grep vs egrep vs fgrep

egrep is equivalent to grep -E [1]. It interprets the pattern as an “extended regular expression”. In extended mode, some backslashes to escape certain characters are removed [2]:

Pattern with grep	Pattern with egrep
abc\{3\}	abc{3}
a\(e\)+g	a(e)g

It looks like a good idea to always use egrep or grep -e first, since the regular expressions I’ve alredy used (for example in JavaScript) are extended regular expressions and not basic regular expressions.

fgrep is equivalent to grep -F [1]. It interprets the pattern as a list of fixed strings, separated by new lines, any of which is to be matched.

For example, if you have a file users.txt with a list of users (one user on each line) and want to search the group file to see if any of the users are in it, you would use:

$ grep -F -f users.txt /etc/group

Sources:

[1] https://unix.stackexchange.com/questions/17949/what-is-the-difference-between-grep-egrep-and-fgrep

[2] https://en.wikibooks.org/wiki/Regular_Expressions/POSIX-Extended_Regular_Expressions