Difference between egrep and grep
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.
Extracted from grep explained and man pages.
grep provides matcher selection options.-E Interpret pattern as an Extended Regular Expression (ERE)-G Interpret pattern as a Basic Regular Expression (BRE). This is the default when no option is specified.
The variant program egrep is the same as grep -E. The variant is deprecated, but is provided for backward compatibility.
Therefore,grep implies grep -Gegrep implies grep -E
There are two interpretations of the syntax in regex patterns. The difference is in the behavior of a few special characters, ?, +, (), {}, and |.
- BRE (Basic Regular Expression) – these characters do not have special meaning unless prefixed with a backslash
\. - ERE (Extended Regular Expression) – these characters are special, unless prefixed with a backslash
\.
Since ^ has the same interpretation by grep(BRE) and egrep(ERE) it works the same in both.
However, | is one of those characters which are interpreted differently by grep(BRE) and egrep(ERE) so it requires to be escaped with a \ depending on the regex intent.
The difference between grep and egrep is:
grep
- It uses Basic Regular Expression which means if you use
grep 'a|b'it will not not use this "|" as OR operator without using this "\" prefix. - It searches for PATTERN in each FILE.
egrep
- It uses Extended Regular Expression and in this you can use commands like this
egrep 'a|b' - It treats meta-character as it is and does not substitute them as strings like grep.