PERLRUN(1)PERLRUN(1)NAMEperlrun - how to execute the Perl interpreter
SYNOPSIS
perl [switches] filename args
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.)
3. Passed in implicitly via standard input. This only works 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.)
As of Perl 5, the #! line is always examined for switches as the line
is being parsed. Thus, if you're on a machine that only allows 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.
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 perl $0 -S ${1+"$@"}'
if 0;
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 partway 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.
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:
-0digits
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 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 since
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 and use
blocks, since these are considered part of the compilation.
-d runs the script under the Perl debugger. See the perldebug
manpage.
-Dnumber
-Dlist
sets debugging flags. To watch how it executes your script, use
-D14. (This only works if debugging is compiled into your Perl.)
Another nice value is -D1024, which lists your compiled syntax
tree. And -D512 displays compiled regular expressions. As an
alternative specify a list of letters instead of numbers (e.g.
-D14 is equivalent to -Dtls):
1 p Tokenizing and Parsing
2 s Stack Snapshots
4 l Label Stack Processing
8 t Trace Execution
16 o Operator Node Construction
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
1024 x Syntax Tree Dump
2048 u Tainting Checks
4096 L Memory Leaks (not supported anymore)
8192 H Hash Dump -- usurps values()
16384 X Scratchpad Allocation
32768 D Cleaning Up
-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.
-Fregexp
specifies a regular expression to split on if -a is also in
effect. If regexp has // around it, the slashes will be ignored.
-iextension
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 added to the name of the old file to make a backup
copy. If no extension is supplied, no backup is made. 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
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 $oldargv
to know when the filename has changed. It does, however, use
ARGVOUT for the selected filehandle. Note that STDOUT is restored
as the default output filehandle after the loop.
You can use eof without parenthesis to locate the end of each
input file, in case you want to append to each file, or reset line
numbering (see example in the eof entry in the perlfunc manpage).
-Idirectory
may be used in conjunction with -P to tell the C preprocessor
where to look for include files. By default /usr/include and
/usr/lib/perl are searched.
-loctnum
enables automatic line-ending processing. It has two effects:
first, it automatically chomps the line terminator when used with
-n or -p, and second, it assigns "$\" to have the value of octnum
so that any print statements will have that line terminator 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 $/ to the null character.
-n causes Perl to assume the following loop around your script, which
makes it iterate over filename arguments somewhat like sed -n or
awk:
while (<>) {
... # your script goes here
}
Note that the lines are not printed by default. See -p to have
lines printed. Here is an efficient way to delete all files older
than a week:
find . -mtime +7 -print ⎪ perl -nle 'unlink;'
This is faster than using the -exec switch of find because you
don't have to start a process on every filename found.
BEGIN and END blocks may be used to capture control before or
after the implicit loop, just as in awk.
-p causes Perl to assume the following loop around your script, which
makes it iterate over filename arguments somewhat like sed:
while (<>) {
... # your script goes here
} continue {
print;
}
Note that the lines are printed automatically. To suppress
printing use the -n switch. A -p overrides a -n switch.
BEGIN and END blocks may be used to capture control before or
after the implicit loop, just as in awk.
-P causes your script to be run through the C preprocessor before
compilation by Perl. (Since both comments and cpp directives
begin with the # character, you should avoid starting comments
with any words recognized by the C preprocessor such as "if",
"else" or "define".)
-s enables some rudimentary switch parsing for switches on the
command line after the script name but before any filename
arguments (or before a --). Any switch found there is removed
from @ARGV and sets the corresponding variable in the Perl script.
The following script prints "true" if and only if the script is
invoked with a -xyz switch.
#!/usr/bin/perl -s
if ($xyz) { print "true\n"; }
-S makes Perl use the PATH environment variable to search for the
script (unless the name of the script starts with a slash).
Typically this is used to emulate #! startup on machines that
don't support #!, in the following 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 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. 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 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 -S $0 ${1+"$@"}'
& eval 'exec /usr/bin/perl -S $0 $argv:q'
if 0;
-T forces "taint" checks to be turned on. Ordinarily these checks
are done only when running setuid or setgid. See the perlsec
manpage.
-u causes Perl to dump core after compiling your script. You can
then 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.
-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.
-v prints the version and patchlevel of your Perl executable.
-w 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 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. See 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 (but only one group of switches, as with
normal #! processing). If a directory name is specified, Perl
will switch to that directory before running the script. The -x
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).
3rd Berkeley DistributionPERLRUN(1)