Saturday, May 3, 2008

Perl Script To Mirror Lines In A File On Linux Or Unix

Good afternoon-morning-day-evening :)

In much the same vein as our previously posted script to do weak encryption with Octal Dump, today we're throwing out another Perl script to do something possibly equally worthless, but still somewhat entertaining ;) Since it's written in Perl, and uses that language's specific constructs (i.e. No "system" calls), it should run equally well on Linux or Unix. The logic in the script is simple enough that it should probably work going back a few major versions.

As much as this may not seem to be of any use to you now, knowing how to mirror (or reverse, since many people, including myself, don't have any mirror-friendly fonts ;) each line in a file can be beneficial. Although no one in the office is likely to come up to you and say "Mr. sysadmin, sir (of course, I'm exaggerating. People are much more formal than that normally ;), Can you print every single line of this report reading from right-to-left instead of the standard left-to-right?", it's even more unlikely that, if this were to happen, any business/war jargon would be used. I can't imagine something like this ever being "mission critical" or required for any sort of "code red" situation. If it ever is, you'll be really glad you know how to do this...

But, at a fundamental level, it's good to understand the basic functionality of Perl and how to deal with scalar variables (or string variables), arrays and how to muck around with them (or make them work for you ;) In this particular script, the building blocks of some very useful functions of Perl are employed (to a dubious end, I'll admit) and, hopefully, presented in an easy to understand fashion. Over time, we'll dig into every little thing there is to know (If that can be cranked out in one life time ;) with regards to each function. In the mean time, check out the use of these four functions (Note that, for all, we'll assume that the variable $variable is defined as "abcd" and the array @array consists of four members: a, b, c and d:

split - This function will take a scalar variable and "split" it, on a delimiter, into an array:

Ex: @array = split(//, $variable); <--- Now @array has four members (a, b, c and d) that it got from $variable

join - This function will take an array and "join" it into one scalar variable:

Ex: $variable = join(/ /, @array); <--- Now $variable equals "abcd" since it contains all 4 members of @array joined together by an empty delimiter "/ /" (that is, each character, or space, is considered a separate member of the array)

undef - This function will "undefine" our array, in this case. It can be used on scalar, array and hash variables as well.

Ex: undef(@array); <--- Now @array is not just empty, it isn't even defined. It may as well not exist.

push - This function will "push" a variable (scalar, array, hash, or references to same, and more -- getting way off-topic ;) onto the left side of an array. Subsequent pushes of extra variables are added from the left, so if you push three variables into an array (a, b and c, for instance) in one order, they'll actually end up in the array in the opposite order.

push(@array, "a");
<--- @array now equals "a"
push(@array, "b"); <--- @array now equals "b" "a"
push(@array, "c"); <--- @array now equals "c" "b" "a"

In any event, enjoy, and I hope this helps you out if you're beginning to learn the Perl scripting language!

Best wishes,


host # cat words|head -3;cat words|tail -3
<--- The beginning and end of the file we're going to "mirror"
host # ./ words
host # ls
. .. words words.mirror
<--- Our newly created file is called "words.mirror"
host # cat words.mirror|head -3;cat words.mirror|tail -3 <--- And (good deal), the mirroring seems to have worked!
host # wc -l words*
<--- Just double checking here to make sure that the number of characters in our original file and the mirror file are exactly the same, which they should be.
45378 words
45378 words.mirror
90756 total

Creative Commons License

This work is licensed under a
Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License


# - print an entire file backward, line by line
# 2008 - Mike Golvach -
# Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License

if ( $#ARGV != 0 ) {
print "Usage: $0 text_file\n";


open(TXT, "<$text_file");
@txt = <TXT>;

open(RTXT, ">${text_file}.mirror");
foreach $ln (@txt) {
@ln = split(//, $ln);
$ln_len = @ln;
while ( $ln_len > 0 ) {
push(@rev, $ln[${ln_len}-1]);
$rev = join(/ /, @rev);
print RTXT "$rev\n";

, Mike