md5sum generates or checks MD5 checksums. The algorithm to generate the checksum is reasonably fast and strong enough for most cases. Exact specification of the algorithm is in RFC 1321.
Normally md5sum generates checksums of all files given to it as a parameter and prints the checksums followed by the filenames. If, however, -c is specified, only one filename parameter is allowed. This file should contain checksums and filenames to which these checksums refer to, and the files listed in that file are checked against the checksums listed there. See option -c for more information.
Use binary mode. In unix environment, only difference between this and the normal mode is an asterix preceding the filename in the output.
Check md5sum of all files listed in file against the checksum listed in the same file. The actual format of that file is the same as output of md5sum. That is, each line in the file describes a file. A line looks like:
So, for example, if a file was created and its message digest calculated like so:
md5sum would report:
The related MD4 message digest algorithm was broken in October 1995. MD5 isn't looking as secure as it used to.
This manpage is not quite accurate and has formatting inconsistent with other manpages.