diff is a Command-line utility that allows you to compare two files line by line. It can also compare the contents of directories. The Diff Command is most commonly used to create a patch containing the differences between one or more files that can be applied using the patch command. How to Use the diff Command The syntax for the diff command is as follows: diff [OPTION]... FILES The diff command can display the output in several formats with the normal, context, and unified format being the most common ones.