How can I process options using Perl in -n or -p mode?

Question

When running perl -n or perl -p, each command line argument is taken as a file to be opened and processed line by line. If you want to pass command

Answer 1

There are three primary ways of passing information to Perl without using STDIN or external storage.

• Arguments

  When using -n or -p, extract the arguments in the BEGIN block.

    perl -ne'BEGIN { ($x,$y)=splice(@ARGV,0,2) } f($x,$y)' -- "$x" "$y" ...
  

• Command-line options

  In a full program, you'd use Getopt::Long, but perl -s will do fine here.

    perl -sne'f($x,$y)' -- -x="$x" -y="$y" -- ...
  

• Environment variables

    X="$x" Y="$y" perl -ne'f($ENV{X},$ENV{Y})' -- ...
  

Answer 2

Here is a short example program (name it t.pl), how you can do it:

#!/bin/perl
use Getopt::Std;

BEGIN {
  my %opts;
  getopts('p', \%opts);
  $prefix = defined($opts{'p'}) ? 'prefix -> ' : '';
}

print $prefix, $_;

Call it like that:

perl -n t.pl file1 file2 file3

or (will add a prefix to every line):

perl -n t.pl -p file1 file2 file3