Reverse grepping

tac/grep Solution

tac file | grep whatever 

Or a bit more effective:

grep whatever < <(tac file) 

Time with a 500MB file:

real 0m1.225s user 0m1.164s sys 0m0.516s 

sed/grep Solution:

sed '1!G;h;$!d' | grep whatever 

Time with a 500MB file: Aborted after 10+ minutes.

awk/grep Solution:

awk '{x[NR]=$0}END{while (NR) print x[NR--]}' file | grep whatever 

Time with a 500MB file:

real 0m5.626s user 0m4.964s sys 0m1.420s 

perl/grep Solution:

perl -e 'print reverse <>' file | grep whatever 

Time with a 500MB file:

real 0m3.551s user 0m3.104s sys 0m1.036s 

This solution might help:

tac file_name | grep -e expression 

This one exits as soon as it finds the first match:

 tac hugeproduction.log | grep -m1 WhatImLookingFor 

The following gives the 5 lines before and after the first two matches:

 tac hugeproduction.log | grep -m2 -A 5 -B 5 WhatImLookingFor 

Remember not to use -i (case insensitive) unless you have to as that will slow down the grep.

If you know the exact string you are looking for then consider fgrep (Fixed String)

 tac hugeproduction.log | grep -F -m2 -A 5 -B 5 'ABC1234XYZ' 

If the file is really big, can not fit in memory, I will use Perl with File::ReadBackwards module from CPAN:

$ cat reverse-grep.pl #!/usr/bin/perl use strict; use warnings; use File::ReadBackwards; my $pattern = shift; my $rev = File::ReadBackwards->new(shift) or die "$!"; while (defined($_ = $rev->readline)) { print if /$pattern/; } $rev->close; 

Then:

$ ./reverse-grep.pl pattern file