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perlrun - how to execute the Perl interpreter |
perlrun - how to execute the Perl interpreter
| perl | [ -CsTuUWX ] |
| [ -hv ] [ -V[:configvar] ] | |
| [ -cw ] [ -d[:debugger] ] [ -D[number/list] ] | |
| [ -pna ] [ -Fpattern ] [ -l[octal] ] [ -0[octal] ] | |
| [ -Idir ] [ -m[-]module ] [ -M[-]'module...' ] | |
| [ -P ] | |
| [ -S ] | |
| [ -x[dir] ] | |
| [ -i[extension] ] | |
| [ -e 'command' ] [ -- ] [ programfile ] [ argument ]... |
The normal way to run a Perl program is by making it directly executable, or else by passing the name of the source file as an argument on the command line. (An interactive Perl environment is also possible--see the perldebug manpage for details on how to do that.) Upon startup, Perl looks for your program 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 program
embedded in a larger message. (In this case you would indicate the end
of the program using the __END__ token.)
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 program.
Because historically some operating systems silently chopped 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 program. And a partial -I switch could also cause odd results.
Some switches do care if they are processed twice, for instance
combinations of -l and -0. Either put all the switches after
the 32-character boundary (if applicable), or replace the use of
-0digits by BEGIN{ $/ = "\0digits"; }.
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 -wS $0 ${1+"$@"}'
if $running_under_some_shell;
to let Perl see the -p switch.
A similar trick involves the env program, if you have it.
#!/usr/bin/env perl
The examples above use a relative path to the perl interpreter, getting whatever version is first in the user's path. If you want a specific version of Perl, say, perl5.005_57, you should place that directly in the #! line's path.
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 program, Perl compiles the entire program to an internal form. If there are any compilation errors, execution of the program is not attempted. (This is unlike the typical shell script, which might run part-way through before finding a syntax error.)
If the program is syntactically correct, it is executed. If the program
runs off the end without hitting an exit() or die() operator, an implicit
exit(0) is provided to indicate successful completion.
Unix's #! technique can be simulated on other systems:
extproc perl -S -your_switches
as the first line in *.cmd file (-S due to a bug in cmd.exe's
`extproc' handling).
ALTERNATIVE_SHEBANG (see the dosish.h file in the source
distribution for more information).
$ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
$ exit++ + ++$status != 0 and $exit = $status = undef;
at the top of your program, where -mysw are any command line switches you
want to pass to Perl. You can now invoke the program directly, by saying
perl program, or as a DCL procedure, by saying @program (or implicitly
via DCL$PATH by just using the name of the program).
This incantation is a bit much to remember, but Perl will display it for
you if you say perl "-V:startperl".
Command-interpreters on non-Unix systems have rather different ideas
on quoting than Unix shells. You'll need to learn the special
characters in your command-interpreter (*, \ and " are
common) and how to protect whitespace and these characters to run
one-liners (see -e below).
On some systems, you may have to change single-quotes to double ones, which you must not do on Unix or Plan9 systems. You might also have to change a single % to a %%.
For example:
# Unix
perl -e 'print "Hello world\n"'
# MS-DOS, etc.
perl -e "print \"Hello world\n\""
# Macintosh
print "Hello world\n"
(then Run "Myscript" or Shift-Command-R)
# VMS
perl -e "print ""Hello world\n"""
The problem is that none of this is reliable: it depends on the command and it is entirely possible neither works. If 4DOS were the command shell, this would probably work better:
perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""
CMD.EXE in Windows NT slipped a lot of standard Unix functionality in when nobody was looking, but just try to find documentation for its quoting rules.
Under the Macintosh, it depends which environment you are using. The MacPerl shell, or MPW, is much like Unix shells in its support for several quoting variants, except that it makes free use of the Macintosh's non-ASCII characters as control characters.
There is no general solution to all of this. It's just a mess.
It may seem obvious to say, but Perl is useful only when users can easily find it. When possible, it's good for both /usr/bin/perl and /usr/local/bin/perl to be symlinks to the actual binary. If that can't be done, system administrators are strongly encouraged to put (symlinks to) perl and its accompanying utilities into a directory typically found along a user's PATH, or in some other obvious and convenient place.
In this documentation, #!/usr/bin/perl on the first line of the program
will stand in for whatever method works on your system. You are
advised to use a specific path if you care about a specific version.
#!/usr/local/bin/perl5.00554
or if you just want to be running at least version, place a statement like this at the top of your program:
use 5.005_54;
As with all standard commands, a single-character switch may be clustered with the following switch, if any.
#!/usr/bin/perl -spi.orig # same as -s -p -i.orig
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 '*.orig' -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.
perl -ane 'print pop(@F), "\n";'
is equivalent to
while (<>) {
@F = split(' ');
print pop(@F), "\n";
}
An alternate delimiter may be specified using -F.
${^WIDE_SYSTEM_CALLS} reflects the state of
this switch. See ${^WIDE_SYSTEM_CALLS} in the perlvar manpage.
This feature is currently only implemented on the Win32 platform.
BEGIN, CHECK, and
use blocks, because these are considered as occurring outside the
execution of your program. INIT and END blocks, however, will
be skipped.
= character.
See the perldebug manpage.
1 p Tokenizing and parsing
2 s Stack snapshots
4 l Context (loop) stack processing
8 t Trace execution
16 o Method and overloading resolution
32 c String/numeric conversions
64 P Print preprocessor command for -P, source file input state
128 m Memory allocation
256 f Format processing
512 r Regular expression parsing and execution
1024 x Syntax tree dump
2048 u Tainting checks
4096 L Memory leaks (needs -DLEAKTEST when compiling Perl)
8192 H Hash dump -- usurps values()
16384 X Scratchpad allocation
32768 D Cleaning up
65536 S Thread synchronization
131072 T Tokenising
All these flags require -DDEBUGGING when you compile the Perl
executable. See the INSTALL file in the Perl source distribution
for how to do this. This flag is automatically set if you include -g
option when Configure asks you about optimizer/debugger flags.
If you're just trying to get a print out of each line of Perl code
as it executes, the way that sh -x provides for shell scripts,
you can't use Perl's -D switch. Instead do this
# Bourne shell syntax $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
# csh syntax % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)
See the perldebug manpage for details and variations.
//, "", 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 used to
modify the name of the old file to make a backup copy, following these
rules:
If no extension is supplied, no backup is made and the current file is overwritten.
If the extension doesn't contain a *, then it is appended to the
end of the current filename as a suffix. If the extension does
contain one or more * characters, then each * is replaced
with the current filename. In Perl terms, you could think of this
as:
($backup = $extension) =~ s/\*/$file_name/g;
This allows you to add a prefix to the backup file, instead of (or in addition to) a suffix:
$ perl -pi 'orig_*' -e 's/bar/baz/' fileA # backup to 'orig_fileA'
Or even to place backup copies of the original files into another directory (provided the directory already exists):
$ perl -pi 'old/*.orig' -e 's/bar/baz/' fileA # backup to 'old/fileA.orig'
These sets of one-liners are equivalent:
$ perl -pi -e 's/bar/baz/' fileA # overwrite current file
$ perl -pi '*' -e 's/bar/baz/' fileA # overwrite current file
$ perl -pi '.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
$ perl -pi '*.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
From the shell, saying
$ perl -p -i.orig -e "s/foo/bar/; ... "
is the same as using the program:
#!/usr/bin/perl -pi.orig
s/foo/bar/;
which is equivalent to
#!/usr/bin/perl
$extension = '.orig';
LINE: while (<>) {
if ($ARGV ne $oldargv) {
if ($extension !~ /\*/) {
$backup = $ARGV . $extension;
}
else {
($backup = $extension) =~ s/\*/$ARGV/g;
}
rename($ARGV, $backup);
open(ARGVOUT, ">$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.
As shown above, Perl creates the backup file whether or not any output is actually changed. So this is just a fancy way to copy files:
$ perl -p -i '/some/file/path/*' -e 1 file1 file2 file3...
or
$ perl -p -i '.orig' -e 1 file1 file2 file3...
You can use eof without parentheses 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).
If, for a given file, Perl is unable to create the backup file as specified in the extension then it will skip that file and continue on with the next one (if it exists).
For a discussion of issues surrounding file permissions and -i, see Why does Perl let me delete read-only files? Why does -i clobber protected files? Isn't this a bug in Perl? in the perlfaq5 manpage.
You cannot use -i to create directories or to strip extensions from files.
Perl does not expand ~ in filenames, which is good, since some
folks use it for their backup files:
$ perl -pi~ -e 's/foo/bar/' file1 file2 file3...
Finally, the -i switch does not impede execution when no files are given on the command line. In this case, no backup is made (the original file cannot, of course, be determined) and processing proceeds from STDIN to STDOUT as might be expected.
@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. 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.
use module (); before executing your
program.
-Mmodule executes use module ; before executing your
program. You can use quotes to add extra code after the module name,
e.g., '-Mmodule qw(foo bar)'.
If the first character after the -M or -m is a dash (-)
then the 'use' is replaced with 'no'.
A little builtin syntactic sugar means you can also say
-mmodule=foo,bar or -Mmodule=foo,bar as a shortcut for
'-Mmodule qw(foo bar)'. This avoids the need to use quotes when
importing symbols. The actual code generated by -Mmodule=foo,bar is
use module split(/,/,q{foo,bar}). Note that the = form
removes the distinction between -m and -M.
LINE:
while (<>) {
... # your program goes here
}
Note that the lines are not printed by default. See -p to have lines printed. If a file named by an argument cannot be opened for some reason, Perl warns you about it and moves on to the next file.
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. It does suffer from the bug of mishandling newlines in pathnames, which you can fix if you
BEGIN and END blocks may be used to capture control before or after
the implicit program loop, just as in awk.
LINE:
while (<>) {
... # your program goes here
} continue {
print or die "-p destination: $!\n";
}
If a file named by an argument cannot be opened for some reason, Perl warns you about it, and moves on to the next file. Note that the lines are printed automatically. An error occurring during printing is treated as fatal. To suppress printing use the -n switch. A -p overrides a -n switch.
BEGIN and END blocks may be used to capture control before or after
the implicit loop, just as in awk.
"if", "else", or "define".
Also, in some platforms the C preprocessor knows too much: it knows
about the C++ -style until-end-of-line comments starting with "//".
This will cause problems with common Perl constructs like
s/foo//;
because after -P this will became illegal code
s/foo
The workaround is to use some other quoting separator than "/",
like for example "!":
s!foo!!;
#!/usr/bin/perl -s
if ($xyz) { print "$xyz\n" }
Do note that --help creates the variable ${-help}, which is not compliant
with strict refs.
On some platforms, this also makes Perl append suffixes to the filename while searching for it. For example, on Win32 platforms, the ``.bat'' and ``.cmd'' suffixes are appended if a lookup for the original name fails, and if the name does not already end in one of those suffixes. If your Perl was compiled with DEBUGGING turned on, using the -Dp switch to Perl shows how the search progresses.
Typically this is used to emulate #! startup on platforms that don't support #!. This example works on many platforms that have a shell compatible with Bourne shell:
#!/usr/bin/perl
eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
if $running_under_some_shell;
The system ignores the first line and feeds the program to /bin/sh,
which proceeds to try to execute the Perl program 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
program if necessary. After Perl locates the program, it parses the
lines and ignores them because the variable $running_under_some_shell
is never true. If the program will be interpreted by csh, you will need
to replace ${1+"$@"} with $*, even though that doesn't understand
embedded spaces (and such) in the argument list. 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 perl -wS $0 ${1+"$@"}'
& eval 'exec /usr/bin/perl -wS $0 $argv:q'
if $running_under_some_shell;
If the filename supplied contains directory separators (i.e., is an absolute or relative pathname), and if that file is not found, platforms that append file extensions will do so and try to look for the file with those extensions added, one by one.
On DOS-like platforms, if the program does not contain directory separators, it will first be searched for in the current directory before being searched for on the PATH. On Unix platforms, the program will be searched for strictly on the PATH.
dump()
operator instead. Note: availability of undump is platform
specific and may not be available for a specific port of Perl.
This switch has been superseded in favor of the new Perl code generator backends to the compiler. See the B manpage and the B::Bytecode manpage for details.
$^W variable) must
be used along with this option to actually generate the
taint-check warnings.
$ perl -V:man.dir
will provide strong clues about what your MANPATH variable should be set to in order to access the Perl documentation.
This switch really just enables the internal ^$W variable. You
can disable or promote into fatal errors specific warnings using
__WARN__ hooks, as described in the perlvar manpage and warn in the perlfunc manpage.
See also the perldiag manpage and the perltrap manpage. A new, fine-grained warning
facility is also available if you want to manipulate entire classes
of warnings; see the warnings manpage or the perllexwarn manpage.
no warnings or $^W.
See the perllexwarn manpage.
use warnings or $^W.
See the perllexwarn manpage.
__END__ if there is trailing garbage to be ignored (the program
can process any or all of the trailing garbage via the DATA filehandle
if desired).
When running taint checks (either because the program was running setuid or setgid, or the -T switch was used), neither variable is used. The program should instead say:
use lib "/my/directory";
BEGIN { require 'perl5db.pl' }
cmd.exe /x/c
on WindowsNT and command.com /c on Windows95. The value is considered
to be space-separated. Precede any character that needs to be protected
(like a space or backslash) with a backslash.
Note that Perl doesn't use COMSPEC for this purpose because COMSPEC has a high degree of variability among users, leading to portability concerns. Besides, perl can use a shell that may not be fit for interactive use, and setting COMSPEC to such a shell may interfere with the proper functioning of other programs (which usually look in COMSPEC to find a shell fit for interactive use).
perl -V:d_mymalloc is 'define').
If set, this causes memory statistics to be dumped after execution. If set
to an integer greater than one, also causes memory statistics to be dumped
after compilation.
Perl also has environment variables that control how Perl handles data specific to particular natural languages. See the perllocale manpage.
Apart from these, Perl uses no other environment variables, except to make them available to the program being executed, and to child processes. However, programs 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 exists $ENV{SHELL};
delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};
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perlrun - how to execute the Perl interpreter |