Question

I have a file that looks like:

SECTION1 id name  
 sub section1
 sub section2
SECTION2 id name  
 sub section3
 sub section4
 sub section6
SECTION1 id name  
 sub section7
 sub section8
SECTION3 id name  
 sub section9
 sub section10
 sub section11
 sub section12
SECTION2 id name  
 sub section13
 sub section14
SECTION1 id name  
 sub section15
 sub section16
SECTION3 id name  
 sub section17
 sub section18

I need to sort this file section wise. The only thing I know is that I have ‘SECTION1’, ‘SECTION2’ AND ‘SECTION3’.
Expected output after sorting is:

SECTION1 id name  
 sub section1
 sub section2
SECTION1 id name  
 sub section7
 sub section8
SECTION1 id name  
 sub section15
 sub section16
SECTION2 id name  
 sub section3
 sub section4
 sub section6
SECTION2 id name  
 sub section13
 sub section14
SECTION3 id name  
 sub section9
 sub section10
 sub section11
 sub section12
SECTION3 id name  
 sub section17
 sub section18

Is there any easy way to do this in perl or using the utilities like grep,sed etc?

Answer 1

One more way using perl:

Assuming infile has the content of the question and following content of script.pl:

use warnings;
use strict;
use sort qw/stable/;

my ($section, @section);

while ( <> ) { 

    ## Save text if first line or when line doesn't begin with 'SECTION' word.
    if ( $. == 1 || $_ !~ m/\ASECTION\d+/ ) { 
        $section .= $_; 
        next unless eof;
    }   

    ## Save the text and the number of section.
    if ( $section =~ m/\ASECTION(\d+)/ ) { 
        push @section, [ $1, $section ];
        $section = q||;
    }   

    ## Begin to save next section.
    $section .= $_; 
}

## Print them sorted by section number.
for ( sort { $a->[0] <=> $b->[0] } @section ) { 
    printf qq|%s|, $_->[1];
}

Run it like:

perl script.pl infile

With following output:

SECTION1 id name  
 sub section1
 sub section2
SECTION1 id name  
 sub section7
 sub section8
SECTION1 id name  
 sub section15
 sub section16
SECTION2 id name  
 sub section3
 sub section4
 sub section6
SECTION2 id name  
 sub section13
 sub section14
SECTION3 id name  
 sub section9
 sub section10
 sub section11
 sub section12
SECTION3 id name  
 sub section17
 sub section18