While we have already discussed the cksum command line utility, there’s another tool you can use in scenarios where you need to verify the integrity of files during transfers. The tool we’re talking about here is md5sum. Another frequently used too to compute checksums for files is the sha1sum command.
This tutorial will discuss this command’s basics using easy-to-understand examples.
But before we do that, it’s worth mentioning that all examples in this article have been tested on Ubuntu 22.04 LTS.
Linux md5sum command
The md5sum command computes and checks an MD5 (128-bit) message digest for files. Here’s the syntax of the command:
md5sum [OPTION]... [FILE]...
The following Q&A-styled examples should give you a better idea of how md5sum works.
Q1. How does the md5sum command work?
Basic usage is pretty simple – if you want to compute the MD5 checksum for a file, you must pass the file name as input to the command. For example:
md5sum testfile.txt
You can also redirect the output to a text file to check the digest later against any change/corruption in the file.
md5sum testfile.txt > digest.md5
Use the -c command line option to check the digest.
md5sum -c digest.md5
And if there’s any change or corruption, here’s the kind of output md5sum produces:
Q2. How to change the way files are read checksum is created?
By default, the md5sum command reads input in text mode. However, if you want, you can also make the tool read input in binary mode. This you can do using the -b command line option.
md5sum -b [filename]
You can also force md5sum to create a BSD-style checksum using the –tag command line option.
Q3. How to make md5sum ignore missing files?
While verifying checksums, if you want md5sum to neither fail nor report the status for missing files, you can use the –ignore-missing option. The following screenshot shows this option in action:
So you can see that the error and the notification weren’t produced in the second case.
Q4. How to make md5sum avoid printing OK for each successfully verified file?
Following is the default behavior of md5sum:
However, if you do not want to see OK for each successfully verified file, you can use the –quiet option. So in our case, the above command would become:
md5sum -c --quiet digest.md5
Q5. How md5sum computes checksums?
The sums are computed by the tool as described in RFC 1321. Here’s what the man page says:
The sums are computed as described in RFC 1321. When checking, the
input should be a former output of this program. The default mode is
to print a line with checksum, a space, a character indicating input
mode ('*' for binary, ' ' for text or where binary is insignificant),
and name for each FILE.The MD5 algorithm should not be used any more for security related pur?
poses. Instead, better use an SHA-2 algorithm, implemented in the pro?
grams sha224sum(1), sha256sum(1), sha384sum(1), sha512sum(1)
If you are a Linux command line newbie, you’ll have fewer chances to use md5sum in your early days. It’s primarily aimed at system admins/pro users. But there’s no harm in developing a basic understanding of how the tool works, which is precisely what this tutorial is focused on. If you want to know more, you can head to md5sum’s man page.
