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 ;)
Ex:
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 ;)
Cheers,
SAMPLE RUN:
host # cat words|head -3;cat words|tail -3 <--- The beginning and end of the file we're going to "reverse"
Aarhus
Aaron
Ababa
Zulu
Zulus
Zurich
host # ./mirror.pl words
host # ls
. .. reverse.pl 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!
hciruZ
suluZ
uluZ
ababA
noraA
suhraA
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
This work is licensed under a
Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License#!/usr/bin/perl
# reverse.pl - reverse each line, and its position in a file
#
# 2008 - Mike Golvach - eggi@comcast.net
#
# Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License
#
if ( $#ARGV != 0 ) {
print "Usage: $0 text_file\n";
exit(1);
}
$text_file=$ARGV[0];
open(TXT, "<$text_file");
@txt = <TXT>;
close(TXT);
foreach $ln (@txt) {
chomp($ln);
@ln = split(//, $ln);
$ln_len = @ln;
undef(@rev);
while ( $ln_len > 0 ) {
push(@rev, $ln[${ln_len}-1]);
$ln_len--;
}
$rev = join(/ /, @rev);
unshift(@reverse, "$rev");
}
open(RTXT, ">${text_file}.reverse");
foreach $backward (@reverse) {
print RTXT "$backward\n";
}
close(RTXT);
exit(0);
, Mike
linux unix internet technology
Sunday, May 4, 2008
Reversing All Lines In A File On Linux Or Unix Using Perl
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.
Ex:
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,
SAMPLE RUN:
host # cat words|head -3;cat words|tail -3 <--- The beginning and end of the file we're going to "mirror"
Aarhus
Aaron
Ababa
Zulu
Zulus
Zurich
host # ./mirror.pl words
host # ls
. .. mirror.pl 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!
suhraA
noraA
ababA
uluZ
suluZ
hciruZ
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
This work is licensed under a
Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License#!/usr/bin/perl
#
# mirror.pl - print an entire file backward, line by line
#
# 2008 - Mike Golvach - eggi@comcast.net
#
# Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License
#
if ( $#ARGV != 0 ) {
print "Usage: $0 text_file\n";
exit(1);
}
$text_file=$ARGV[0];
open(TXT, "<$text_file");
@txt = <TXT>;
close(TXT);
open(RTXT, ">${text_file}.mirror");
foreach $ln (@txt) {
chomp($ln);
@ln = split(//, $ln);
$ln_len = @ln;
undef(@rev);
while ( $ln_len > 0 ) {
push(@rev, $ln[${ln_len}-1]);
$ln_len--;
}
$rev = join(/ /, @rev);
print RTXT "$rev\n";
}
close(RTXT);
exit(0);
, Mike
linux unix internet technology
