sha1sum is a computer program that calculates and verifies SHA-1 hashes.
It (or a variant) is installed by default on most Linux distributions.
[6] Ports to a wide variety of systems are available, including Microsoft Windows.
(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: To verify that a file was downloaded correctly or that it has not been tampered with: 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.