Checksum a file with md5sum

md5sum creates file hashes. The hash is solely based on the file contents (even if you change the file name, the hash remains the same).

$ md5sum README.md 
bb4955b6d0855ce20c30223273124dd7  README.md

To create a file containing hashes, do:

$ md5sum README.md > MD5SUM.txt

To verify if the file was not tampered:

$ md5sum -c MD5SUM.txt
README.md: OK