Difference between egrep and grep

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The egrep command is a shortcut for the grep binary, but with one exception: when grep is invoked as egrep, the grep binary activates its internal logic to run as if it were called as grep -E.

The difference is that -E option enables usage of extended regexp patterns. This allows use of meta-symbols such as +, ? or |. These aren’t ordinary characters like we may use in words or filenames but are control commands for the grep binary itself. Thus, with egrep, the character | means logical OR.

So, for example, you want to list files in a directory and see only those which contain “mp4” or “avi” as filename extensions. With egrep you will do:

ls | egrep "mp4|avi"

In this example | acts like an OR command. It will grab to output from ls all names which contain either “mp4” or “avi” strings. If you run it with a plain grep command you will get nothing, because grep doesn’t know such thing as | command. Instead, grep will search for “mp4|avi” as a whole text string (with pipe symbol). E.g. if you have a file named |mp4|avi|cool-guy.q2.stats in your dir, you will get it with plain grep searching with pipes.

So, that is why you should escape | in your egrep command to achieve the same effect as in grep. Escaping will screen off the special meaning of | command for grep binary.

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