Go to the first, previous, next, last section, table of contents.

Perl Startup

Upon startup, perl looks for your script in one of the following places:

  1. Specified line by line via `-e' switches on the command line.
  2. Contained in the file specified by the first filename on the command line. (Note that systems supporting the `#!' notation invoke interpreters this way.)
  3. Passed in implicitly via standard input. This only works if there are no filename arguments--to pass arguments to a stdin script you must explicitly specify a `-' for the script name.

After locating your script, perl compiles it to an internal form. If the script is syntactically correct, it is executed.


Note: on first reading this section may not make much sense to you. It's here at the front for easy reference.

A single-character option may be combined with the following option, if any. This is particularly useful when invoking a script using the `#!' construct which only allows one argument. Example:

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

Options include:

`-0 digits'
specifies the record separator ($/) as an octal number. If there are no digits, the null character is the separator. Other switches may precede or follow the digits. For example, if you have a version of find which can print filenames terminated by the null character, you can say this:
find . -name '*.bak' -print0 | perl -n0e unlink
The special value `00' will cause Perl to slurp files in paragraph mode. The value `0777' will cause Perl to slurp files whole since there is no legal character with that value.
turns on autosplit mode when used with a `-n' or `-p'. An implicit split command to the `@F' array is done as the first thing inside the implicit while loop produced by the `-n' or `-p'.
perl -ane 'print pop(@F), "\n";'
is equivalent to
while (<>) {
        @F = split(' ');
        print pop(@F), "\n";
causes perl to check the syntax of the script and then exit without executing it.
runs the script under the perl debugger. See section Debugging, for more info.
`-D number'
sets debugging flags. To watch how it executes your script, use `-D14'. (This only works if debugging is compiled into your perl.) Another nice value is `-D1024', which lists your compiled syntax tree. And `-D512' displays compiled regular expressions.
`-e commandline'
may be used to enter one line of script. Multiple `-e' commands may be given to build up a multi-line script. If `-e' is given, perl will not look for a script filename in the argument list.
`-i extension'
specifies that files processed by the `<>' construct are to be edited in-place. It does this by renaming the input file, opening the output file by the same name, and selecting that output file as the default for print statements. The extension, if supplied, is added to the name of the old file to make a backup copy. If no extension is supplied, no backup is made. Saying
perl -p -i.bak -e "s/foo/bar/;" ...
is the same as using the script:
#!/usr/bin/perl -pi.bak
which is equivalent to
while (<>) {
        if ($ARGV ne $oldargv) {
                rename($ARGV, $ARGV . '.bak');
                open(ARGVOUT, ">$ARGV");
                $oldargv = $ARGV;
continue {
    print;      # this prints to original filename
except that the `-i' form doesn't need to compare `$ARGV' to `$oldargv' to know when the filename has changed. It does, however, use `ARGVOUT' for the selected filehandle. Note that `STDOUT' is restored as the default output filehandle after the loop. You can use eof to locate the end of each input file, in case you want to append to each file, or reset line numbering (see section Input/Output, for an example).
`-I directory'
may be used in conjunction with `-P' to tell the C preprocessor where to look for include files. By default `/usr/include' and `/usr/lib/perl' are searched.
`-l octnum'
enables automatic line-ending processing. It has two effects: first, it automatically chops the line terminator when used with `-n' or `-p', and second, it assigns `$\' to have the value of octnum so that any print statements will have that line terminator added back on. If octnum is omitted, sets `$\' to the current value of `$/'. 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 `$/' to the null character.
causes perl to assume the following loop around your script, which makes it iterate over filename arguments somewhat like sed -n or awk:
while (<>) {
        ...         # your script goes here
Note that the lines are not printed by default. See `-p' option to have lines printed. Here is an efficient way to delete all files older than a week: [ before version 4.003 ]
find . -mtime +7 -print | perl -ne 'chop;unlink;'
[ version 4.003 and beyond ]
find . -mtime +7 -print | perl -nle 'unlink;'
This is faster than using the `-exec' switch of find because you don't have to start a process on every filename found.
causes perl to assume the following loop around your script, which makes it iterate over filename arguments somewhat like sed:
while (<>) {
        ...         # your script goes here
} continue {
Note that the lines are printed automatically. To suppress printing use the `-n' switch. A `-p' overrides a `-n' switch.
causes your script to be run through the C preprocessor before compilation by perl. (Since both comments and cpp directives begin with the `#' character, you should avoid starting comments with any words recognized by the C preprocessor such as if, else or define.)
enables some rudimentary switch parsing for switches on the command line after the script name but before any filename arguments (or before a `--'). Any switch found there is removed from `@ARGV' and sets the corresponding variable in the perl script. The following script prints `true' if and only if the script is invoked with a `-xyz' switch.
#!/usr/bin/perl -s
if ($xyz) { print "true\n"; }
makes perl use the `PATH' environment variable to search for the script (unless the name of the script starts with a slash). Typically this is used to emulate `#!' startup on machines that don't support `#!', in the following manner:
eval "exec /usr/bin/perl -S $0 $*"
        if $running_under_some_shell;
The system ignores the first line and feeds the script to `/bin/sh', which proceeds to try to execute the perl script as a shell script. The shell executes the second line as a normal shell command, and thus starts up the perl interpreter. On some systems `$0' doesn't always contain the full pathname, so the `-S' tells perl to search for the script if necessary. After perl locates the script, it parses the lines and ignores them because the variable `$running_under_some_shell' is never true. A better construct than `$*' would be `${1+"$@"}', which handles embedded spaces and such in the filenames, but doesn't work if the script is being interpreted by csh. In order to start up sh rather than csh, some systems may have to replace the `#!' line with a line containing just a colon, which will be politely ignored by perl. Other systems can't control that, and need a totally devious construct that will work under any of csh, sh or perl, such as the following:
eval '(exit $?0)' && eval 'exec /usr/bin/perl -S $0 ${1+"$@"}'
& eval 'exec /usr/bin/perl -S $0 $argv:q'
        if 0;
causes perl to dump core after compiling your script. You can then take this core dump and turn it into an executable file by using the undump program (not supplied). This speeds startup at the expense of some disk space (which you can minimize by stripping the executable). (Still, a "hello world" executable comes out to about 200K on my machine.) If you are going to run your executable as a set-id program then you should probably compile it using taintperl rather than normal perl. If you want to execute a portion of your script before dumping, use the dump operator instead. Note: availability of undump is platform specific and may not be available for a specific port of perl.
allows perl to do unsafe operations. Currently the only `unsafe' operations are the unlinking of directories while running as superuser, and running setuid programs with fatal taint checks turned into warnings.
prints the version and patchlevel of your perl executable.
prints warnings about identifiers that are mentioned only once, and scalar variables that are used before being set. Also warns about redefined subroutines, and references to undefined filehandles or filehandles opened readonly that you are attempting to write on. Also warns you if you use `==' on values that don't look like numbers, and if your subroutines recurse more than 100 deep.
`-x directory'
tells perl that the script is embedded in a message. Leading garbage will be discarded until the first line that starts with `#!' and contains the string "perl". Any meaningful switches on that line will be applied (but only one group of switches, as with normal `#!' processing). If a directory name is specified, perl will switch to that directory before running the script. The `-x' switch only controls the the disposal of leading garbage. The script must be terminated with `__END__' if there is trailing garbage to be ignored (the script can process any or all of the trailing garbage via the `DATA' filehandle if desired).

Go to the first, previous, next, last section, table of contents.