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Full Discussion: cksum parts of a file
Top Forums UNIX for Dummies Questions & Answers cksum parts of a file Post 25441 by auswipe on Tuesday 30th of July 2002 11:38:27 AM
Old 07-30-2002
I would write a routine in C (or C++) that would open the source files and then fseek offsets from 0 to (n-1) (where n is the length of the file) and just skip over or substitute a common character (say, captial X) for bytes 223 and 224 and write it out to a temporary binary file and run checksum against it and then perform the same with the second file and then compare checksums.

You could probably also write a similiar routine in perl as well but C seems to appropriate.
 

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SHASUM(1)						 Perl Programmers Reference Guide						 SHASUM(1)

NAME
shasum - Print or Check SHA Checksums SYNOPSIS
Usage: shasum [OPTION]... [FILE]... Print or check SHA checksums. With no FILE, or when FILE is -, read standard input. -a, --algorithm 1 (default), 224, 256, 384, 512, 512224, 512256 -b, --binary read in binary mode -c, --check read SHA sums from the FILEs and check them -p, --portable read files in portable mode produces same digest on Windows/Unix/Mac -t, --text read in text mode (default) The following two options are useful only when verifying checksums: -s, --status don't output anything, status code shows success -w, --warn warn about improperly formatted checksum lines -h, --help display this help and exit -v, --version output version information and exit When verifying SHA-512/224 or SHA-512/256 checksums, indicate the algorithm explicitly using the -a option, e.g. shasum -a 512224 -c checksumfile The sums are computed as described in FIPS-180-4. When checking, the input should be a former output of this program. The default mode is to print a line with checksum, a character indicating type (`*' for binary, ` ' for text, `?' for portable), and name for each FILE. Report shasum bugs to mshelor@cpan.org DESCRIPTION
Running shasum is often the quickest way to compute SHA message digests. The user simply feeds data to the script through files or standard input, and then collects the results from standard output. The following command shows how easy it is to compute digests for typical inputs such as the NIST test vector "abc": perl -e "print qq(abc)" | shasum Or, if you want to use SHA-256 instead of the default SHA-1, simply say: perl -e "print qq(abc)" | shasum -a 256 Since shasum mimics the behavior of the combined GNU sha1sum, sha224sum, sha256sum, sha384sum, and sha512sum programs, you can install this script as a convenient drop-in replacement. AUTHOR
Copyright (c) 2003-2011 Mark Shelor <mshelor@cpan.org>. SEE ALSO
shasum is implemented using the Perl module Digest::SHA or Digest::SHA::PurePerl. perl v5.14.2 2014-09-30 SHASUM(1)
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