Jtdylktuy

How to do this best depends on what you want:

• Do you want to extract a single part of the large file?


• Or do you want to create all the parts in one go?

If you want a single part of the file, your idea to use gunzip and head is right. You can use:

gunzip -c hugefile.txt.gz | head -n 4000000

That would output the first 4000000 lines on standard out - you probably want to append another pipe to actually do something with the data.

To get the other parts, you'd use a combination of head and tail, like:

gunzip -c hugefile.txt.gz | head -n 8000000 |tail -n 4000000

to get the second block.

Is perhaps doing a series of these a solution or would the gunzip -c
require enough space for the entire file to be unzipped

No, the gunzip -c does not require any disk space - it does everything in memory, then streams it out to stdout.

If you want to create all the parts in one go, it is more efficient to create them all with a single command, because then the input file is only read once. One good solution is to use split; see jim mcnamara's answer for details.

pipe to split use either gunzip -c or zcat to open the file

gunzip -c bigfile.gz | split -l 400000

Add output specifications to the split command.

As you are working on a (non-rewindable) stream, you will want to use the '+N' form of tail to get lines starting from line N onwards.

zcat hugefile.txt.gz | head -n 40000000

zcat hugefile.txt.gz | tail -n +40000001 | head -n 40000000

zcat hugefile.txt.gz | tail -n +80000001 | head -n 40000000

I'd consider using split.

split a file into pieces

Here's a python script to open a globbed set of files from a directory, gunzip them if necessary, and read through them line by line. It only uses the space necessary in memory for holding the filenames, and the current line, plus a little overhead.

#!/usr/bin/env python

import gzip, bz2

import os

import fnmatch



def gen_find(filepat,top):

    for path, dirlist, filelist in os.walk(top):

        for name in fnmatch.filter(filelist,filepat):

            yield os.path.join(path,name)



def gen_open(filenames):

    for name in filenames:

        if name.endswith(".gz"):

            yield gzip.open(name)

        elif name.endswith(".bz2"):

            yield bz2.BZ2File(name)

        else:

            yield open(name)



def gen_cat(sources):

    for s in sources:

        for item in s:

            yield item



def main(regex, searchDir):

    fileNames = gen_find(regex,searchDir)

    fileHandles = gen_open(fileNames)

    fileLines = gen_cat(fileHandles)

    for line in fileLines:

        print line



if __name__ == '__main__':

    parser = argparse.ArgumentParser(description='Search globbed files line by line', version='%(prog)s 1.0')

    parser.add_argument('regex', type=str, default='*', help='Regular expression')

    parser.add_argument('searchDir', , type=str, default='.', help='list of input files')

    args = parser.parse_args()

    main(args.regex, args.searchDir)

The print line command will send every line to std out, so you can redirect to a file. Alternatively, if you let us know what you want done with the lines, I can add it to the python script and you won't need to leave chunks of the file laying around.

Here's a perl program that can be used to read stdin, and split the lines, piping each clump to a separate command that can use a shell variable $SPLIT to route it to a different destination. For your case, it would be invoked with

zcat hugefile.txt.gz | perl xsplit.pl 40000000 'cat > tmp$SPLIT.txt; do_something tmp$SPLIT.txt; rm tmp$SPLIT.txt'

Sorry the command-line processing is a little kludgy but you get the idea.

#!/usr/bin/perl -w

#####

# xsplit.pl: like xargs but instead of clumping input into each command's args, clumps it into each command's input.

# Usage: perl xsplit.pl LINES 'COMMAND'

# where: 'COMMAND' can include shell variable expansions and can use $SPLIT, e.g.

#   'cat > tmp$SPLIT.txt'

# or:

#   'gzip > tmp$SPLIT.gz'

#####

use strict;



sub pipeHandler {

    my $sig = shift @_;

    print " Caught SIGPIPE: $sign";

    exit(1);

}

$SIG{PIPE} = &pipeHandler;



my $LINES = shift;

die "LINES must be a positive numbern" if ($LINES <= 0);

my $COMMAND = shift || die "second argument should be COMMANDn";



my $line_number = 0;



while (<STDIN>) {

    if ($line_number%$LINES == 0) {

        close OUTFILE;

        my $split = $ENV{SPLIT} = sprintf("%05d", $line_number/$LINES+1);

        print "$splitn";

        my $command = $COMMAND;

        open (OUTFILE, "| $command") or die "failed to write to command '$command'n";

    }

    print OUTFILE $_;

    $line_number++;

}



exit 0;