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Man Page for PERLRUN
NAME
perlrun - 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.
-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 "$
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 a an array as though
it were a scalar, if your subroutines recurse more than
100 deep, and innumeriable 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).
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