Shell Programming and Scripting

awk -F '' '{getline s<f;split(s,T);for(i=1;i<=NF;i++)if($i==T[i])$i=" "}1' OFS= f=file2 file1

awk '{getline s<f;split(s,T);for(i=1;i<=NF;i++)if($i==T[i])$i=" "}1' FS= OFS= f=<(fold -w80 file2) <(fold -w80 file1)

use strict;
use warnings;
use File::Basename;

my $NAME = basename $0;

$\ = "\n";
$, = '';
$" = '';

if (2 != @ARGV) {
    print STDERR 'USAGE: ', $NAME, '<file1> <file2>';
    exit 1;
}

my $F1 = shift @ARGV;
my $F2 = shift @ARGV;

my $len = length($F1) > length($F2) ? length($F1) : length($F2);
my $fmt = "\%-${len}s(%d): \%s\n";

open F1, '<', $F1 or die $F1;
open F2, '<', $F2 or die $F2;

my $L1;
my $L2;

while (1) {
    $L1 = <F1>;
    $L2 = <F2>; 

    last unless defined $L1 && defined $L2;

    if ($L1 eq $L2) {
        print '';
    next;
    }

    chomp $L1;
    chomp $L2;

    my @L1 = split //, $L1;
    my @L2 = split //, $L2;

    my @R = ();

    while (0 < @L1 || 0 < @L2 ) {
        my $c1 = shift @L1; $c1 = ' ' unless defined $c1;
        my $c2 = shift @L2; $c2 = ''  unless defined $c2;

    push @R, $c1 eq $c2 ? ' ' : $c1;
    }

    print @R;
}

# if file-1 is longer than file-2

while (defined $L1) {
    chomp $L1;
    print $L1;
    $L1 = <F1>;
}

# if file-2 is longer than file-1

while (defined $L2) {
    print ' ' x length($L2);
    $L2 = <F2>;
}