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PERL(1) UNIX Programmer's Manual PERL(1) NAME perl - Practical Extraction and Report Language SYNOPSIS perl [options] filename args DESCRIPTION Perl is a 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, sed, awk, and sh, so people familiar with those languages should have little difficulty with it. (Language historians will also note some vestiges of csh, Pascal, and even BASIC-PLUS.) Expression syntax corresponds quite closely to C expression syntax. If you have a problem that would ordinarily use sed or awk or sh, but it exceeds their capabilities or must run a little fas- ter, and you don't want to write the silly thing in C, then perl may be for you. There are also translators to turn your sed and awk scripts into perl scripts. OK, enough hype. Upon startup, perl looks for your script in one of the fol- lowing places: 1. Specified line by line via -e switches on the command line. 2. Contained in the file specified by the first filename on the command line. (Note that systems supporting the #! notation invoke interpreters this way.) 3. Passed in via standard input. After locating your script, perl compiles it to an internal form. If the script is syntactically correct, it is exe- cuted. Options 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: Printed 7/26/88 LOCAL 1 PERL(1) UNIX Programmer's Manual PERL(1) #!/bin/perl -spi.bak # same as -s -p -i.bak ... Options include: -D<number> sets debugging flags. To watch how it executes your script, use -D14. (This only works if debugging is com- piled into your perl.) -e commandline may be used to enter one line of script. Multiple -e commands may be given to build up a multi-line script. If -e is given, perl will not look for a script filename in the argument list. -i<extension> 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: #!/bin/perl -pi.bak s/foo/bar/; which is equivalent to #!/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 -i form doesn't need to compare $ARGV to $oldargv to know when the filename has changed. It does, however, use ARGVOUT for the selected filehandle. Note that stdout is restored as the default output filehandle after the loop. Printed 7/26/88 LOCAL 2 PERL(1) UNIX Programmer's Manual PERL(1) -I<directory> may be used in conjunction with -P to tell the C preprocessor where to look for include files. By default /usr/include and /usr/lib/perl are searched. -n causes perl to assume the following loop around your script, which makes it iterate over filename arguments somewhat like "sed -n" or awk: while (<>) { ... # your script goes here } Note that the lines are not printed by default. See -p to have lines printed. -p causes perl to assume the following loop around your script, which makes it iterate over filename arguments somewhat like sed: while (<>) { ... # your script goes here } continue { print; } Note that the lines are printed automatically. To suppress printing use the -n switch. A -p overrides a -n switch. -P causes your script to be run through the C preprocessor before compilation by perl. (Since both comments and cpp directives begin with the # character, you should avoid starting comments with any words recognized by the C preprocessor such as "if", "else" or "define".) -s enables some rudimentary switch parsing for switches on the command line after the script name but before any filename arguments (or before a --). Any switch found there is removed from @ARGV and sets the corresponding variable in the perl script. The following script prints "true" if and only if the script is invoked with a -xyz switch. #!/bin/perl -s if ($xyz) { print "true\n"; } -v prints the version and patchlevel of your perl execut- able. Printed 7/26/88 LOCAL 3 PERL(1) UNIX Programmer's Manual PERL(1) Data Types and Objects Perl has about two and a half data types: strings, arrays of strings, and associative arrays. Strings and arrays of strings are first class objects, for the most part, in the sense that they can be used as a whole as values in an expression. Associative arrays can only be accessed on an association by association basis; they don't have a value as a whole (at least not yet). Strings are interpreted numerically as appropriate. A string 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. References to string variables always begin with '$', even when referring to a string that is part of an array. Thus: $days # a simple string variable $days[28] # 29th element of array @days $days{'Feb'} # one value from an associative array but entire arrays are denoted by '@': @days # ($days[0], $days[1],... $days[n]) Any of these four constructs may be assigned to (in compiler lingo, may serve as an lvalue). (Additionally, you may find the length of array @days by evaluating "$#days", as in csh. [Actually, it's not the length of the array, it's the sub- script of the last element, since there is (ordinarily) a 0th element.]) Every data type has its own namespace. You can, without fear of conflict, use the same name for a string variable, an array, an associative array, a filehandle, a subroutine name, and/or a label. Since variable and array references always start with '$' or '@', the "reserved" words aren't in fact reserved with respect to variable names. (They ARE reserved with respect to labels and filehandles, however, which don't have an initial special character.) Case IS significant--"FOO", "Foo" and "foo" are all different names. Names which start with a letter may also contain digits and underscores. Names which do not start with a letter are limited to one character, e.g. "$%" or "$$". (Many one character names have a predefined significance to perl. More later.) 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 Printed 7/26/88 LOCAL 4 PERL(1) UNIX Programmer's Manual PERL(1) substitution; single-quoted strings are not. The usual backslash rules apply for making characters such as newline, tab, etc. 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 perl finds another line containing the quote character, which may be much further on in the script. Variable substitution inside strings is lim- ited (currently) to simple string variables. The following 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. Array literals are denoted by separating individual values by commas, and enclosing the list in parentheses. In a con- text not requiring an array value, the value of the array literal is the value of the final element, as in the C 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. 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); Numeric literals are specified in any of the usual floating point or integer formats. There are several other pseudo-literals that you should know about. If a string is enclosed by backticks (grave accents), it is interpreted as a command, and the output of that command is the value of the pseudo-literal, just like in any of the standard shells. The command is executed each time the pseudo-literal is evaluated. Unlike in csh, no interpretation is done on the data--newlines remain new- lines. The status value of the command is returned in $?. Evaluating a filehandle in angle brackets yields the next line from that file (newline included, so it's never false Printed 7/26/88 LOCAL 5 PERL(1) UNIX Programmer's Manual PERL(1) until EOF). Ordinarily you must assign that value to a variable, but there is one situation where in which an automatic assignment happens. If (and only if) the input symbol is the only thing inside the conditional of a while 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 perl script you write.) Anyway, the following lines are equivalent to each other: while ($_ = <stdin>) { while (<stdin>) { for (;<stdin>;) { The filehandles stdin, stdout and stderr are predefined. Additional filehandles may be created with the open func- tion. The null filehandle <> is special and can be used to emulate the behavior of sed and awk. 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. 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: Printed 7/26/88 LOCAL 6 PERL(1) UNIX Programmer's Manual PERL(1) 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 stdin. Syntax A perl script consists of a sequence of declarations and commands. The only things that need to be declared in perl are report formats and subroutines. See the sections below for more information on those declarations. All objects are assumed to start with a null or 0 value. The sequence of commands is executed just once, unlike in sed and awk 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 control over which files and which lines you look at. (Actually, I'm lying--it is possible to do an implicit loop with either the -n or -p switch.) A declaration can be put anywhere a command can, but has no effect on the execution of the primary sequence of commands. Typically all the declarations are put at the beginning or the end of the script. Perl 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. Compound statements In perl, 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: Printed 7/26/88 LOCAL 7 PERL(1) UNIX Programmer's Manual PERL(1) 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 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 required--no dangling statements allowed. If you want to write conditionals without curly brackets there are 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) ? die "Can't open $foo" : 'hi mom'; # a bit exotic, that last one The if statement is straightforward. Since BLOCKs are always bounded by curly brackets, there is never any ambi- guity about which if an else goes with. If you use unless in place of if, the sense of the test is reversed. The while 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 next, last and redo (see below). If there is a continue BLOCK, it is always executed just before the conditional is about to be evaluated again, similarly to the third part of a for loop in C. Thus it can be used to increment a loop variable, even when the loop has been continued via the next statement (similar to the C "continue" statement). If the word while is replaced by the word until, the sense of the test is reversed, but the conditional is still tested before the first iteration. In either the if or the while 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 for loop works exactly like the corresponding while loop: Printed 7/26/88 LOCAL 8 PERL(1) UNIX Programmer's Manual PERL(1) for ($i = 1; $i < 10; $i++) { ... } is the same as $i = 1; while ($i < 10) { ... } continue { $i++; } 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 continue 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; } Simple statements 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 awk). Any simple statement may optionally be followed by a single modifier, just before the terminating semicolon. The possi- ble modifiers are: if EXPR unless EXPR while EXPR until EXPR The if and unless modifiers have the expected semantics. The while and unless modifiers also have the expected seman- tics (conditional evaluated first), except when applied to a do-BLOCK command, in which case the block executes once before the conditional is evaluated. This is so that you can write loops like: Printed 7/26/88 LOCAL 9 PERL(1) UNIX Programmer's Manual PERL(1) do { $_ = <stdin>; ... } until $_ eq ".\n"; (See the do 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.) Expressions Since perl expressions work almost exactly like C expres- sions, only the differences will be mentioned here. Here's what perl has that C doesn't: () The null list, used to initialize an array to null. . Concatenation of two strings. .= The corresponding assignment operator. eq String equality (== is numeric equality). For a mnemonic just think of "eq" as a string. (If you are used to the awk 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. le String less than or equal. ge String greater than or equal. =~ 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 Printed 7/26/88 LOCAL 10 PERL(1) UNIX Programmer's Manual PERL(1) precedence of this operator is lower than unary minus and autoincrement/decrement, but higher than everything 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. print '-' x 80; # print row of dashes print '-' x80; # illegal, x80 is identifier print "\t" x ($tab/8), ' ' x ($tab%8); # tab over x= The corresponding assignment operator. .. The range operator, which is bistable. It is false as long as its left argument is false. Once the left argument is true, it stays true until the right argument is true, AFTER which it becomes false again. (It doesn't become false till the next time it's evaluated. It can become false on the same evaluation it became true, but it still returns true once.) The .. operator is primarily intended for doing line number ranges after the fashion of sed or awk. The precedence is a little 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 argument to .. is static, that argu- ment is implicitly compared to the $. variable, the current line number. Examples: if (101 .. 200) { print; } # print 2nd hundred lines next line if (1 .. /^$/); # skip header lines s/^/> / if (/^$/ .. eof()); # quote body -x A file test. This unary operator takes one argu- ment, a filename, and tests the file to see if some- thing is true about it. It returns 1 for true and '' for false. Precedence is higher than logical and Printed 7/26/88 LOCAL 11 PERL(1) UNIX Programmer's Manual PERL(1) relational operators, but lower than arithmetic operators. The operator may be any of: -r File is readable by effective uid. -w File is writeable 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 writeable 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. -f File is a plain file. -d File is a directory. -l File is a symbolic link. Example: while (<>) { chop; next unless -f $_; # ignore specials ... } Note that -s/a/b/ does not do a negated substitu- tion. Here is what C has that perl doesn't: unary & Address-of operator. unary * Dereference-address operator. Like C, perl 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. Along with the literals and variables mentioned earlier, the following operations can serve as terms in an expression: /PATTERN/i Searches a string for a pattern, and returns true Printed 7/26/88 LOCAL 12 PERL(1) UNIX Programmer's Manual PERL(1) (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 you prepend an `m' you can use any pair of char- acters as delimiters. This is particularly useful for matching Unix path names that contain `/'. If the final delimiter is followed by the optional letter `i', the matching is done in a case- insensitive manner. 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#; ?PATTERN? This is just like the /pattern/ search, except that it matches only once between calls to the reset operator. This is a useful optimization when you only want to see the first occurence of something in each of a set of files, for instance. chdir EXPR Changes the working directory to EXPR, if possible. Returns 1 upon success, 0 otherwise. See example under die(). chmod LIST Changes the permissions of a list of files. The first element of the list must be the numerical mode. LIST may be an array, in which case you may wish to use the unshift() command to put the mode on the front of the array. Returns the number of files successfully changed. Note: in order to use the value you must put the whole thing in parentheses. $cnt = (chmod 0755,'foo','bar'); chop(VARIABLE) Printed 7/26/88 LOCAL 13 PERL(1) UNIX Programmer's Manual PERL(1) chop Chops off the last character of a string and returns it. It's used primarily to remove the newline from the end of an input record, but is much more effi- cient 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(/:/); ... } chown LIST Changes the owner (and group) of a list of files. LIST may be an array. The first two elements of the list must be the NUMERICAL uid and gid, in that order. Returns the number of files successfully changed. Note: in order to use the value you must put the whole thing in parentheses. $cnt = (chown $uid,$gid,'foo'); Here's an example of looking up non-numeric uids: print "User: "; $user = <stdin>; chop($user); open(pass,'/etc/passwd') || die "Can't open passwd"; while (<pass>) { ($login,$pass,$uid,$gid) = split(/:/); $uid{$login} = $uid; $gid{$login} = $gid; } @ary = ('foo','bar','bie','doll'); if ($uid{$user} eq '') { die "$user not in passwd file"; } else { unshift(@ary,$uid{$user},$gid{$user}); chown @ary; } 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 open.) Printed 7/26/88 LOCAL 14 PERL(1) UNIX Programmer's Manual PERL(1) However, an explicit close on an input file resets the line counter ($.), while the implicit close done by open 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. 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 crypt(PLAINTEXT,SALT) Encrypts a string exactly like the crypt() function in the C library. Useful for checking the password file for lousy passwords. Only the guys wearing white hats should do this. die EXPR Prints the value of EXPR to stderr and exits with a non-zero status. Equivalent examples: die "Can't cd to spool." unless chdir '/usr/spool/news'; (chdir '/usr/spool/news') || die "Can't cd to spool." Note that the parens are necessary above due to pre- cedence. See also exit. 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 state- ments the loop modifiers test the conditional first.) do SUBROUTINE (LIST) Executes a SUBROUTINE declared by a sub declaration, and returns the value of the last expression evaluated in SUBROUTINE. (See the section on sub- routines later on.) 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. Printed 7/26/88 LOCAL 15 PERL(1) UNIX Programmer's Manual PERL(1) The iterator can be reset only by reading all the elements from the array. You should not modify the array while iterating over it. 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(). eof(FILEHANDLE) eof Returns 1 if the next read on FILEHANDLE will return end of file, or if FILEHANDLE is not open. If (FILEHANDLE) is omitted, the eof status is returned for the last file read. The null filehandle 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. Example: # insert dashes just before last line while (<>) { if (eof()) { print "--------------\n"; } print; } eval EXPR EXPR is parsed and executed as if it were a little perl program. It is executed in the context of the current perl 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, a null string is returned by eval, and $@ is set to the error mes- sage. If there was no error, $@ is null. If EXPR is omitted, evaluates $_. exec LIST If there is more than one argument in LIST, calls execvp() with the arguments in LIST. If there is only one argument, the argument is checked for shell metacharacters. If there are any, the entire argu- ment is passed to /bin/sh -c for parsing. If there are none, the argument is split into words and passed directly to execvp(), which is more effi- cient. Note: exec (and system) do not flush your Printed 7/26/88 LOCAL 16 PERL(1) UNIX Programmer's Manual PERL(1) output buffer, so you may need to set $| to avoid lost output. exit EXPR Evaluates EXPR and exits immediately with that value. Example: $ans = <stdin>; exit 0 if $ans =~ /^[Xx]/; See also die. exp(EXPR) Returns e to the power of EXPR. fork Does a fork() call. Returns the child pid to the parent process and 0 to the child process. Note: unflushed buffers remain unflushed in both processes, which means you may need to set $| to avoid duplicate output. 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. 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 sed-to-perl translator easier. Use at your own risk. 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().) index(STR,SUBSTR) Returns the position of SUBSTR in STR, based at 0, or whatever you've set the $[ variable to. If the substring is not found, returns one less than the base, ordinarily -1. Printed 7/26/88 LOCAL 17 PERL(1) UNIX Programmer's Manual PERL(1) int(EXPR) Returns the integer portion of EXPR. 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 split. 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"; } kill LIST Sends a signal to a list of processes. The first element of the list must be the (numerical) signal to send. LIST may be an array, in which case you may wish to use the unshift command to put the sig- nal on the front of the array. Returns the number of processes successfully signaled. Note: in order to use the value you must put the whole thing in parentheses: $cnt = (kill 9,$child1,$child2); If the signal is negative, kills process groups instead of processes. (On System V, a negative pro- cess number will also kill process groups, but that's not portable.) last LABEL last The last command is like the break statement in C (as used in loops); it immediately exits the loop in question. If the LABEL is omitted, the command Printed 7/26/88 LOCAL 18 PERL(1) UNIX Programmer's Manual PERL(1) refers to the innermost enclosing loop. The con- tinue block, if any, is not executed: line: while (<stdin>) { last line if /^$/; # exit when done with header ... } 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: ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime(time); All array elements are numeric. log(EXPR) Returns logarithm (base e) of EXPR. next LABEL next The next command is like the continue 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 continue 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. length(EXPR) Returns the length in characters of the value of EXPR. link(OLDFILE,NEWFILE) Creates a new filename linked to the old filename. Returns 1 for success, 0 otherwise. 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 fol- lowing will handle decimal, octal and hex in the standard notation: Printed 7/26/88 LOCAL 19 PERL(1) UNIX Programmer's Manual PERL(1) $val = oct($val) if $val =~ /^0/; open(FILEHANDLE,EXPR) open(FILEHANDLE) open FILEHANDLE Opens the file whose filename is given by EXPR, and associates it with FILEHANDLE. If EXPR is omitted, the string variable of the same name as the FILEHAN- DLE contains the filename. If the filename begins with ">", the file is opened for output. If the filename begins with ">>", the file is opened for appending. If the filename begins with "|", the filename is interpreted 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 stdin and open- ing '>-' opens stdout. Open returns 1 upon success, '' otherwise. Examples: $article = 100; open article || die "Can't find article $article"; while (<article>) {... open(log, '>>/usr/spool/news/twitlog'); open(article, "caeser <$article |"); # decrypt article open(extract, "|sort >/tmp/Tmp$$"); # $$ is our process# ord(EXPR) Returns the ascii value of the first character of EXPR. pop ARRAY pop(ARRAY) Pops and returns the last value of the array, shor- tening the array by 1. print FILEHANDLE LIST print LIST print Prints a string or comma-separated list of strings. If FILEHANDLE is omitted, prints by default to stan- dard output (or to the last selected output channel--see select()). If LIST is also omitted, Printed 7/26/88 LOCAL 20 PERL(1) UNIX Programmer's Manual PERL(1) prints $_ to stdout. LIST may also be an array value. To set the default output channel to some- thing other than stdout use the select operation. printf FILEHANDLE LIST printf LIST Equivalent to a "print FILEHANDLE sprintf(LIST)". push(ARRAY,EXPR) Treats ARRAY (@ is optional) as a stack, and pushes the value of EXPR onto the end of ARRAY. The length of ARRAY increases by 1. Has the same effect as $ARRAY[$#ARRAY+1] = EXPR; but is more efficient. redo LABEL redo The redo command restarts the loop block without evaluating the conditional again. The continue 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, 0 otherwise. reset EXPR Generally used in a continue block at the end of a Printed 7/26/88 LOCAL 21 PERL(1) UNIX Programmer's Manual PERL(1) 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 string variables beginning with one of those letters are set to the null string. If the expression is omitted, one-match searches (?pat- tern?) are reset to match again. Always returns 1. Examples: reset 'X'; # reset all X variables reset 'a-z'; # reset lower case variables reset; # just reset ?? searches s/PATTERN/REPLACEMENT/gi 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 occurences of the pat- tern are to be replaced. The "i" is also optional, and if present, indicates that matching is to be done in a case-insensitive manner. Any delimiter may replace the slashes; if single quotes are used, no interpretation is done on the replacement string. If no string is specified via the =~ or !~ operator, the $_ string is searched and modified. (The string specified with =~ must be a string variable or array element, 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. 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 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields (Note the use of $ instead of \ in the last example. See section on regular expressions.) seek(FILEHANDLE,POSITION,WHENCE) Randomly positions the file pointer for FILEHANDLE, just like the fseek() call of stdio. Returns 1 upon success, 0 otherwise. select(FILEHANDLE) Sets the current default filehandle for output. Printed 7/26/88 LOCAL 22 PERL(1) UNIX Programmer's Manual PERL(1) This has two effects: first, a write or a print without a filehandle will default to this FILEHAN- DLE. Second, references to variables related to output will refer to this output channel. For exam- ple, if you have to set the top of form format for more than one output channel, you might do the fol- lowing: select(report1); $^ = 'report1_top'; select(report2); $^ = 'report2_top'; Select happens to return TRUE if the file is currently open and FALSE otherwise, but this has no effect on its operation. 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 ARRAY is omitted, shifts the ARGV array. 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. 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. Returns the number of seconds actually slept. split(/PATTERN/,EXPR) split(/PATTERN/) split Splits a string into an array of strings, and returns it. If EXPR is omitted, splits the $_ string. If PATTERN is also omitted, splits on whi- tespace (/[ \t\n]+/). Anything matching PATTERN is taken to be a delimiter separating the fields. (Note that the delimiter may be longer than one character.) Trailing null fields are stripped, which potential users of pop() would do well to remember. A pattern matching the null string (not to be con- fused with a null pattern) will split the value of EXPR into separate characters at each point it matches that way. For example: Printed 7/26/88 LOCAL 23 PERL(1) UNIX Programmer's Manual PERL(1) print join(':',split(/ */,'hi there')); produces the output 'h:i:t:h:e:r:e'. The pattern /PATTERN/ may be replaced with an expression to specify patterns that vary at runtime. 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 awk's default behavior, whereas split(/ /) will give you as many null initial fields as there are leading spaces. Example: 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 join. sprintf(FORMAT,LIST) Returns a string formatted by the usual printf con- ventions. The * character is not supported. sqrt(EXPR) Return the square root of EXPR. stat(FILEHANDLE) stat(EXPR) 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); substr(EXPR,OFFSET,LEN) Extracts a substring out of EXPR and returns it. First character is at offset 0, or whatever you've set $[ to. system LIST Does exactly the same thing as "exec LIST" except Printed 7/26/88 LOCAL 24 PERL(1) UNIX Programmer's Manual PERL(1) 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 exec. symlink(OLDFILE,NEWFILE) Creates a new filename symbolically linked to the old filename. Returns 1 for success, 0 otherwise. On systems that don't support symbolic links, pro- duces a fatal error. tell(FILEHANDLE) tell Returns the current file position for FILEHANDLE. If FILEHANDLE is omitted, assumes the file last read. time Returns the number of seconds since 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. ($user,$system,$cuser,$csystem) = times; tr/SEARCHLIST/REPLACEMENTLIST/ y/SEARCHLIST/REPLACEMENTLIST/ Translates all occurences of the characters found in the search list with the corresponding character in the replacement list. It returns the number of characters replaced. If no string is specified via the =~ or !~ operator, the $_ string is translated. (The string specified with =~ must be a string vari- able or array element, i.e. an lvalue.) For sed devotees, y is provided as a synonym for tr. Exam- ples: $ARGV[1] =~ y/A-Z/a-z/; # canonicalize to lower case $cnt = tr/*/*/; # count the stars in $_ umask(EXPR) Sets the umask for the process and returns the old one. Printed 7/26/88 LOCAL 25 PERL(1) UNIX Programmer's Manual PERL(1) unlink LIST Deletes a list of files. LIST may be an array. Returns the number of files successfully deleted. Note: in order to use the value you must put the whole thing in parentheses: $cnt = (unlink 'a','b','c'); unshift(ARRAY,LIST) Does the opposite of a shift. Prepends list to the front of the array, and returns the number of ele- ments in the new array. unshift(ARGV,'-e') unless $ARGV[0] =~ /^-/; 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 produces (given that the associative array has not been modified). See also keys() and each(). 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 select) 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, a spe- cial top-of-page format is used to format the new page header, and then the record is written. By default the top-of-page format is "top", but it may be set to the format of your choice by assigning the name to the $^ variable. If FILEHANDLE is unspecified, output goes to the current default output channel, which starts out as stdout but may be changed by the select 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 Printed 7/26/88 LOCAL 26 PERL(1) UNIX Programmer's Manual PERL(1) on formats, see the section on formats later on. Subroutines A subroutine may be declared as follows: sub NAME BLOCK Any arguments passed to the routine come in as array @_, that is ($_[0], $_[1], ...). The return value of the sub- routine is the value of the last expression evaluated. There are no local variables--everything is a global vari- able. A subroutine is called using the do operator. (CAVEAT: For efficiency reasons recursive subroutine calls are not currently supported. This restriction may go away in the future. Then again, it may not.) Example: sub MAX { $max = pop(@_); while ($foo = pop(@_)) { $max = $foo if $max < $foo; } $max; } ... $bestday = do MAX($mon,$tue,$wed,$thu,$fri); Printed 7/26/88 LOCAL 27 PERL(1) UNIX Programmer's Manual PERL(1) 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 ($_ = get_line()) { ... } Use array assignment to name your formal arguments: sub maybeset { ($key,$value) = @_; $foo{$key} = $value unless $foo{$key}; } Regular Expressions The patterns used in pattern matching are regular expres- sions such as those used by egrep(1). In addition, \w matches an alphanumeric character and \W a nonalphanumeric. Word boundaries may be matched by \b, and non-boundaries by \B. The bracketing construct ( ... ) may also be used, $<digit> matches the digit'th substring, where digit can range from 1 to 9. (You can also use the old standby \<digit> in search patterns, but $<digit> also works in replacement patterns and in the block controlled by the current conditional.) $+ returns whatever the last bracket match matched. $& returns the entire matched string. Up to 10 alternatives may given in a pattern, separated by |, with the caveat that ( ... | ... ) is illegal. Examples: s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words Printed 7/26/88 LOCAL 28 PERL(1) UNIX Programmer's Manual PERL(1) if (/Time: (..):(..):(..)/) { $hours = $1; $minutes = $2; $seconds = $3; } By default, the ^ character matches only the beginning of the string, and perl does certain optimizations with the assumption that the string contains only one line. You may, however, wish to treat a string as a multi-line buffer, such that the ^ will match after any newline within the string. At the cost of a little more overhead, you can do this by setting the variable $* to 1. Setting it back to 0 makes perl revert to its old behavior. Formats Output record formats for use with the write 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 justfication, right justification, or centering. 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. They must currently be either string variable names or string literals (or pseudo- literals). Currently you can separate values with spaces, Printed 7/26/88 LOCAL 29 PERL(1) UNIX Programmer's Manual PERL(1) but commas may be placed between values to prepare for pos- sible future versions in which full expressions are allowed as values. Picture fields that begin with ^ rather than @ are treated specially. The value supplied must be a string variable name which contains a text string. Perl 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 string 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. 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 possi- ble.) The tilde will be translated to a space upon output. Examples: # a report on the /etc/passwd file format top = Passwd File Name Login Office Uid Gid Home ------------------------------------------------------------------ . format stdout = @<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<< $name $login $office $uid $gid $home . Printed 7/26/88 LOCAL 30 PERL(1) UNIX Programmer's Manual PERL(1) # a report from a bug report form format 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. Predefined Names The following names have special meaning to perl. 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 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 ($_ = <>) {... Printed 7/26/88 LOCAL 31 PERL(1) UNIX Programmer's Manual PERL(1) /^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 file that was read. Readonly. (Mnemonic: many programs use . to mean the current line number.) $/ The input record separator, newline by default. Works like awk's RS variable, including treating blank lines as delimiters if set to the null string. If set to a value longer than one character, only the first character is used. (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 awk, set this variable as you would set awk's OFS variable to specify what is printed between fields. (Mnemonic: what is printed when there is a , in your print statement.) $\ 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 get behavior more like awk, set this variable as you would set awk'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 perl.) $# The output format for printed numbers. This vari- able is a half-hearted attempt to emulate awk's OFMT variable. There are times, however, when awk and perl 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 awk's value. (Mnemonic: # is the number sign.) $% The current page number of the currently selected output channel. (Mnemonic: % is page number in Printed 7/26/88 LOCAL 32 PERL(1) UNIX Programmer's Manual PERL(1) 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. (Mnemonic: brother to $^.) $^ The name of the current top-of-page format for the currently selected output channel. (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 stdout 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 perl 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 perl running this script. (Mnemonic: same as shells.) $? The status returned by the last backtick (``) com- mand. (Mnemonic: same as sh and ksh.) $+ The last bracket matched by the last search pattern. This is useful if you don't know which of a set of alternative patterns matched. For example: /Version: (.*)|Revision: (.*)/ && ($rev = $+); (Mnemonic: be positive and forward looking.) $* Set to 1 to do multiline matching within a string, 0 to assume strings contain a single line. Default is 0. (Mnemonic: * matches multiple things.) $0 Contains the name of the file containing the perl script being executed. The value should be copied elsewhere before any pattern matching happens, which clobbers $0. (Mnemonic: same as sh and ksh.) Printed 7/26/88 LOCAL 33 PERL(1) UNIX Programmer's Manual PERL(1) $<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 perl behave more like awk (or Fortran) when subscripting and when evaluating the index() and substr() functions. (Mnemonic: [ begins subscripts.) $! The current value of errno, with all the usual caveats. (Mnemonic: What just went bang?) $@ The error message from the last eval command. If null, the last eval parsed and executed correctly. (Mnemonic: Where was the syntax error "at"?) @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 $ARGV[0] is the first argument, NOT the command name. See $0 for the command name. $ENV{expr} The associative array ENV contains your current environment. Setting a value in ENV changes the environment for child processes. $SIG{expr} The associative array SIG is used to set signal handlers for various signals. Example: sub handler { # 1st argument is signal name ($sig) = @_; print "Caught a SIG$sig--shutting down0; close(log); exit(0); } $SIG{'INT'} = 'handler'; $SIG{'QUIT'} = 'handler'; ... $SIG{'INT'} = 'DEFAULT'; # restore default action $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT ENVIRONMENT Perl currently uses no environment variables, except to make them available to the script being executed, and to child Printed 7/26/88 LOCAL 34 PERL(1) UNIX Programmer's Manual PERL(1) processes. However, scripts running setuid would do well to execute the following lines before doing anything else, just to keep people honest: $ENV{'PATH'} = '/bin:/usr/bin'; # or whatever you need $ENV{'SHELL'} = '/bin/sh' if $ENV{'SHELL'}; $ENV{'IFS'} = '' if $ENV{'IFS'}; AUTHOR Larry Wall <lwall@jpl-devvax.Jpl.Nasa.Gov> FILES /tmp/perl-eXXXXXX temporary file for -e commands. SEE ALSO a2p awk to perl translator s2p sed to perl translator perldb interactive perl debugger DIAGNOSTICS 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 perl via -e switches, each -e is counted as one line.) TRAPS Accustomed awk users should take special note of the follow- ing: * Semicolons are required after all simple statements in perl. Newline is not a statement delimiter. * Curly brackets are required on ifs and whiles. * Variables begin with $ or @ in perl. * 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. * 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 split has different arguments. * The current input line is normally in $_, not $0. It generally does not have the newline stripped. ($0 is Printed 7/26/88 LOCAL 35 PERL(1) UNIX Programmer's Manual PERL(1) initially the name of the program executed, then the last matched string.) * The current filename is $ARGV, not $FILENAME. NR, RS, ORS, OFS, and OFMT have equivalents with other symbols. FS doesn't have an equivalent, since you have to be explicit about split statements. * $<digit> does not refer to fields--it refers to sub- strings matched by the last match pattern. * The print 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.) * The match operator is "=~", not "~". ("~" is the one's complement operator.) * The concatenation operator is ".", not the null string. (Using the null string would render "/pat/ /pat/" unparseable, 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.) * The \nnn construct in patterns must be given as [\nnn] to avoid interpretation as a backreference. * Next, exit, and continue work differently. * When in doubt, run the awk 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" * Break and continue become last and next, respectively. * There's no switch statement. * Variables begin with $ or @ in perl. * Printf does not implement *. Printed 7/26/88 LOCAL 36 PERL(1) UNIX Programmer's Manual PERL(1) * Comments begin with #, not /*. * You can't take the address of anything. * Subroutines are not reentrant. * ARGV must be capitalized. * The "system" calls link, unlink, rename, etc. return 1 for success, not 0. * Signal handlers deal with signal names, not numbers. Seasoned sed programmers should take note of the following: * Backreferences in substitutions use $ rather than \. * The pattern matching metacharacters (, ), and | do not have backslashes in front. BUGS You can't currently dereference array elements inside a double-quoted string. You must assign them to a temporary and interpolate that. Associative arrays really ought to be first class objects. Recursive subroutines are not currently supported, due to the way temporary values are stored in the syntax tree. Arrays ought to be passable to subroutines just as strings are. The array literal consisting of one element is currently misinterpreted, i.e. @array = (123); doesn't work right. Perl actually stands for Pathologically Eclectic Rubbish Lister, but don't tell anyone I said that. Printed 7/26/88 LOCAL 37