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NAME
perlrun - how to execute the Perl interpreter
SYNOPSIS
perl [ -sTuU ] [ -hv ] [ -V[:*configvar*] ] [ -cw ] [
-d[:*debugger*] ] [ -D[*number/list*] ] [ -pna ] [ -F*pattern* ]
[ -l[*octal*] ] [ -0[*octal*] ] [ -I*dir* ] [ -m[-]*module* ] [
-M[-]*'module...'* ] [ -P ] [ -S ] [ -x[*dir*] ] [
-i[*extension*] ] [ -e *'command'* ] [ -- ] [ *programfile* ] [
*argument* ]...
DESCRIPTION
Upon startup, Perl looks for your script in one of the following
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. See the Location of Perl
manpage.)
3. Passed in implicitly via standard input. This works only if
there are no filename arguments--to pass arguments to a
STDIN script you must explicitly specify a "-" for the
script name.
With methods 2 and 3, Perl starts parsing the input file from
the beginning, unless you've specified a -x switch, in which
case it scans for the first line starting with #! and containing
the word "perl", and starts there instead. This is useful for
running a script embedded in a larger message. (In this case you
would indicate the end of the script using the `__END__' token.)
The #! line is always examined for switches as the line is being
parsed. Thus, if you're on a machine that allows only one
argument with the #! line, or worse, doesn't even recognize the
#! line, you still can get consistent switch behavior regardless
of how Perl was invoked, even if -x was used to find the
beginning of the script.
Because many operating systems silently chop off kernel
interpretation of the #! line after 32 characters, some switches
may be passed in on the command line, and some may not; you
could even get a "-" without its letter, if you're not careful.
You probably want to make sure that all your switches fall
either before or after that 32 character boundary. Most switches
don't actually care if they're processed redundantly, but
getting a - instead of a complete switch could cause Perl to try
to execute standard input instead of your script. And a partial
-I switch could also cause odd results.
Some switches do care if they are processed twice, for instance
combinations of -l and -0. Either put all the switches after the
32 character boundary (if applicable), or replace the use of -
0*digits* by `BEGIN{ $/ = "\0digits"; }'.
Parsing of the #! switches starts wherever "perl" is mentioned
in the line. The sequences "-*" and "- " are specifically
ignored so that you could, if you were so inclined, say
#!/bin/sh -- # -*- perl -*- -p
eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
if $running_under_some_shell;
to let Perl see the -p switch.
If the #! line does not contain the word "perl", the program
named after the #! is executed instead of the Perl interpreter.
This is slightly bizarre, but it helps people on machines that
don't do #!, because they can tell a program that their SHELL is
/usr/bin/perl, and Perl will then dispatch the program to the
correct interpreter for them.
After locating your script, Perl compiles the entire script to
an internal form. If there are any compilation errors, execution
of the script is not attempted. (This is unlike the typical
shell script, which might run part-way through before finding a
syntax error.)
If the script is syntactically correct, it is executed. If the
script runs off the end without hitting an exit() or die()
operator, an implicit `exit(0)' is provided to indicate
successful completion.
#! and quoting on non-Unix systems
Unix's #! technique can be simulated on other systems:
OS/2
Put
extproc perl -S -your_switches
as the first line in `*.cmd' file (`-S' due to a bug in
cmd.exe's `extproc' handling).
MS-DOS
Create a batch file to run your script, and codify it in
`ALTERNATIVE_SHEBANG' (see the dosish.h file in the source
distribution for more information).
Win95/NT
The Win95/NT installation, when using the Activeware port of
Perl, will modify the Registry to associate the .pl
extension with the perl interpreter. If you install another
port of Perl, including the one in the Win32 directory of
the Perl distribution, then you'll have to modify the
Registry yourself. Note that this means you can no longer
tell the difference between an executable Perl program and a
Perl library file.
Macintosh
Macintosh perl scripts will have the appropriate Creator and
Type, so that double-clicking them will invoke the perl
application.
VMS Put
$ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
$ exit++ + ++$status != 0 and $exit = $status = undef;
at the top of your script, where `-mysw' are any command
line switches you want to pass to Perl. You can now invoke
the script directly, by saying `perl script', or as a DCL
procedure, by saying `@script' (or implicitly via DCL$PATH
by just using the name of the script).
This incantation is a bit much to remember, but Perl will
display it for you if you say `perl "-V:startperl"'.
Command-interpreters on non-Unix systems have rather different
ideas on quoting than Unix shells. You'll need to learn the
special characters in your command-interpreter (`*', `\' and `"'
are common) and how to protect whitespace and these characters
to run one-liners (see `-e' below).
On some systems, you may have to change single-quotes to double
ones, which you must *NOT* do on Unix or Plan9 systems. You
might also have to change a single % to a %%.
For example:
# Unix
perl -e 'print "Hello world\n"'
# MS-DOS, etc.
perl -e "print \"Hello world\n\""
# Macintosh
print "Hello world\n"
(then Run "Myscript" or Shift-Command-R)
# VMS
perl -e "print ""Hello world\n"""
The problem is that none of this is reliable: it depends on the
command and it is entirely possible neither works. If 4DOS was
the command shell, this would probably work better:
perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""
CMD.EXE in Windows NT slipped a lot of standard Unix
functionality in when nobody was looking, but just try to find
documentation for its quoting rules.
Under the Macintosh, it depends which environment you are using.
The MacPerl shell, or MPW, is much like Unix shells in its
support for several quoting variants, except that it makes free
use of the Macintosh's non-ASCII characters as control
characters.
There is no general solution to all of this. It's just a mess.
Location of Perl
It may seem obvious to say, but Perl is useful only when users
can easily find it. When possible, it's good for both
/usr/bin/perl and /usr/local/bin/perl to be symlinks to the
actual binary. If that can't be done, system administrators are
strongly encouraged to put (symlinks to) perl and its
accompanying utilities, such as perldoc, into a directory
typically found along a user's PATH, or in another obvious and
convenient place.
In this documentation, `#!/usr/bin/perl' on the first line of
the script will stand in for whatever method works on your
system.
Switches
A single-character switch may be combined with the following
switch, if any.
#!/usr/bin/perl -spi.bak # same as -s -p -i.bak
Switches include:
-0[*digits*]
specifies the input 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 find which can print
filenames terminated by the null character, 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 because there is no legal character with that
value.
-a turns on autosplit mode when used with a -n or -p. An implicit
split command to the @F array is done as the first thing
inside the implicit while loop produced by the -n or -p.
perl -ane 'print pop(@F), "\n";'
is equivalent to
while (<>) {
@F = split(' ');
print pop(@F), "\n";
}
An alternate delimiter may be specified using -F.
-c causes Perl to check the syntax of the script and then exit
without executing it. Actually, it *will* execute `BEGIN',
`END', and `use' blocks, because these are considered as
occurring outside the execution of your program.
-d runs the script under the Perl debugger. See the perldebug
manpage.
-d:*foo*
runs the script under the control of a debugging or tracing
module installed as Devel::foo. E.g., -d:DProf executes the
script using the Devel::DProf profiler. See the perldebug
manpage.
-D*letters*
-D*number*
sets debugging flags. To watch how it executes your script,
use -Dtls. (This works only if debugging is compiled into
your Perl.) Another nice value is -Dx, which lists your
compiled syntax tree. And -Dr displays compiled regular
expressions. As an alternative, specify a number instead of
list of letters (e.g., -D14 is equivalent to -Dtls):
1 p Tokenizing and parsing
2 s Stack snapshots
4 l Context (loop) stack processing
8 t Trace execution
16 o Method and overloading resolution
32 c String/numeric conversions
64 P Print preprocessor command for -P
128 m Memory allocation
256 f Format processing
512 r Regular expression parsing and execution
1024 x Syntax tree dump
2048 u Tainting checks
4096 L Memory leaks (needs C<-DLEAKTEST> when compiling Perl)
8192 H Hash dump -- usurps values()
16384 X Scratchpad allocation
32768 D Cleaning up
65536 S Thread synchronization
All these flags require `-DDEBUGGING' when you compile the
Perl executable. This flag is automatically set if you
include `-g' option when `Configure' asks you about
optimizer/debugger flags.
-e *commandline*
may be used to enter one line of script. If -e is given,
Perl will not look for a script filename in the argument
list. Multiple -e commands may be given to build up a
multi-line script. Make sure to use semicolons where you
would in a normal program.
-F*pattern*
specifies the pattern to split on if -a is also in effect.
The pattern may be surrounded by `//', `""', or `''',
otherwise it will be put in single quotes.
-h prints a summary of the options.
-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 original name, and
selecting that output file as the default for print()
statements. The extension, if supplied, is used to modify
the name of the old file to make a backup copy, following
these rules:
If no extension is supplied, no backup is made and the
current file is overwritten.
If the extension doesn't contain a `*' then it is appended
to the end of the current filename as a suffix.
If the extension does contain one or more `*' characters,
then each `*' is replaced with the current filename. In
perl terms you could think of this as:
($backup = $extension) =~ s/\*/$file_name/g;
This allows you to add a prefix to the backup file, instead
of (or in addition to) a suffix:
$ perl -pi'bak_*' -e 's/bar/baz/' fileA # backup to 'bak_fileA'
Or even to place backup copies of the original files into
another directory (provided the directory already exists):
$ perl -pi'old/*.bak' -e 's/bar/baz/' fileA # backup to 'old/fileA.bak'
These sets of one-liners are equivalent:
$ perl -pi -e 's/bar/baz/' fileA # overwrite current file
$ perl -pi'*' -e 's/bar/baz/' fileA # overwrite current file
$ perl -pi'.bak' -e 's/bar/baz/' fileA # backup to 'fileA.bak'
$ perl -pi'*.bak' -e 's/bar/baz/' fileA # backup to 'fileA.bak'
From the shell, 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
#!/usr/bin/perl
$extension = '.bak';
while (<>) {
if ($ARGV ne $oldargv) {
if ($extension !~ /\*/) {
$backup = $ARGV . $extension;
}
else {
($backup = $extension) =~ s/\*/$ARGV/g;
}
rename($ARGV, $backup);
open(ARGVOUT, ">$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.
As shown above, Perl creates the backup file whether or not
any output is actually changed. So this is just a fancy way
to copy files:
$ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...
or
$ perl -p -i'.bak' -e 1 file1 file2 file3...
You can use `eof' without parentheses to locate the end of
each input file, in case you want to append to each file,
or reset line numbering (see example in the "eof" entry in
the perlfunc manpage).
If, for a given file, Perl is unable to create the backup
file as specified in the extension then it will skip that
file and continue on with the next one (if it exists).
For a discussion of issues surrounding file permissions and
`-i', see the "Why does Perl let me delete read-only files?
Why does -i clobber protected files? Isn't this a bug in
Perl?" entry in the perlfaq5 manpage.
You cannot use -i to create directories or to strip
extensions from files.
Perl does not expand `~', so don't do that.
Finally, note that the -i switch does not impede execution
when no files are given on the command line. In this case,
no backup is made (the original file cannot, of course, be
determined) and processing proceeds from STDIN to STDOUT as
might be expected.
-I*directory*
Directories specified by -I are prepended to the search
path for modules (`@INC'), and also tells the C
preprocessor where to search for include files. The C
preprocessor is invoked with -P; by default it searches
/usr/include and /usr/lib/perl.
-l[*octnum*]
enables automatic line-ending processing. It has two
effects: first, it automatically chomps "`$/'" (the input
record separator) when used with -n or -p, and second, it
assigns "`$\'" (the output record separator) to have the
value of *octnum* so that any print statements will have
that separator added back on. If *octnum* 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 -l switch
is followed by a -0 switch:
gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
This sets `$\' to newline and then sets `$/' to the null
character.
-m[-]*module*
-M[-]*module*
-M[-]*'module ...'*
-[mM][-]*module=arg[,arg]...*
`-m'*module* executes `use' *module* `();' before executing
your script.
`-M'*module* executes `use' *module* `;' before executing
your script. You can use quotes to add extra code after the
module name, e.g., `-M'module qw(foo bar)''.
If the first character after the `-M' or `-m' is a dash (`-
') then the 'use' is replaced with 'no'.
A little builtin syntactic sugar means you can also say `-
mmodule=foo,bar' or `-Mmodule=foo,bar' as a shortcut for `-
M'module qw(foo bar)''. This avoids the need to use quotes
when importing symbols. The actual code generated by `-
Mmodule=foo,bar' is `use module split(/,/,q{foo,bar})'.
Note that the `=' form removes the distinction between `-m'
and `-M'.
-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. If a file named by an argument cannot
be opened for some reason, Perl warns you about it, and
moves on to the next file.
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.
`BEGIN' and `END' blocks may be used to capture control
before or after the implicit loop, just as in awk.
-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 or die "-p destination: $!\n";
}
If a file named by an argument cannot be opened for some
reason, Perl warns you about it, and moves on to the next
file. Note that the lines are printed automatically. An
error occurring during printing is treated as fatal. To
suppress printing use the -n switch. A -p overrides a -n
switch.
`BEGIN' and `END' blocks may be used to capture control
before or after the implicit loop, just as in awk.
-P causes your script to be run through the C preprocessor before
compilation by Perl. (Because both comments and cpp
directives begin with the # character, you should avoid
starting comments with any words recognized by the C
preprocessor such as "if", "else", or "define".)
-s enables some rudimentary switch parsing for switches on the
command line after the script name but before any filename
arguments (or before a --). Any switch found there is
removed from @ARGV and sets the corresponding variable in
the Perl script. The following script prints "true" if and
only if the script is invoked with a -xyz switch.
#!/usr/bin/perl -s
if ($xyz) { print "true\n"; }
-S makes Perl use the PATH environment variable to search for the
script (unless the name of the script contains directory
separators). On some platforms, this also makes Perl append
suffixes to the filename while searching for it. For
example, on Win32 platforms, the ".bat" and ".cmd" suffixes
are appended if a lookup for the original name fails, and
if the name does not already end in one of those suffixes.
If your Perl was compiled with DEBUGGING turned on, using
the -Dp switch to Perl shows how the search progresses.
If the filename supplied contains directory separators
(i.e. it is an absolute or relative pathname), and if the
file is not found, platforms that append file extensions
will do so and try to look for the file with those
extensions added, one by one.
On DOS-like platforms, if the script does not contain
directory separators, it will first be searched for in the
current directory before being searched for on the PATH. On
Unix platforms, the script will be searched for strictly on
the PATH.
Typically this is used to emulate #! startup on platforms
that don't support #!. This example works on many platforms
that have a shell compatible with Bourne shell:
#!/usr/bin/perl
eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
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 Perl script
as a shell script. The shell executes the second line as a
normal shell command, and thus starts up the Perl
interpreter. On some systems $0 doesn't always contain the
full pathname, so the -S tells Perl to search for the
script if necessary. After Perl locates the script, it
parses the lines and ignores them because the variable
$running_under_some_shell is never true. If the script will
be interpreted by csh, you will need to replace `${1+"$@"}'
with `$*', even though that doesn't understand embedded
spaces (and such) in the argument list. 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
politely 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 following:
eval '(exit $?0)' && eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
& eval 'exec /usr/bin/perl -wS $0 $argv:q'
if $running_under_some_shell;
-T forces "taint" checks to be turned on so you can test them.
Ordinarily these checks are done only when running setuid
or setgid. It's a good idea to turn them on explicitly for
programs run on another's behalf, such as CGI programs. See
the perlsec manpage. Note that (for security reasons) this
option must be seen by Perl quite early; usually this means
it must appear early on the command line or in the #! line
(for systems which support that).
-u causes Perl to dump core after compiling your script. You can
then in theory take this core dump and turn it into an
executable file by using the undump program (not supplied).
This speeds startup at the expense of some disk space
(which you can minimize by stripping the executable).
(Still, a "hello world" executable comes out to about 200K
on my machine.) If you want to execute a portion of your
script before dumping, use the dump() operator instead.
Note: availability of undump is platform specific and may
not be available for a specific port of Perl. It has been
superseded by the new perl-to-C compiler, which is more
portable, even though it's still only considered beta.
-U allows Perl to do unsafe operations. Currently the only
"unsafe" operations are the unlinking of directories while
running as superuser, and running setuid programs with
fatal taint checks turned into warnings. Note that the -w
switch (or the `$^W' variable) must be used along with this
option to actually generate the taint-check warnings.
-v prints the version and patchlevel of your Perl executable.
-V prints summary of the major perl configuration values and the
current value of @INC.
-V:*name*
Prints to STDOUT the value of the named configuration
variable.
-w prints warnings about variable names 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 read-only that
you are attempting to write on. Also warns you if you use
values as a number that doesn't look like numbers, using an
array as though it were a scalar, if your subroutines
recurse more than 100 deep, and innumerable other things.
You can disable specific warnings using `__WARN__' hooks,
as described in the perlvar manpage and the "warn" entry in
the perlfunc manpage. See also the perldiag manpage and the
perltrap manpage.
-x *directory*
tells Perl 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. If a
directory name is specified, Perl will switch to that
directory before running the script. The -x switch controls
only 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).
ENVIRONMENT
HOME Used if chdir has no argument.
LOGDIR Used if chdir has no argument and HOME is not set.
PATH Used in executing subprocesses, and in finding the
script if -S is used.
PERL5LIB A colon-separated list of directories in which to look
for Perl library files before looking in the
standard library and the current directory. If
PERL5LIB is not defined, PERLLIB is used. When
running taint checks (because the script was running
setuid or setgid, or the -T switch was used),
neither variable is used. The script should instead
say
use lib "/my/directory";
PERL5OPT Command-line options (switches). Switches in this
variable are taken as if they were on every Perl
command line. Only the -[DIMUdmw] switches are
allowed. When running taint checks (because the
script was running setuid or setgid, or the -T
switch was used), this variable is ignored. If
PERL5OPT begins with -T, tainting will be enabled,
and any subsequent options ignored.
PERLLIB A colon-separated list of directories in which to look
for Perl library files before looking in the
standard library and the current directory. If
PERL5LIB is defined, PERLLIB is not used.
PERL5DB The command used to load the debugger code. The default
is:
BEGIN { require 'perl5db.pl' }
PERL5SHELL (specific to WIN32 port)
May be set to an alternative shell that perl must
use internally for executing "backtick" commands or
system(). Default is `cmd.exe /x/c' on WindowsNT and
`command.com /c' on Windows95. The value is
considered to be space delimited. Precede any
character that needs to be protected (like a space
or backslash) with a backslash.
Note that Perl doesn't use COMSPEC for this purpose
because COMSPEC has a high degree of variability
among users, leading to portability concerns.
Besides, perl can use a shell that may not be fit
for interactive use, and setting COMSPEC to such a
shell may interfere with the proper functioning of
other programs (which usually look in COMSPEC to
find a shell fit for interactive use).
PERL_DEBUG_MSTATS
Relevant only if perl is compiled with the malloc
included with the perl distribution (that is, if
`perl -V:d_mymalloc' is 'define'). If set, this
causes memory statistics to be dumped after
execution. If set to an integer greater than one,
also causes memory statistics to be dumped after
compilation.
PERL_DESTRUCT_LEVEL
Relevant only if your perl executable was built with
-DDEBUGGING, this controls the behavior of global
destruction of objects and other references.
Perl also has environment variables that control how Perl
handles data specific to particular natural languages. See the
perllocale manpage.
Apart from these, Perl uses no other environment variables,
except to make them available to the script being executed, and
to child 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 exists $ENV{SHELL};
delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};