field

#!/usr/bin/perl -wnl

# Reads colum-formatted files and reoutputs requested fields, 
# possibly reordered
# Usage: field.pl 3 1 5..7 file.txt 
# # outputs the 3rd and 1st then 5th 6th 7th cols from file.txt
# Field numbering begins at 1
# Use negative numbers to start counting from right (-1 downwards). E.g.:
# cat file.csv | field -s , -- -1 -2 1
# This uses comma as a separator, then prints the last, 2nd-to-last, first cols
# The -- is necessary so that the -1 won't be considered a command line option

use strict;
use Getopt::Long;

# Field indices to select and ouptu
our @a;
# Field separator, default (any amount of) whitespace
our $sep;

BEGIN {
    GetOptions("separator|sep|s=s" => \$sep);
    $sep ||= '\s+';
    # Read all integers from the command line
    while(@ARGV) {
        if ($ARGV[0] =~ /^[\d-]+$/) {
            # Adjusted for 1-based counting
            $ARGV[0] -= 1 if $ARGV[0] > 0;
            push @a, shift;
        } elsif ($ARGV[0] =~ /^[\d]+\.\.[\d]+$/) {
            # Allow ranges like 3..5
            my ($a, $b) = split(/\.\./, shift());
            ($a, $b) = map { $_ - 1 } ($a, $b);
            push @a, ($a .. $b);
        } else {
            print STDERR "Invalid column number: $ARGV[0]\n";
        }
    } 
}

# Replace undefined with empty string
sub fix { defined($_[0]) ? $_[0] : "" }

# splits input stream
my @l = split /$sep/o;
# Select all fields if none specified
@a = (0 .. scalar(@l-1)) unless @a;

# Don't consider any columns that doesn't actually exist for this row
my @b = grep { defined($l[$_]) } @a;

# Put fields back together single-space separated. 
# See column(1) for a way to get this back into a pretty table format
# paste(1) is also relevant
#
print join(" ", map { fix($l[$_]) } @b);