Linux Shell Scripting Cookbook - Third Edition

Checksum and verification

Checksum programs are used to generate a relatively small unique key from files. We can recalculate the key to confirm that a file has not changed. Files may be modified deliberately (adding a new user changes the password file), accidentally (a data read error from a CD-ROM drive), or maliciously (a virus is inserted). Checksums let us verify that a file contains the data we expect it to.

Checksums are used by backup applications to check whether a file has been modified and needs to be backed up.

Most software distributions also have a checksum file available. Even robust protocols such as TCP can allow a file to be modified in transit. Hence, we need to know whether the received file is the original one or not ...