diff is a program that creates specially formed patch files. It compares two files, and outputs the differences in a format that can be used to recreate one of the files from the other. (These special files are called either patch-files or diff-files.) The usual invocation of the program is something like this: diff -Nurd original-nethack-dir/ changed-nethack-dir/ This page is a . You could probably [ expand this page] should you wish to do so.