Perl flags -pe, -pi, -p, -w, -d, -i, -t?

Question

I have seen lots of ways of running Perl code or scripts, with different flags. However, when I try to google for what each flag means, I mainly get results to generic Perl sites and no specific information regarding the flags or their use is found there.

Below are the flags that I encounter most often, and I don't have a clue what they mean:

• perl -pe
• perl -pi
• perl -p
• perl -w
• perl -d
• perl -i
• perl -t

I will be very grateful if you tell me what each of those mean and some use cases for them, or at least tell me a way of finding out their meaning.

Answer 1

Yes, Google is notoriously difficult for looking up punctuation and, unfortunately, Perl does seem to be mostly made up of punctuation :-)

The command line switches are all detailed in perlrun (available from the command line by calling perldoc perlrun). Going into the options briefly, one-by-one:

• -p: Places a printing loop around your command so that it acts on each line of standard input. Used mostly so Perl can beat the pants off Awk in terms of power AND simplicity :-)
• -n: Places a non-printing loop around your command.
• -e: Allows you to provide the program as an argument rather than in a file. You don't want to have to create a script file for every little Perl one-liner.
• -i: Modifies your input file in-place (making a backup of the original). Handy to modify files without the {copy, delete-original, rename} process.
• -w: Activates some warnings. Any good Perl coder will use this.
• -d: Runs under the Perl debugger. For debugging your Perl code, obviously.
• -t: Treats certain "tainted" (dubious) code as warnings (proper taint mode will error on this dubious code). Used to beef up Perl security, especially when running code for other users, such as setuid scripts or web stuff.

Answer 2

The -p flag basically runs the script with

while (<>) {
# exec here
}
continue {
    print or die "-p destination: $!\n";
}

-e allows you to pass a script to as a parameter rather than as a file:

perl -e '$x = "Hello world!\n"; print $x;'

-i directs the interpreter that all data passed to STDIN by the executing script is to be done inplace.

-w is the same as use warnings;, but in a global rather than local scope

-d runs the Perl debugger

Answer 3

Other have mentioned perlrun. If you use B::Deparse, you can see what it means (for most things):

$ perl -MO=Deparse   -p  -e 1
LINE: while (defined($_ = <ARGV>)) {
    '???';
}
continue {
    die "-p destination: $!\n" unless print $_;
}
-e syntax OK

1 is represented by '???', because it is optimized away.

$ perl -MO=Deparse   -p -i  -e 1
BEGIN { $^I = ""; }
LINE: while (defined($_ = <ARGV>)) {
    '???';
}
continue {
    die "-p destination: $!\n" unless print $_;
}
-e syntax OK

-i sets $^I, like

$ perl -MO=Deparse   -p -i.bak  -e 1
BEGIN { $^I = ".bak"; }
LINE: while (defined($_ = <ARGV>)) {
    '???';
}
continue {
    die "-p destination: $!\n" unless print $_;
}
-e syntax OK

But remember, <ARGV> uses 2-argument open, so don't have filenames that start with > < or start/end with |.

Answer 4

There is also one important flag -n which is not mentioned in the list.

-n works the same as -p, only it does not print $_ by default. This can be very useful in filtering text files.

In this way Perl can replace grep | sed in a single one-liner.

For example:

perl -ne 'print "$1\n" if /Messages read: (\d+)/' <my_input.txt

Will print out every integer value found after "Messages read: ", and nothing more.