Last updated

Linux and Unix sha1sum command tutorial with examples

Tutorial on using sha1sum, a UNIX and Linux command to compute and check a SHA-1 message digest. Examples of reading a SHA-1 message digest, writing a SHA-1 message digest to a file, and checking a SHA-1 message digest.

Estimated reading time: 3 minutes

Table of contents

Terminal showing sha1sum man page

What is the sha1sum command in UNIX?

The sha1sum command computes the SHA-1 message digest of a file. This allows it be compared to a published message digest to check whether the file is unmodified from the original. As such the sha1sum command can be used to attempt to verify the integrity of a file. SHA-1 produces a 160-bit (20 byte) hash value known as a message digest. Although SHA-1 is no longer considered secure against well funded opponents it is still widely used to verify files.

How to get the SHA-1 of a file

To get the SHA-1 of a file pass the path of a file to the sha1sum command. The SHA-1 will be printed to standard output printing first the SHA-1 checksum then the name of the file.

sha1sum somefile.txt
da39a3ee5e6b4b0d3255bfef95601890afd80709  somefile.txt

How to write the SHA-1 of a file

To write the SHA-1 of a file standard shell redirection can be used.

sha1sum somefile.txt > somefile.txt.sha1
cat somefile.txt.sha1
da39a3ee5e6b4b0d3255bfef95601890afd80709  somefile.txt

If the file somefile.txt is to be distributed on the Internet the accompanying SHA-1 file can be distributed with it. This allows anyone downloading or receiving the file to verify (to some extent) that the file has not been tampered with. Normally the canonical author of a file will also publish the SHA-1 of the file. It is worth verifying that the published SHA-1 also matches the one published by the author on any webpage relating to the download.

How to check the SHA-1 of a file

If a SHA-1 file has been provided with a download this can be used to check the integrity of a downloaded file. To check the SHA-1 of a file use the -c option and pass the SHA-1 checksum file that corresponds to the file or files you wish to check. If not file has been provided with the download the author of the file will normally publish a SHA-1 message digest and this can be checked manually by comparing the output of sha1sum [file] with the published message digest.

somefile.txt somefile.txt.sha1
sha1sum -c somefile.txt.sha1
somefile.txt: OK

If the SHA-1 code matches an OK will be printed to standard output along with the filename verified. If the SHA-1 code fails to match a failure message will be printed to standard output and the file should not be trusted.

sha1sum -c somefile.txt.sha1
somefile.txt: FAILED
sha1sum: WARNING: 1 computed checksum did NOT match

A note on hashing algorithms

With all issues relating to security things move fast. The md5 hashing algorithm is now widely considered to be insecure. The sha1 hashing algorithm is also expected to be insecure. As such it is expected that new algorithms will eventually emerge and be widely used. Already there are many checksum commands distributed with Linux and it is expected this will evolve.

sha1sum    sha224sum  sha256sum  sha384sum  sha512sum  shasum

For each of these hashing algorithms the command options and behaviour is the same so when a hashing algorithm changes it is a drop-in replacement.

Further reading

Have an update or suggestion for this article? You can edit it here and send me a pull request.


Recent Posts