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.
#!/usr/bin/perl -spi.bak # same as -s -p -i.bak
Switches include:
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
.
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
$ 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 $oldargv to
know when the filename has changed. It does, however, use ARGVOUT for
the selected filehandle. Note that STDOUT is restored as the
default output filehandle after the loop.
You can use eof without parenthesis to locate the end of each input file, in case you want to append to each file, or reset line numbering (see example in perlfunc/eof).
perl -lpe 'substr($_, 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;