print a file from Bottom to Top (reverse order)

theravadamonk has asked for the wisdom of the Perl Monks concerning the following question:

Hi, Monks, I want to know how to print a file from Bottom to Top (reverse order).

I found this below link with below code

https://www.perlmonks.org/?node_id=361397

open(FILE, "<myfile.txt");
@file = reverse <FILE>;
foreach (@file) {
    #process line
}
[download]

Given below is My CODE with while loop, I want to go with while loop. How can I print from Bottom to Top ?

How can I insert reverse order in to while loop?

What's the easiest way ? I DO hope U monks will come with ideas...


#!/usr/bin/perl
use CGI ':standard';
use strict;
use warnings;
use CGI::Carp 'fatalsToBrowser'; # use only for testing

#my $date = localtime();

$ENV{"PATH"} = "/usr/sbin:/usr/bin:/sbin:/bin";

open FILE, '<', '/var/log/maillog' or die $!;

while (<FILE>){
   chomp;
   next unless /\S/; # skip blank lines
   if (/Passed CLEAN/) { #search string Passed CLEAN
      print "$_ \n"; # print ALL lines containing Passed CLEAN

   }
}

close FILE;
[download]

Comment on print a file from Bottom to Top (reverse order) Select or Download Code

Replies are listed 'Best First'.
Re: print a file from Bottom to Top (reverse order) by haukex (Archbishop) on Jun 20, 2018 at 16:59 UTC
See File::ReadBackwards: `use warnings; use strict; use File::ReadBackwards; my $bw = File::ReadBackwards->new('/var/log/maillog') or die $!; while (defined( my $line = $bw->readline )) { chomp($line); print "<$line>\n"; } $bw->close;` [download]	[reply] [d/l]
Re: print a file from Bottom to Top (reverse order) by thanos1983 (Parson) on Jun 20, 2018 at 16:33 UTC
Hello theravadamonk, You can read this Is there a way to read a file backwards? or this How do I read a file line by line in reverse order (from EOF to start of file) previously asked question(s) they contain different answers to your problem. Hope this helps, BR. Seeking for Perl wisdom...on the process of learning...not there...yet!	[reply] [d/l] [select]
Re: print a file from Bottom to Top (reverse order) by Laurent_R (Canon) on Jun 20, 2018 at 17:59 UTC
I want to go with while loop. Why? Given what you're doing in your second code sample, you could do it as well on the items of the `@file` array in your first code sample. For example: `open(FILE, "<myfile.txt"); @file = reverse <FILE>; foreach (@file) { chomp; # --> Note: no need to chomp, i +t just forces you to add the \n again when you print next unless /\S/; # skip blank lines --> Note: this is useless giv +en that you print only line matching the /Passed CLEAN/ pattern if (/Passed CLEAN/) { #search string Passed CLEAN print "$_ \n"; # print ALL lines containing Passed CLEAN } }` [download] Or, simplifying a bit the syntax: `open(FILE, "<myfile.txt"); @file = reverse <FILE>; foreach (@file) { print if /Passed CLEAN/; }` [download] That should work fine unless your file is truly huge and won't fit into memory. If your file is too big, then the `File::ReadBackwards` is the only simple solution;	[reply] [d/l] [select]
Re^2: print a file from Bottom to Top (reverse order) by theravadamonk (Scribe) on Jun 20, 2018 at 18:29 UTC
Thank you very much for your code. it works as expected. my file is actually maillog file. It's sometimes huge. So File::ReadBackwards is much better. But, the problem is I am trying to insert below lines in to that code. `if (/Passed CLEAN/) { #search string Passed CLEAN print "$_ \n"; # print ALL lines containing Passed CLEAN }` [download] But, still no success. I am learning perl. I am still a novice. I googled how to insert it, But, still not lucky.	[reply] [d/l]
Re^3: print a file from Bottom to Top (reverse order) by Laurent_R (Canon) on Jun 20, 2018 at 18:45 UTC
Changing just a couple of small things to haukex's solution above should give you what you need: `use warnings; use strict; use File::ReadBackwards; my $bw = File::ReadBackwards->new('/var/log/maillog') or die $!; while (defined( my $line = $bw->readline )) { print $line if $line =~ /Passed CLEAN/; } $bw->close;` [download]	[reply] [d/l]
Re^4: print a file from Bottom to Top (reverse order) by theravadamonk (Scribe) on Jun 22, 2018 at 15:45 UTC
Re^3: print a file from Bottom to Top (reverse order) by AnomalousMonk (Archbishop) on Jun 20, 2018 at 19:04 UTC
Given Laurent_R's comprehensive reply, let's move on to the next question: How do I match one or more arbitrary whitespace characters between the `'Passed'` and the `'CLEAN'` in the pattern match? Please see the `+` quantifier in perlre, and `\s` in perlre and Whitespace in perlrecharclass. In general, please see perlretut. Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re: print a file from Bottom to Top (reverse order) by afoken (Chancellor) on Jun 20, 2018 at 18:49 UTC
tac Alexander -- Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)	[reply]