How to split a text file based on the content into multiple text files

And here's a nice, simple, gawk one-liner :

$ gawk '/^\[/{match($0, /^\[ (.+?) \]/, k)} {print >k[1]".txt" }' entry.txt

This will work for any file size, irrespective of the number of lines in each entry, as long as each entry header looks like [ blahblah blah blah ]. Notice the space just after the opening [ and just before the closing ].

EXPLANATION:

awk and gawk read an input file line by line. As each line is read, its contents are saved in the $0 variable. Here, we are telling gawk to match anything within square brackets, and save its match into the array k.

So, every time that regular expression is matched, that is, for every header in your file, k[1] will have the matched region of the line. Namely, "entry1", "entry2" or "entry3" or "entryN".

Finally, we print each line into a file called <whatever value k currently has>.txt, ie entry1.txt, entry2.txt ... entryN.txt.

This method will be much faster than perl for larger files.

This will work for the given format. This assumes the file will always start with 00:00:00:00.

#!/usr/bin/env perl

use strict;
use warnings;

open(my $infh, '<', 'ABC_TabDelim.txt') or die $!;

my $outfh;
my $filecount = 0;
while ( my $line = <$infh> ) {
    if ( $line =~ /^00:00:00:00/ ) {
        close($outfh) if $outfh;
        open($outfh, '>', sprintf('ABC%02d_TabDelim.txt', ++$filecount)) or die $!;        
    }
    print {$outfh} $line or die "Failed to write to file: $!";
}

close($outfh);
close($infh);