check md5 of compressed file without unpacking it completely

This:

gzip -d -c myfile.gz | md5sum

will stream the decompressed content into md5sum, rather than loading the whole thing into memory.

If it's a zip file, the command is unzip -p myfile.zip | md5sum

The simple answer using gzip/zcat and piping to md5sum (which someone already posted while I was writing this) will work, but if you want to have more fun, here is a short Perl script which will do the same thing.

#!/usr/bin/perl

use strict;
use warnings;

use Archive::Zip qw/:ERROR_CODES :CONSTANTS/;
use Digest::MD5;

die "Usage: $0 zipfile filename" unless @ARGV == 2;

my $zipfile = $ARGV[0];
my $filename = $ARGV[1];

my $z = Archive::Zip->new();
die "Error reading $zipfile" unless $z->read($zipfile) == AZ_OK;
my $member = $z->memberNamed($filename);
die "Could not find $filename in $zipfile" unless $member;
$member->desiredCompressionMethod(COMPRESSION_STORED);
$member->rewindData();

my $md5 = Digest::MD5->new;
while(1) {
        my($buf,$status) = $member->readChunk();
        $md5->add($$buf) if $status == AZ_STREAM_END || $status == AZ_OK;
        last if $status == AZ_STREAM_END;
        die "IO Error" if $status != AZ_OK;
}
my $digest = $md5->hexdigest;
print "$digest  $zipfile/$filename\n";