Intro To 'md5sum' Command In Linux

2023-05-11 - By Robert Elder

     I use the 'md5sum' command to calculate the md5 checksum of files:

md5sum some-file.txt

     and the output will look something like this:

ed076287532e86365e841e92bfc50d8c  some-file.txt

     You can run the 'md5sum' command with one or more arguments to calculate the checksum of all of the files at once:

md5sum some-file.txt other-file.txt cat.mp4

     and the output will look something like this:

ed076287532e86365e841e92bfc50d8c  some-file.txt
e83e2f16a9b7646803c105099234ad17  other-file.txt
d077f244def8a70e5ea758bd8352fcd8  cat.mp4

Verifying MD5 Checksums In A File With 'md5sum'

     You can use the '-c' flag with the 'md5sum' command to specify a file that contains a list of file names and checksums.  Here, I have a file called 'checksums.txt' that contains the following list of checksums.  The format of these checksums is the same as the regular output from the 'md5sum' command:

d077f244def8a70e5ea758bd8352fcd8  cat.mp4
e83e2f16a9b7646803c105099234ad17  other-file.txt
ed076287532e86365e841e92bfc50d8c  some-file.txt

     If I run the 'md5sum' command like this:

md5sum -c checksums.txt

     the output will look something like this:

cat.mp4: OK
other-file.txt: OK
some-file.txt: OK

     Each file name listed in the checksum file will be compared against its expected checksum.  If the checksum fails to match, a message stating so will be displayed.  To test this, we can add some extra data to one of the files so that it's checksum doesn't match:

echo 'foo' >> cat.mp4

     And now, running the same command again:

md5sum -c checksums.txt

     produces the following output:

cat.mp4: FAILED
other-file.txt: OK
some-file.txt: OK
md5sum: WARNING: 1 computed checksum did NOT match

     The md5 checksum algorithm was historically considered to be cryptographically secure, however this is no longer the case as major security flaws in the algorithm have been found.  The md5 hash function is still useful for non-cryptographic purposes.

     And that's why the 'md5sum' command is my favourite Linux command.