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perlrun - how to execute the Perl interpreter
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.
extproc perl -S -your_switches
as the first line in "*.cmd" file (-S due to a bug in cmd.exe's `extproc' handling).
$ 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 Plan 9 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.
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;
#!/usr/bin/perl -spi.orig # same as -s -p -i.orig
Switches include:
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 byte with that value.
If you want to specify any Unicode character, use the hexadecimal format: "-0xHHH...", where the "H" are valid hexadecimal digits. (This means that you cannot use the "-x" with a directory name that consists of hexadecimal digits.)
perl -ane 'print pop(@F), "\n";'
is equivalent to
while (<>) {
@F = split(' ');
print pop(@F), "\n";
}
An alternate delimiter may be specified using -F.
As of 5.8.1, the "-C" can be followed either by a number or a list of option letters. The letters, their numeric values, and effects are as follows; listing the letters is equal to summing the numbers.
I 1 STDIN is assumed to be in UTF-8
O 2 STDOUT will be in UTF-8
E 4 STDERR will be in UTF-8
S 7 I + O + E
i 8 UTF-8 is the default PerlIO layer for input streams
o 16 UTF-8 is the default PerlIO layer for output streams
D 24 i + o
A 32 the @ARGV elements are expected to be strings encoded in UTF-8
L 64 normally the "IOEioA" are unconditional,
the L makes them conditional on the locale environment
variables (the LC_ALL, LC_TYPE, and LANG, in the order
of decreasing precedence) -- if the variables indicate
UTF-8, then the selected "IOEioA" are in effect
For example, "-COE" and "-C6" will both turn on UTF-8-ness on both STDOUT and STDERR. Repeating letters is just redundant, not cumulative nor toggling.
The "io" options mean that any subsequent open() (or similar I/O operations) will have the ":utf8" PerlIO layer implicitly applied to them, in other words, UTF-8 is expected from any input stream, and UTF-8 is produced to any output stream. This is just the default, with explicit layers in open() and with binmode() one can manipulate streams as usual.
"-C" on its own (not followed by any number or option list), or the empty string "" for the "PERL_UNICODE" environment variable, has the same effect as "-CSDL". In other words, the standard I/O handles and the default "open()" layer are UTF-8-fied but only if the locale environment variables indicate a UTF-8 locale. This behaviour follows the implicit (and problematic) UTF-8 behaviour of Perl 5.8.0.
You can use "-C0" (or "0" for "PERL_UNICODE") to explicitly disable all the above Unicode features.
The read-only magic variable "${^UNICODE}" reflects the numeric value of this setting. This is variable is set during Perl startup and is thereafter read-only. If you want runtime effects, use the three-arg open() (see ``open'' in perlfunc), the two-arg binmode() (see ``binmode'' in perlfunc), and the "open" pragma (see open).
(In Perls earlier than 5.8.1 the "-C" switch was a Win32-only switch that enabled the use of Unicode-aware ``wide system call'' Win32 APIs. This feature was practically unused, however, and the command line switch was therefore ``recycled''.)
As an alternative, specify a number instead of list of letters (e.g., -D14 is equivalent to -Dtls):
1 p Tokenizing and parsing
2 s Stack snapshots (with v, displays all stacks)
4 l Context (loop) stack processing
8 t Trace execution
16 o Method and overloading resolution
32 c String/numeric conversions
64 P Print profiling info, 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 (Obsolete, previously used for LEAKTEST)
8192 H Hash dump -- usurps values()
16384 X Scratchpad allocation
32768 D Cleaning up
65536 S Thread synchronization
131072 T Tokenising
262144 R Include reference counts of dumped variables (eg when using -Ds)
524288 J Do not s,t,P-debug (Jump over) opcodes within package DB
1048576 v Verbose: use in conjunction with other flags
8388608 q quiet - currently only suppresses the "EXECUTING" message
All these flags require -DDEBUGGING when you compile the Perl executable (but see Devel::Peek, re which may change this). 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
# If you have "env" utility env PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
# 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 perldebug for details and variations.
Perl can be built so that it by default will try to execute $Config{sitelib}/sitecustomize.pl at startup. This is a hook that allows the sysadmin to customize how perl behaves. It can for instance be used to add entries to the @INC array to make perl find modules in non-standard locations.
If no extension is supplied, no backup is made and the current file is overwritten.
If the extension doesn't contain a "*", then it is appended to the end of the current filename as a suffix. If the extension does contain one or more "*" characters, then each "*" is replaced with the current filename. In Perl terms, you could think of this as:
($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 perlfunc).
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 perlfaq5.
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...
Note that because -i renames or deletes the original file before creating a new file of the same name, UNIX-style soft and hard links will not be preserved.
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.
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.
-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.
A consequence of this is that -MFoo=number never does a version check (unless "Foo::import()" itself is set up to do a version check, which could happen for example if Foo inherits from Exporter.)
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 that haven't been modified for at least 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 follow the example under -0.
"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: $!\n";
}
If a file named by an argument cannot be opened for some reason, Perl warns you about it, and moves on to the next file. Note that the lines are printed automatically. An error occurring during printing is treated as fatal. To suppress printing use the -n switch. A -p overrides a -n switch.
"BEGIN" and "END" blocks may be used to capture control before or after the implicit loop, just as in awk.
This option causes your program to be run through the C preprocessor before compilation by Perl. Because both comments and cpp directives begin with the # character, you should avoid starting comments with any words recognized by the C preprocessor such as "if", "else", or "define".
If you're considering using "-P", you might also want to look at the Filter::cpp module from CPAN.
The problems of -P include, but are not limited to:
s/foo//;
because after -P this will became illegal code
s/foo
The workaround is to use some other quoting separator than "/", like for example "!":
s!foo!!;
#!/usr/bin/perl -s
if ($xyz) { print "$xyz\n" }
Do note that a switch like --help creates the variable ${-help}, which is not compliant with "strict refs". Also, when using this option on a script with warnings enabled you may get a lot of spurious ``used only once'' warnings.
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 #!. Its also convenient when debugging a script that uses #!, and is thus normally found by the shell's $PATH search mechanism.
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.
By default, the Red Hat perl distribution will prepend to the default
search path (@INC) the -V:archname subdirectory of each member of
the -V:inc_version_list under the perl vendor and site installation
directories.
i.e. in shell notation:
{-V:vendorlib_stem,-V:sitelib_stem}/{-V:inc_version_list}/-V:archname
where inc_version_list includes every previous perl version shipped
by Red Hat, to provide compatibility for binary modules installed under
previous perl versions. This can be quite a long list of directories
to search, which can slow down module loading. You can disable searching
these previous perl version architecture specific directories by specifying
the -R switch - then the default search path will be as for the default
upstream perl release.
NOTE: this is not a substitute for -T. This is meant only to be used as a temporary development aid while securing legacy code: for real production code and for new secure code written from scratch always use the real -T.
This switch has been superseded in favor of the new Perl code generator backends to the compiler. See B and B::Bytecode for details.
$ perl -V:libc
libc='/lib/libc-2.2.4.so';
$ perl -V:lib.
libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
libc='/lib/libc-2.2.4.so';
$ perl -V:lib.*
libpth='/usr/local/lib /lib /usr/lib';
libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
lib_ext='.a';
libc='/lib/libc-2.2.4.so';
libperl='libperl.a';
....
Additionally, extra colons can be used to control formatting. A trailing colon suppresses the linefeed and terminator ';', allowing you to embed queries into shell commands. (mnemonic: PATH separator ':'.)
$ echo "compression-vars: " `perl -V:z.*: ` " are here !"
compression-vars: zcat='' zip='zip' are here !
A leading colon removes the 'name=' part of the response, this allows you to map to the name you need. (mnemonic: empty label)
$ echo "goodvfork="`./perl -Ilib -V::usevfork`
goodvfork=false;
Leading and trailing colons can be used together if you need positional parameter values without the names. Note that in the case below, the PERL_API params are returned in alphabetical order.
$ echo building_on `perl -V::osname: -V::PERL_API_.*:` now
building_on 'linux' '5' '1' '9' now
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 perlvar and ``warn'' in perlfunc. See also perldiag and perltrap. A new, fine-grained warning facility is also available if you want to manipulate entire classes of warnings; see warnings or perllexwarn.
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";
It is conventional to start layer names with a colon e.g. ":perlio" to emphasise their similarity to variable ``attributes''. But the code that parses layer specification strings (which is also used to decode the PERLIO environment variable) treats the colon as a separator.
An unset or empty PERLIO is equivalent to ":stdio".
The list becomes the default for all perl's IO. Consequently only built-in layers can appear in this list, as external layers (such as :encoding()) need IO in order to load them!. See ``open pragma'' for how to add external encodings as defaults.
The layers that it makes sense to include in the PERLIO environment variable are briefly summarised below. For more details see PerlIO.
Unlike in the earlier versions of Perl ":raw" is not just the inverse of ":crlf" - other layers which would affect the binary nature of the stream are also removed or disabled.
On all platforms the default set of layers should give acceptable results.
For UNIX platforms that will equivalent of ``unix perlio'' or ``stdio''. Configure is setup to prefer ``stdio'' implementation if system's library provides for fast access to the buffer, otherwise it uses the ``unix perlio'' implementation.
On Win32 the default in this release is ``unix crlf''. Win32's ``stdio'' has a number of bugs/mis-features for perl IO which are somewhat C compiler vendor/version dependent. Using our own "crlf" layer as the buffer avoids those issues and makes things more uniform. The "crlf" layer provides CRLF to/from ``\n'' conversion as well as buffering.
This release uses "unix" as the bottom layer on Win32 and so still uses C compiler's numeric file descriptor routines. There is an experimental native "win32" layer which is expected to be enhanced and should eventually be the default under Win32.
PERLIO_DEBUG=/dev/tty perl script ...
and Win32 approximate equivalent:
set PERLIO_DEBUG=CON perl script ...
This functionality is disabled for setuid scripts and for scripts run with -T.
BEGIN { require 'perl5db.pl' }
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).
The default behaviour is to randomise unless the PERL_HASH_SEED is set. If Perl has been compiled with "-DUSE_HASH_SEED_EXPLICIT", the default behaviour is not to randomise unless the PERL_HASH_SEED is set.
If PERL_HASH_SEED is unset or set to a non-numeric string, Perl uses the pseudorandom seed supplied by the operating system and libraries. This means that each different run of Perl will have a different ordering of the results of keys(), values(), and each().
Please note that the hash seed is sensitive information. Hashes are randomized to protect against local and remote attacks against Perl code. By manually setting a seed this protection may be partially or completely lost.
See ``Algorithmic Complexity Attacks'' in perlsec and ``PERL_HASH_SEED_DEBUG'' for more information.
Note that the hash seed is sensitive information: by knowing it one can craft a denial-of-service attack against Perl code, even remotely, see ``Algorithmic Complexity Attacks'' in perlsec for more information. Do not disclose the hash seed to people who don't need to know it. See also hash_seed() of Hash::Util.
Perl also has environment variables that control how Perl handles data specific to particular natural languages. See perllocale.
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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