perlrun



PERLRUN(1)             Perl Programmers Reference Guide             PERLRUN(1)




NAME

       perlrun - how to execute the Perl interpreter


SYNOPSIS

       perl [ -sTtuUWX ]      [ -hv ] [ -V[:configvar] ]
            [ -cw ] [ -d[t][:debugger] ] [ -D[number/list] ]
            [ -pna ] [ -Fpattern ] [ -l[octal] ] [ -0[octal/hexadecimal] ]
            [ -Idir ] [ -m[-]module ] [ -M[-]module... ]      [ -P ]
            [ -S ]      [ -x[dir] ]      [ -i[extension] ]      [ -e com-
       mand ] [ -- ] [ programfile ] [ argument ]...       [ -C [nnuumm--
       bbeerr//lliisstt] ] ]>


DESCRIPTION

       The normal way to run a Perl program is by making it directly exe-
       cutable, or else by passing the name of the source file as an argument
       on the command line.  (An interactive Perl environment is also possi-
       ble--see perldebug for details on how to do that.)  Upon startup, Perl
       looks for your program 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 com-
           mand line.  (Note that systems supporting the #! notation invoke
           interpreters this way. See "Location of Perl".)

       3.  Passed in implicitly via standard input.  This works only if there
           are no filename arguments--to pass arguments to a STDIN-read pro-
           gram you must explicitly specify a "-" for the program 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 program embedded in
       a larger message.  (In this case you would indicate the end of the pro-
       gram 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 pro-
       gram.  And a partial -I switch could also cause odd results.

       Some switches do care if they are processed twice, for instance combi-
       nations of -l and -0.  Either put all the switches after the 32-charac-
       ter 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 pro-
       gram runs off the end without hitting an exit() or die() operator, an
       implicit exit(0) is provided to indicate successful completion.

       #! and quoting on non-Unix systems

       Unix’s #! technique can be simulated on other systems:

       OS/2
           Put

               extproc perl -S -your_switches

           as the first line in "*.cmd" file (-S due to a bug in cmd.exe’s
           ‘extproc’ handling).

       MS-DOS
           Create a batch file to run your program, and codify it in "ALTER-
           NATE_SHEBANG" (see the dosish.h file in the source distribution for
           more information).

       Win95/NT
           The Win95/NT installation, when using the ActiveState installer for
           Perl, will modify the Registry to associate the .pl extension with
           the perl interpreter.  If you install Perl by other means (includ-
           ing building from the sources), you may have to modify the Registry
           yourself.  Note that this means you can no longer tell the differ-
           ence between an executable Perl program and a Perl library file.

       Macintosh
           A Macintosh perl program will have the appropriate Creator and
           Type, so that double-clicking them will invoke the perl applica-
           tion.

       VMS Put

               $ 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 say-
           ing @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 sev-
       eral 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.

       Location of Perl

       It may seem obvious to say, but Perl is useful only when users can eas-
       ily 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 pro-
       gram 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;

       Command Switches

       As with all standard commands, a single-character switch may be clus-
       tered with the following switch, if any.

           #!/usr/bin/perl -spi.orig   # same as -s -p -i.orig

       Switches include:

       -0[octal/hexadecimal]
            specifies the input record separator ($/) as an octal or hexadeci-
            mal 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 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.)

       -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 [nnuummbbeerr//lliisstt]
            The "-C" flag controls some Unicode of the Perl Unicode features.

            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 num-
            bers.

                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 cumu-
            lative 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 dis-
            able 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".)

       -c   causes Perl to check the syntax of the program and then exit with-
            out executing it.  Actually, it will execute "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.

       -d
       -dt  runs the program under the Perl debugger.  See perldebug.  If t is
            specified, it indicates to the debugger that threads will be used
            in the code being debugged.

       -d:foo[=bar,baz]
       -dt:foo[=bar,baz]
            runs the program under the control of a debugging, profiling, or
            tracing module installed as Devel::foo. E.g., -d:DProf executes
            the program using the Devel::DProf profiler.  As with the -M flag,
            options may be passed to the Devel::foo package where they will be
            received and interpreted by the Devel::foo::import routine.  The
            comma-separated list of options must follow a "=" character.  If t
            is specified, it indicates to the debugger that threads will be
            used in the code being debugged.  See perldebug.

       -Dletters
       -Dnumber
            sets debugging flags.  To watch how it executes your program, use
            -Dtls.  (This works only if debugging is compiled into your Perl.)
            Another nice value is -Dx, which lists your compiled syntax tree.
            And -Dr displays compiled regular expressions; the format of the
            output is explained in perldebguts.

            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 exe-
            cutable (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 "Con-
            figure" 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.

       -e commandline
            may be used to enter one line of program.  If -e is given, Perl
            will not look for a 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.

       -Fpattern
            specifies the pattern to split on if -a is also in effect.  The
            pattern may be surrounded by "//", "", or ’’, otherwise it will be
            put in single quotes.

       -h   prints a summary of the options.

       -i[extension]
            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 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 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 con-
            tinue 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.

       -Idirectory
            Directories specified by -I are prepended to the search path for
            modules (@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.

       -l[octnum]
            enables automatic line-ending processing.  It has two separate
            effects.  First, it automatically chomps $/ (the input record sep-
            arator) when used with -n or -p.  Second, it assigns "$\" (the
            output record separator) to have the value of octnum so that any
            print statements will have that separator added back on.  If oct-
            num 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 pro-
            cessed, 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.

       -m[-]module
       -M[-]module
       -M[-]module ...
       -[mM][-]module=arg[,arg]...
            -mmodule executes "use" module "();" before executing your pro-
            gram.

            -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 -mmod-
            ule=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 dis-
            tinction between -m and -M.

       -n   causes Perl to assume the following loop around your program,
            which makes it iterate over filename arguments somewhat like sed
            -n or awk:

              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.

       -p   causes Perl to assume the following loop around your program,
            which makes it iterate over filename arguments somewhat like sed:

              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.

       -P   NOTE: Use of -P is strongly discouraged because of its inherent
            problems, including poor portability.

            This option causes your program to be run through the C preproces-
            sor 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:

            *         The "#!" line is stripped, so any switches there don’t
                      apply.

            *         A "-P" on a "#!" line doesn’t work.

            *         All lines that begin with (whitespace and) a "#" but do
                      not look like cpp commands, are stripped, including any-
                      thing inside Perl strings, regular expressions, and
                      here-docs .

            *         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 com-
                      mon 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!!;

            *         It requires not only a working C preprocessor but also a
                      working sed.  If not on UNIX, you are probably out of
                      luck on this.

            *         Script line numbers are not preserved.

            *         The "-x" does not work with "-P".

       -s   enables rudimentary switch parsing for switches on the command
            line after the program name but before any filename arguments (or
            before an argument of --).  This means you can have switches with
            two leading dashes (--help).  Any switch found there is removed
            from @ARGV and sets the corresponding variable in the Perl pro-
            gram.  The following program prints "1" if the program is invoked
            with a -xyz switch, and "abc" if it is invoked with -xyz=abc.

                #!/usr/bin/perl -s
                if ($xyz) { print "$xyz\n" }

            Do note that --help creates the variable ${-help}, which is not
            compliant with "strict refs".

       -S   makes Perl use the PATH environment variable to search for the
            program (unless the name of the program contains directory separa-
            tors).

            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 pro-
            gresses.

            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 $running_under_some_shell is never true.  If the pro-
            gram will be interpreted by csh, you will need to replace
            "${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 con-
            struct 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.

       -t   Like -T, but taint checks will issue warnings rather than fatal
            errors.  These warnings can be controlled normally with "no warn-
            ings qw(taint)".

            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.

       -T   forces "taint" checks to be turned on so you can test them.  Ordi-
            narily these checks are done only when running setuid or setgid.
            It’s a good idea to turn them on explicitly for programs that run
            on behalf of someone else whom you might not necessarily trust,
            such as CGI programs or any internet servers you might write in
            Perl.  See perlsec for details.  For security reasons, this option
            must be seen by Perl quite early; usually this means it must
            appear early on the command line or in the #! line for systems
            which support that construct.

       -u   This obsolete switch causes Perl to dump core after compiling your
            program.  You can then in theory take this core dump and turn it
            into an executable file by using the undump program (not sup-
            plied).  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 program 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.

            This switch has been superseded in favor of the new Perl code gen-
            erator backends to the compiler.  See B and B::Bytecode for
            details.

       -U   allows Perl to do unsafe operations.  Currently the only "unsafe"
            operations are the unlinking of directories while running as supe-
            ruser, and running setuid programs with fatal taint checks turned
            into warnings.  Note that the -w switch (or the $^W variable) must
            be used along with this option to actually generate the taint-
            check warnings.

       -v   prints the version and patchlevel of your perl executable.

       -V   prints summary of the major perl configuration values and the cur-
            rent values of @INC.

       -V:configvar
            Prints to STDOUT the value of the named configuration variable(s),
            with multiples when your configvar argument looks like a regex
            (has non-letters).  For example:

                $ 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 ’;’, allow-
            ing 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 posi-
            tional 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

       -w   prints warnings about dubious constructs, such as variable names
            that are mentioned only once and scalar variables that are used
            before being set, redefined subroutines, references to undefined
            filehandles or filehandles opened read-only that you are attempt-
            ing to write on, values used as a number that doesn’t look like
            numbers, using an array as though it were a scalar, if your sub-
            routines recurse more than 100 deep, and innumerable other things.

            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.

       -W   Enables all warnings regardless of "no warnings" or $^W.  See per-
            llexwarn.

       -X   Disables all warnings regardless of "use warnings" or $^W.  See
            perllexwarn.

       -x
       -x directory
            tells Perl that the program is embedded in a larger chunk of unre-
            lated ASCII text, such as in a mail 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.  If a directory name is specified, Perl will switch to
            that directory before running the program.  The -x switch controls
            only the disposal of leading garbage.  The program must be termi-
            nated with "__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).


ENVIRONMENT

       HOME        Used if chdir has no argument.

       LOGDIR      Used if chdir has no argument and HOME is not set.

       PATH        Used in executing subprocesses, and in finding the program
                   if -S is used.

       PERL5LIB    A list of directories in which to look for Perl library
                   files before looking in the standard library and the cur-
                   rent directory.  Any architecture-specific directories
                   under the specified locations are automatically included if
                   they exist.  If PERL5LIB is not defined, PERLLIB is used.
                   Directories are separated (like in PATH) by a colon on
                   unixish platforms and by a semicolon on Windows (the proper
                   path separator being given by the command "perl
                   -V:path_sep").

                   When running taint checks (either because the program was
                   running setuid or setgid, or the -T switch was used), nei-
                   ther variable is used.  The program should instead say:

                       use lib "/my/directory";

       PERL5OPT    Command-line options (switches).  Switches in this variable
                   are taken as if they were on every Perl command line.  Only
                   the -[DIMUdmtw] switches are allowed.  When running taint
                   checks (because the program was running setuid or setgid,
                   or the -T switch was used), this variable is ignored.  If
                   PERL5OPT begins with -T, tainting will be enabled, and any
                   subsequent options ignored.

       PERLIO      A space (or colon) separated list of PerlIO layers. If perl
                   is built to use PerlIO system for IO (the default) these
                   layers effect perl’s IO.

                   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 environ-
                   ment 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. Conse-
                   quently 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.

                   :bytes  A pseudolayer that turns off the ":utf8" flag for
                           the layer below.  Unlikely to be useful on its own
                           in the global PERLIO environment variable.  You
                           perhaps were thinking of ":crlf:bytes" or ":per-
                           lio:bytes".

                   :crlf   A layer which does CRLF to "\n" translation distin-
                           guishing "text" and "binary" files in the manner of
                           MS-DOS and similar operating systems.  (It cur-
                           rently does not mimic MS-DOS as far as treating of
                           Control-Z as being an end-of-file marker.)

                   :mmap   A layer which implements "reading" of files by
                           using "mmap()" to make (whole) file appear in the
                           process’s address space, and then using that as
                           PerlIO’s "buffer".

                   :perlio This is a re-implementation of "stdio-like" buffer-
                           ing written as a PerlIO "layer".  As such it will
                           call whatever layer is below it for its operations
                           (typically ":unix").

                   :pop    An experimental pseudolayer that removes the top-
                           most layer.  Use with the same care as is reserved
                           for nitroglycerin.

                   :raw    A pseudolayer that manipulates other layers.
                           Applying the ":raw" layer is equivalent to calling
                           "binmode($fh)".  It makes the stream pass each byte
                           as-is without any translation.  In particular CRLF
                           translation, and/or :utf8 intuited from locale are
                           disabled.

                           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.

                   :stdio  This layer provides PerlIO interface by wrapping
                           system’s ANSI C "stdio" library calls. The layer
                           provides both buffering and IO.  Note that ":stdio"
                           layer does not do CRLF translation even if that is
                           platforms normal behaviour. You will need a ":crlf"
                           layer above it to do that.

                   :unix   Low level layer which calls "read", "write" and
                           "lseek" etc.

                   :utf8   A pseudolayer that turns on a flag on the layer
                           below to tell perl that output should be in utf8
                           and that input should be regarded as already in
                           utf8 form.  May be useful in PERLIO environment
                           variable to make UTF-8 the default. (To turn off
                           that behaviour use ":bytes" layer.)

                   :win32  On Win32 platforms this experimental layer uses
                           native "handle" IO rather than unix-like numeric
                           file descriptor layer. Known to be buggy in this
                           release.

                   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" implementa-
                   tion 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 rou-
                   tines. There is an experimental native "win32" layer which
                   is expected to be enhanced and should eventually be the
                   default under Win32.

       PERLIO_DEBUG
                   If set to the name of a file or device then certain opera-
                   tions of PerlIO sub-system will be logged to that file
                   (opened as append). Typical uses are UNIX:

                      PERLIO_DEBUG=/dev/tty perl script ...

                   and Win32 approximate equivalent:

                      set PERLIO_DEBUG=CON
                      perl script ...

       PERLLIB     A list of directories in which to look for Perl library
                   files before looking in the standard library and the cur-
                   rent directory.  If PERL5LIB is defined, PERLLIB is not
                   used.

       PERL5DB     The command used to load the debugger code.  The default
                   is:

                           BEGIN { require ’perl5db.pl’ }

       PERL5DB_THREADED
                   If set to a true value, indicates to the debugger that the
                   code being debugged uses threads.

       PERL5SHELL (specific to the Win32 port)
                   May be set to an alternative shell that perl must use
                   internally for executing "backtick" commands or system().
                   Default is "cmd.exe /x/d/c" on WindowsNT and "command.com
                   /c" on Windows95.  The value is considered to be space-sep-
                   arated.  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, lead-
                   ing to portability concerns.  Besides, perl can use a shell
                   that may not be fit for interactive use, and setting COM-
                   SPEC to such a shell may interfere with the proper func-
                   tioning of other programs (which usually look in COMSPEC to
                   find a shell fit for interactive use).

       PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
                   Set to 1 to allow the use of non-IFS compatible LSP’s.
                   Perl normally searches for an IFS-compatible LSP because
                   this is required for its emulation of Windows sockets as
                   real filehandles.  However, this may cause problems if you
                   have a firewall such as McAfee Guardian which requires all
                   applications to use its LSP which is not IFS-compatible,
                   because clearly Perl will normally avoid using such an LSP.
                   Setting this environment variable to 1 means that Perl will
                   simply use the first suitable LSP enumerated in the cata-
                   log, which keeps McAfee Guardian happy (and in that partic-
                   ular case Perl still works too because McAfee Guardian’s
                   LSP actually plays some other games which allow applica-
                   tions requiring IFS compatibility to work).

       PERL_DEBUG_MSTATS
                   Relevant only if perl is compiled with the malloc included
                   with the perl distribution (that is, if "perl -V:d_mymal-
                   loc" 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_DESTRUCT_LEVEL
                   Relevant only if your perl executable was built with -DDE-
                   BUGGING, this controls the behavior of global destruction
                   of objects and other references.  See "PERL_DESTRUCT_LEVEL"
                   in perlhack for more information.

       PERL_DL_NONLAZY
                   Set to one to have perl resolve all undefined symbols when
                   it loads a dynamic library.  The default behaviour is to
                   resolve symbols when they are used.  Setting this variable
                   is useful during testing of extensions as it ensures that
                   you get an error on misspelled function names even if the
                   test suite doesn’t call it.

       PERL_ENCODING
                   If using the "encoding" pragma without an explicit encoding
                   name, the PERL_ENCODING environment variable is consulted
                   for an encoding name.

       PERL_HASH_SEED
                   (Since Perl 5.8.1.)  Used to randomise Perl’s internal hash
                   function.  To emulate the pre-5.8.1 behaviour, set to an
                   integer (zero means exactly the same order as 5.8.0).
                   "Pre-5.8.1" means, among other things, that hash keys will
                   be ordered the same between different runs of Perl.

                   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.

       PERL_HASH_SEED_DEBUG
                   (Since Perl 5.8.1.)  Set to one to display (to STDERR) the
                   value of the hash seed at the beginning of execution.
                   This, combined with "PERL_HASH_SEED" is intended to aid in
                   debugging nondeterministic behavior caused by hash random-
                   ization.

                   Note that the hash seed is sensitive information: by know-
                   ing 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_ROOT (specific to the VMS port)
                   A translation concealed rooted logical name that contains
                   perl and the logical device for the @INC path on VMS only.
                   Other logical names that affect perl on VMS include PERL-
                   SHR, PERL_ENV_TABLES, and SYS$TIMEZONE_DIFFERENTIAL but are
                   optional and discussed further in perlvms and in README.vms
                   in the Perl source distribution.

       PERL_SIGNALS
                   In Perls 5.8.1 and later.  If set to "unsafe" the
                   pre-Perl-5.8.0 signals behaviour (immediate but unsafe) is
                   restored.  If set to "safe" the safe (or deferred) signals
                   are used.  See "Deferred Signals (Safe Signals)" in per-
                   lipc.

       PERL_UNICODE
                   Equivalent to the -C command-line switch.  Note that this
                   is not a boolean variable-- setting this to "1" is not the
                   right way to "enable Unicode" (whatever that would mean).
                   You can use "0" to "disable Unicode", though (or alterna-
                   tively unset PERL_UNICODE in your shell before starting
                   Perl).  See the description of the "-C" switch for more
                   information.

       SYS$LOGIN (specific to the VMS port)
                   Used if chdir has no argument and HOME and LOGDIR are not
                   set.

       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 pro-
       cesses.  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)};



perl v5.8.6                       2004-11-05                        PERLRUN(1)

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