Sunday, May 4, 2008

Reversing All Lines In A File On Linux Or Unix Using Perl

Good evening-day-morning-afternoon :)

In response to some positive feedback, and some additional questions, prompted by our earlier post on using Perl to mirror lines in a file on Linux or Unix, today we're dispensing with yet another Perl script to do almost the same thing, but with an extra twist. While our original mirror file script reversed each line, this one will attempt to do the same thing while also reversing the order of the lines. If you have a nervous condition you should probably quit reading this is, as it is only going to get more intense ;)

The good news is that I already know it works. The bad news is available at your local newsstand or in that meeting you should really try to blow off ;) Sorry... but only because the bad jokes are intentional.

So, when we're done crunching a file with today's script it will come out with all the lines reading from left to right and with the first line being the last, the second line being the second to last, and so on until the last line, which will be the first. End of story... or is it? ;)

Again, we're going to take a quick look at another useful function in this script. Since the rest of them are the same as in the script that prompted this one (split, join, undef and push), I should refer you to that post on mirroring file lines so that we don't waste too much space with duplicate content. It's interesting to look at the two side-by-side to really see the difference.

In today's post we'll check out the use of this one function (Note that, this time, we'll assume that the array @array consists of no members at all:

unshift - This function does the same thing as the "push" function we used in our last post, except that it will "unshift" a variable on to the "right side" of an array. The name isn't as intuitive as "push," but, if it helps, when you "shift" a variable from an array, you're pulling it out of the "left side." So, naturally, "unshifting" is adding to the "right side." ...No matter how I explain it, it will never make sense. It's just one of those words you have to accept on faith. Like "defenestration." <--- Apologies for the cheap-shot at Microsoft. That word is officially defined as "the act of throwing someone or something out of a window." It's only a few small steps to the jab I was going for ;)

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

Again, enjoy, and I hope this helps you out with whatever you're doing that it might help you out with ;)



host # cat words|head -3;cat words|tail -3
<--- The beginning and end of the file we're going to "reverse"
host # ./ words
host # ls
. .. words words.reverse
<--- Our newly created file is called "words.reverse"
host # cat words.reverse|head -3;cat words.reverse|tail -3 <--- And, once again (since I tested this a few times already), the reversal of the file 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 reverse file are exactly the same, which they should be.
45378 words
45378 words.reverse
90756 total

Creative Commons License

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


# - reverse each line, and its position in a file
# 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>;

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

, Mike