Reverse grepping

Question

Let's say, I have a really big text file (about 10.000.000 lines). I need to grep it from the end and save result to a file. What's the most efficient way to accomplish task?

Answer 1

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

Answer 2

This solution might help:

tac file_name | grep -e expression

Answer 3

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'

Answer 4

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