreta- tion; the perlglob.exe program distributed with perl and the globbing built in to perl.exe use the latter. (Don't install the distributed perlglob.exe if you are using the MKS tools.) 96 PERL(1) USER COMMANDS PERL(1) FFFFiiiilllleeee nnnnaaaammmmeeeessss iiiinnnn MMMMSSSS----DDDDOOOOSSSS _P_e_r_l scripts should use forward slashes to delimit path name components. (C programmers should note that most MS-DOS com- pilers support this, too.) MS-DOS _p_e_r_l will work with backslashes, but they make code unnecessarily messy since they must be escaped any time they might be interpreted to mean an escape sequence. Subprograms may or may not like forward slashes: most programs compiled from C source like them fine, but command.com won't take them. Slashes can be reversed before invoking a subprocess using the perl substu- tion features. Inside _p_e_r_l, the following are equivalent: "c:/new/report" "c:\\new\\report" Drive prefixes are optional. If omitted, the file is sought on the current drive. GGGGlllloooobbbbbbbbiiiinnnngggg <<<<>>>> EEEExxxxpppprrrreeeessssssssiiiioooonnnnssss oooonnnn MMMMSSSS----DDDDOOOOSSSS Expressions like <*.c> are expanded by a subshell in Unix. On MS-DOS there are two cases. If $MKSARGS is defined in the environment and MKS support is compiled in, the $SHELL is used, running _e_c_h_o piped to _t_r as with the Bourne shell on Unix _p_e_r_l. Otherwise, the supplied perlglob.exe program is run. This program simply takes the supplied command line, DOS-globs it, and writes the result, null-separated, to the _p_e_r_l process via a "pipe." TTTThhhheeee MMMMSSSS----DDDDOOOOSSSS EEEEnnnnvvvviiiirrrroooonnnnmmmmeeeennnntttt MS-DOS _p_e_r_l uses a number of environment variables not used by Unix _p_e_r_l. This is because the MS-DOS version is widely distributed in executable form. (Not everyone who wishes to use _p_e_r_l on MS-DOS has the tools to build it from the source code.) Directory and file names in the environment can be delimited with forward or back slashes. Environment vari- ables are interpreted when they are needed. (Exception: creation of $TMP from $TMPDIR is done at startup.) Thus the perl script can change them. For example, if a huge "pipe" file is to be created, and there's room on the hard disk (c:) but not the usual $TMP, which is a RAM disk, one could write: ENV{'TMP'} = "c:/tmp"; open (HANDLE, "program|"); The state of the environment passed to perl.exe determines argument parsing, though, since this is done before compila- tion and interpretation of the perl script. 97 PERL(1) USER COMMANDS PERL(1) PATH This is the semicolon-separated path list. The current directory is search first unless $MKSARGS is defined and MKS support is compiled in, in which case the current directory is searched only in response to an explicit "." or null path component. MKSARGS Set to 1 enable all MKSisms. The MKS tools can assume that the other pieces of the toolkit are lying around, but perl.exe can't. (It may be the only MKS-compatible program on a system.) If have have the MKS tools, you should set this variable. Perl will recognize and generate MKS compatible arguments in any case, but without the switch will default SLASHC to "-c" instead of "-ce", will fail to run MKS glob, and will run perl's glob instead of the Korn shell to expand expressions like <*.c>. (Note that MKS support can be compiled out.) ROOTDIR Full drive and path where MKS toolkit is installed. Example, "c:/mks" or "d:/". Typi- cally is already set in MKS environment. Used only if $MKSARGS is set and $GLOB is not. See $GLOB. Ignored if MKS support compiled out. TMP First choice for temporary files, e.g., "h:\\tmp". If not set, uses $TMPDIR (see below), if that's not set, the current directory is used. Swapping also goes here unless $EXESWAP is defined. Temporary files are pseudo-pipes, the swap file, and the ----eeee file. TMPDIR If $TMPDIR is set and $TMP is not, the following is done internally: ( $ENV{'TMP'} = $ENV{'TMPDIR'} ) =~ s,/,\\,g; (Backslashes are reversed as a gesture to decen- dents of the perl process.) Creation of $ENV{'TMP'} from $TMPDIR is done at perl.exe startup. Note that the MKS tools use $TMPDIR as a first choice; as a gesture of compatibility for non-MKS users, here, it is a second choice. EXESWAP First choice for swap out file location. A RAM disk is a nice choice. $TMP is used if this isn't set. (See also $TMPDIR). The swap file created the first time swapping is invoked and is left open until perl exits or does an exec. Set to ".off" (note illegal DOS name) to inhibit swapping--useful for speedy running of small 98 PERL(1) USER COMMANDS PERL(1) subprocesses. This feature (inhibition) can be turned on and off. The following example runs "ls" using "e:/tmp" as the directory for the swap file. It then runs "who," with swapping disabled. Finally, it runs "ps" with swapping re-enabled. Note that $ENV{'EXESWAP'} is set to 'yes' but anything other than '.off' would have sufficed. $ENV{'EXESWAP'} = 'e:/tmp'; system "ls"; $ENV{'EXESWAP'} = '.off'; system "who"; $ENV{'EXESWAP'} = 'yes'; system "ps"; GLOB First choice for MKS globbing program: full path, name, and extension. Example: "d:/mks/etc/glob.exe". The perl globbing pro- gram (used for <*.c> expansion) is found, as before, via the $PATH. Used only in an MKS environment, and then only when perl is run from a non-MKS program. Ignored if MKS support com- piled out. SHELL Full path name and extension of the shell used used for subprocesses when wildcard expansion is required, e.g., "c:/mks/bin/sh.exe". If unde- fined, COMSPEC is used. Presumably this could be the MKS korn shell, but it can be another shell (e.g., 4DOS) and thus $SHELL is inspected even if MKS support is compiled out. COMSPEC Full path name of DOS command interpreter, e.g., "c:\\command.com" Used only if $SHELL is not defined. If not found, "\\command.com" is used. (It is bad practice to allow $COMSPEC to default or to have it have anything other than a full drive and path name. You don't want your pro- grams looking for command.com on alternate drives.) METACHAR List of characters that are metacharacters to the $SHELL or $COMPSPEC. Used to determine if command can be run directly or if a subshell must be invoked. If undefined, \|<> is used for COMSPEC and *"?<>][`'\ for SHELL. SLASHC The shell option for invoking a command. Defaults: /c for $COMSPEC, MS-DOS version 4.x or better [_s_i_c]; _sc, where _s is the switch char- acter, for $COMSPEC, DOS < 4.0; -ce for $SHELL 99 PERL(1) USER COMMANDS PERL(1) if $MKSARGS is set and MKS support is compiled in. (The -e needed to get the MKS Korn shell to return the status properly). The default is -c for other $SHELLs. (This is a guess.) PERLLIB Directory(ies) containing perl library. Defaults to /usr/local/lib/perl. Directories are separated like $PATH: semicolons in MS-DOS, colons in Unix. For the -P switch, only the first $PERLLIB directory (or the default, if there's no $PERLLIB) is tried. TTTTyyyyppppiiiiccccaaaallll MMMMKKKKSSSS sssseeeettttuuuupppp ((((pppprrrrooooffffiiiilllleeee....kkkksssshhhh)))) # If you're using the MKS stuff, you probably don't have # to do anything other than set MKSARGS and PERLLIB. export MKSARGS=1 # ROOTDIR set by init process or etc.rc or here. export TMPDIR=e:/tmp # EXESWAP left to default to $TMPDIR # GLOB left to default to $ROOTDIR/etc/glob.exe # SHELL set by init process or here. # COMSPEC not used by perl.exe but probably defined for other uses. # METACHAR not defined, left to default. export PERLLIB='c:/lib/perl;d:/usr/me/myperlib' TTTTyyyyppppiiiiccccaaaallll nnnnoooonnnn----MMMMKKKKSSSS sssseeeettttuuuupppp ((((aaaauuuuttttooooeeeexxxxeeeecccc....bbbbaaaatttt)))) Rem You probably don't need to do anything except set $TMP, Rem which you may be doing anyway. Rem MKSARGS not set Rem ROOTDIR not used set TMP=d:mp Rem EXESWAP left to default to $TMP Rem GLOB not used Rem SHELL not set Rem COMPEC set by config.sys SHELL command or by MS-DOS startup. Rem METACHAR not defined, left to default. set PERLLIB=c:/lib/perl RRRRuuuunnnnnnnniiiinnnngggg SSSSuuuubbbbpppprrrroooocccceeeesssssssseeeessss oooonnnn MMMMSSSS----DDDDOOOOSSSS _P_e_r_l will by default swap itself out almost entirely when it runs a subprocess other than MKS $GLOB. (See the $EXESWAP environment variable). The swap file is opened when the first subprocess is run and is left open until _p_e_r_l exits or does an IR exec . The command line to be run is scanned for $METACHARacters as described above. If none are found, the subcommand is invoked directly. If metacharacter(s) are found, a $SHELL or $COMSPEC is invoked to run the command. 100 PERL(1) USER COMMANDS PERL(1) Use of a single |||| in a open() command does not constitute a metacharcter: this is a directive to perl to open a pipe. The following, too, has no SHELL metacharacters since the subprocess is simply pwd: chop($direct = `pwd`); Beware of MS-DOS "internal" commands; i.e., those that are built into command.com. Examples are DIR and COPY. COMMAND.COM users can use these directly if the command has $METACHARacters; if not, you must invoke an explicit command.com. In the first example below, '>' is a metachar- acter. In the second example, there are no metacharacters and so the internal "ver" command must be run with an expli- clit $COMSPEC. system "dir >my.fil"; system "$ENV{'COMSPEC'} /c ver"; Users of $SHELLs other than COMMAND.COM must use the second format for anything to be passed to command.com. Be aware that no wild card expansion is going to be done by command.com usless you're using one of the built-in commands that does it (e.g., COPY). You can use <> expansion to get around this. Even those who have fancy $SHELLs should take note of this, since having _p_e_r_l run the $SHELL and then the command uses less memory than if perl runs the $SHELL which runs the command: $files = <*.c>; `the_com $files`; EEEENNNNVVVVIIIIRRRROOOONNNNMMMMEEEENNNNTTTT _P_e_r_l uses PATH in executing subprocesses, and in finding the script if -S is used. HOME or LOGDIR are used if chdir has no argument. Apart from these, Unix _p_e_r_l uses no environment variables, except to make them available to the script being executed, and to child processes. AAAAUUUUTTTTHHHHOOOORRRR Larry Wall <lwall@jpl-devvax.Jpl.Nasa.Gov> MS-DOS port by Diomidis Spinellis <dds@cc.ic.ac.uk> and Len Reed <holos0!lbr@gatech.edu> FFFFIIIILLLLEEEESSSS /tmp/perl-eXXXXXX temporary file for ----eeee commands. 101 PERL(1) USER COMMANDS PERL(1) SSSSEEEEEEEE AAAALLLLSSSSOOOO a2p awk to perl translator s2p sed to perl translator DDDDIIIIAAAAGGGGNNNNOOOOSSSSTTTTIIIICCCCSSSS Compilation errors will tell you the line number of the error, with an indication of the next token or token type that was to be examined. (In the case of a script passed to _p_e_r_l via ----eeee switches, each ----eeee is counted as one line.) TTTTRRRRAAAAPPPPSSSS Accustomed _a_w_k users should take special note of the follow- ing: * Semicolons are required after all simple statements in _p_e_r_l. Newline is not a statement delimiter. * Curly brackets are required on ifs and whiles. * Variables begin with $ or @ in _p_e_r_l. * Arrays index from 0 unless you set $[. Likewise string positions in substr() and index(). * You have to decide whether your array has numeric or string indices. * Associative array values do not spring into existence upon mere reference. * You have to decide whether you want to use string or numeric comparisons. * Reading an input line does not split it for you. You get to split it yourself to an array. And the _s_p_l_i_t operator has different arguments. * The current input line is normally in $_, not $0. It generally does not have the newline stripped. ($0 is the name of the program executed.) * $<digit> does not refer to fields--it refers to sub- strings matched by the last match pattern. * The _p_r_i_n_t statement does not add field and record separators unless you set $, and $\. * You must open your files before you print to them. * The range operator is "..", not comma. (The comma operator works as in C.) 102 PERL(1) USER COMMANDS PERL(1) * The match operator is "=~", not "~". ("~" is the one's complement operator, as in C.) * The exponentiation operator is "**", not "^". ("^" is the XOR operator, as in C.) * The concatenation operator is ".", not the null string. (Using the null string would render "/pat/ /pat/" unparsable, since the third slash would be interpreted as a division operator--the tokener is in fact slightly context sensitive for operators like /, ?, and <. And in fact, . itself can be the beginning of a number.) * _N_e_x_t, _e_x_i_t and _c_o_n_t_i_n_u_e work differently. * The following variables work differently Awk Perl ARGC $#ARGV ARGV[0] $0 FILENAME $ARGV FNR $. - something FS (whatever you like) NF $#Fld, or some such NR $. OFMT $# OFS $, ORS $\ RLENGTH length($&) RS $/ RSTART length($`) SUBSEP $; * When in doubt, run the _a_w_k construct through a2p and see what it gives you. Cerebral C programmers should take note of the following: * Curly brackets are required on ifs and whiles. * You should use "elsif" rather than "else if" * _B_r_e_a_k and _c_o_n_t_i_n_u_e become _l_a_s_t and _n_e_x_t, respectively. * There's no switch statement. * Variables begin with $ or @ in _p_e_r_l. * Printf does not implement *. * Comments begin with #, not /*. 103 PERL(1) USER COMMANDS PERL(1) * You can't take the address of anything. * ARGV must be capitalized. * The "system" calls link, unlink, rename, etc. return nonzero for success, not 0. * Signal handlers deal with signal names, not numbers. Seasoned _s_e_d programmers should take note of the following: * Backreferences in substitutions use $ rather than \. * The pattern matching metacharacters (, ), and | do not have backslashes in front. * The range operator is .. rather than comma. Sharp shell programmers should take note of the following: * The backtick operator does variable interpretation without regard to the presence of single quotes in the command. * The backtick operator does no translation of the return value, unlike csh. * Shells (especially csh) do several levels of substitu- tion on each command line. _P_e_r_l does substitution only in certain constructs such as double quotes, backticks, angle brackets and search patterns. * Shells interpret scripts a little bit at a time. _P_e_r_l compiles the whole program before executing it. * The arguments are available via @ARGV, not $1, $2, etc. * The environment is not automatically made available as variables. EEEERRRRRRRRAAAATTTTAAAA AAAANNNNDDDD AAAADDDDDDDDEEEENNNNDDDDAAAA The Perl book, _P_r_o_g_r_a_m_m_i_n_g _P_e_r_l , has the following omis- sions and goofs. On page 5, the examples which read eval "/usr/bin/perl should read eval "exec /usr/bin/perl 104 PERL(1) USER COMMANDS PERL(1) On page 195, the equivalent to the System V sum program only works for very small files. To do larger files, use undef $/; $checksum = unpack("%32C*",<>) % 32767; The ----0000 switch to set the initial value of $/ was added to Perl after the book went to press. The ----llll switch now does automatic line ending processing. The qx// construct is now a synonym for backticks. $0 may now be assigned to set the argument displayed by _p_s (_1). The new @###.## format was omitted accidentally from the description on formats. It wasn't known at press time that s///ee caused multiple evaluations of the replacement expression. This is to be construed as a feature. (LIST) x $count now does array replication. There is now no limit on the number of parentheses in a reg- ular expression. In double-quote context, more escapes are supported: \e, \a, \x1b, \c[, \l, \L, \u, \U, \E. The latter five control up/lower case translation. The $$$$//// variable may now be set to a multi-character delim- iter. There is now a g modifier on ordinary pattern matching that causes it to iterate through a string finding multiple matches. All of the $^X variables are new except for $^T. BBBBUUUUGGGGSSSS _P_e_r_l is at the mercy of your machine's definitions of vari- ous operations such as type casting, atof() and sprintf(). If your stdio requires an seek or eof between reads and writes on a particular stream, so does _p_e_r_l. (This doesn't apply to sysread() and syswrite().) While none of the built-in data types have any arbitrary size limits (apart from memory size), there are still a few 105 PERL(1) USER COMMANDS PERL(1) arbitrary limits: a given identifier may not be longer than 255 characters; sprintf is limited on many machines to 128 characters per field (unless the format specifier is exactly %s); and no component of your PATH may be longer than 255 if you use -S. _P_e_r_l actually stands for Pathologically Eclectic Rubbish Lister, but don't tell anyone I said that. 106