The “diff command” compares two files, returns the line(s) that are different, and tells you how to make the files the same.
Here’s an example to use ‘diff’ command
$ diff file1 file2
Here, File1 and File 2 are two files taken for comparison. The UNIX assigns for file 1 ‘A’ and for File 2 ‘B’. Then it gives clear differences.
Refer more for diff commands