Shack is an sha1sum clone. Given a file as input it will calculate the SHA-1 hash of that file. This is typically used for checking that a file has not been tampered with or changed (e.g. when downloading a file from a server).
$ shack ubuntu-12.04.3-desktop-amd64.iso c09241f9a5c1eeae26ebf72116427c5f76177d6a
Disclaimer: I am not a cryptographer. This program should not be used as part of any cryptographic system to provide security or privacy. It likely contains bugs and I will not be held responsible for any consequences incurred through the misuse of this program.