| perlrun - how to execute the Perl interpreter |
perlrun - how to execute the Perl interpreter
perl [switches] filename args
Upon startup, Perl looks for your script in one of the following places:
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.
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:
$/) 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.
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 and use blocks,
since these are considered part of the compilation.
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
// around it, the slashes will be ignored.
<> 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 in the perlfunc manpage).
$\'' to have the value of octnum so that
any print statements will have that line terminator added back on. If
octnum is omitted, sets ``$\'' 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 $/ to the null character.
while (<>) {
... # your script goes here
}
Note that the lines are not printed by default. See -p to have lines printed. Here is an efficient way to delete all files older than a week:
find . -mtime +7 -print | perl -nle 'unlink;'
This is faster than using the -exec switch of find because you don't
have to start a process on every filename found.
BEGIN and END blocks may be used to capture control before or after
the implicit loop, just as in awk.
while (<>) {
... # your script goes here
} continue {
print;
}
Note that the lines are printed automatically. To suppress printing use the -n switch. A -p overrides a -n switch.
BEGIN and END blocks may be used to capture control before or after
the implicit loop, just as in awk.
#!/usr/bin/perl -s
if ($xyz) { print "true\n"; }
#!/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;
dump() operator instead. Note: availability of undump is
platform specific and may not be available for a specific port of
Perl.
__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).
| perlrun - how to execute the Perl interpreter |