diff command compares files line by line and outputs any lines that are not identical. For example:
matthew@seymour:~$ diff file1 file2
This command outputs every line that is different between the two. If
file2 are different versions of a configuration file, say the current and a backup, the output quickly tells you what, if anything, has changed. This can help when a config file is automatically updated during an operating system upgrade or when you make a change that doesn’t work as well as you had planned and then go back a couple of weeks later to change the configuration back.
There are several options you may use when running
diff (the original UNIX style ...