sha1sum is a computer program that calculates and verifies SHA-1 hashes. It is commonly used to verify the integrity of files. It (or a variant) is installed by default on most Linux distributions. Typically distributed alongside sha1sum are sha224sum, sha256sum, sha384sum and sha512sum, which use a specific SHA-2 hash function and b2sum,^[1] which uses the BLAKE2 cryptographic hash function.

The SHA-1 variants are proven vulnerable to collision attacks, and users should instead use, for example, a SHA-2 variant such as sha256sum or the BLAKE2 variant b2sum to prevent tampering by an adversary.^[2][3]

It is included in GNU Core Utilities,^[4] Busybox (excluding b2sum),^[5] and Toybox (excluding b2sum).^[6] Ports to a wide variety of systems are available, including Microsoft Windows.

To create a file with a SHA-1 hash in it, if one is not provided:

$ sha1sum filename [filename2] ... > SHA1SUM

If distributing one file, the .sha1 file extension may be appended to the filename e.g.:

$ sha1sum --binary my-zip.tar.gz > my-zip.tar.gz.sha1

The output contains one line per file of the form "{hash} SPACE (ASTERISK|SPACE) [{directory} SLASH] {filename}". (Note well, if the hash digest creation is performed in text mode instead of binary mode, then there will be two space characters instead of a single space character and an asterisk.) For example:

$ sha1sum -b my-zip.tar.gz
d5db29cd03a2ed055086cef9c31c252b4587d6d0 *my-zip.tar.gz
$ sha1sum -b subdir/filename2
55086cef9c87d6d031cd5db29cd03a2ed0252b45 *subdir/filename2

To verify that a file was downloaded correctly or that it has not been tampered with:

$ sha1sum -c SHA1SUM
filename: OK
filename2: OK
$ sha1sum -c my-zip.tar.gz.sha1
my-zip.tar.gz: OK

sha1sum can only create checksums of one or multiple files inside a directory, but not of a directory tree, i.e. of subdirectories, sub-subdirectories, etc. and the files they contain. This is possible by using sha1sum in combination with the find command with the -exec option, or by piping the output from find into xargs. sha1deep can create checksums of a directory tree.

To use sha1sum with find:

$ find s_* -type f -exec sha1sum '{}' \;
65c23f142ff6bcfdddeccebc0e5e63c41c9c1721  s_1/file_s11
d3d59905cf5fc930cd4bf5b709d5ffdbaa9443b2  s_2/file_s21
5590e00ea904568199b86aee4b770fb1b5645ab8  s_a/file_02

Likewise, piping the output from find into xargs yields the same output:

$ find s_* -type f | xargs sha1sum
65c23f142ff6bcfdddeccebc0e5e63c41c9c1721  s_1/file_s11
d3d59905cf5fc930cd4bf5b709d5ffdbaa9443b2  s_2/file_s21
5590e00ea904568199b86aee4b770fb1b5645ab8  s_a/file_02

• shasum is a Perl program to calculate any of SHA-1, 224, 256, 384, 512 hashes.^[7] It is part of the ActivePerl distribution.
• sha3sum is a similarly named program that calculates SHA-3, HAKE, RawSHAKE, and Keccak functions.^[8]
• The <hash>sum naming convention is also used by the BLAKE team with b2sum and b3sum, by the program tthsum, and many others.
• On FreeBSD and OpenBSD, the utilities are called md5, sha1, sha256, and sha512. These versions offer slightly different options and features. Additionally, FreeBSD offers the Skein family of message digests.^[9]

• cksum
• md5sum
• md5deep
• GNU Core Utilities

• sha1sum(1) – Linux User Commands Manual
• sha1sum for Windows announcement
• FreeBSD Manual Pages md5
• OpenBSD General Commands Manual md5