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.
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 $0 -S ${1+"$@"}'
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 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 is provided to indicate successful completion.
#!/usr/bin/perl -spi.bak # same as -s -p -i.bak
Switches include:
$/) 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.
@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.
BEGIN, END, and use blocks, because these are considered as occurring outside the execution of
your program.
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
//, "", or '', otherwise it will be put in single quotes.
<> 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 eof).
@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.
$/'' (the input record separator) when used with -n or -p, and second, it assigns ``$\'' (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 $/ to the null character.
; before executing your script.
-Mmodule executes use module ; before executing your script. You can use quotes to add extra code after
the module name, e.g., -M'module qw'.
If the first character after the -M or -m is a dash (-) then the 'use' is replaced with 'no'.
A little built-in syntactic sugar means you can also
say
-mmodule=foo,bar or -Mmodule=foo,bar as a shortcut for
-M'module qw'. This avoids the need to use quotes when importing symbols. The actual
code generated by -Mmodule=foo,bar is
use module split. Note that the = form removes the distinction between -m and -M.
while (<>) {
... # your script goes here
}
Note that the lines are not printed by default. See -p to have lines printed. Here is an efficient way to delete all files older than a week:
find . -mtime +7 -print | perl -nle 'unlink;'
This is faster than using the -exec switch of find because you don't have to start a process on every filename found.
BEGIN and END blocks may be used to capture control before or after the implicit loop,
just as in awk.
while (<>) {
... # your script goes here
} continue {
print;
}
Note that the lines are printed automatically. To suppress printing use the -n switch. A -p overrides a -n switch.
BEGIN and END blocks may be used to capture control before or after the implicit loop,
just as in awk.
@ARGV and sets the
corresponding variable in the Perl script. The following script prints
``true'' if and only if the script is invoked with a -xyz switch.
#!/usr/bin/perl -s
if ($xyz) { print "true\n"; }
#!/usr/bin/perl
eval 'exec /usr/bin/perl -S $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.
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. 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 $running_under_some_shell;
dump operator instead. Note: availability of
undump is platform specific and may not be available for a specific port of Perl.
You can disable specific warnings using __WARN__ hooks, as described in the perlvar manpage and warn. See also the perldiag manpage and the perltrap manpage.
__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).
use lib "/my/directory";
BEGIN { require 'perl5db.pl' }
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 defined $ENV{'SHELL'};
$ENV{'IFS'} = '' if defined $ENV{'IFS'};