Using the star sign in grep

regex bash grep
Saobi picture Saobi · Jul 1, 2009 · Viewed 207.6k times · Source

I am trying to search for the substring "abc" in a specific file in linux/bash

So I do:

grep '*abc*' myFile

It returns nothing.

But if I do:

grep 'abc' myFile

It returns matches correctly.

Now, this is not a problem for me. But what if I want to grep for a more complex string, say

*abc * def *

How would I accomplish it using grep?

Answer

Daniel Vandersluis picture Daniel Vandersluis · Jul 1, 2009

The asterisk is just a repetition operator, but you need to tell it what you repeat. /*abc*/ matches a string containing ab and zero or more c's (because the second * is on the c; the first is meaningless because there's nothing for it to repeat). If you want to match anything, you need to say .* -- the dot means any character (within certain guidelines). If you want to just match abc, you could just say grep 'abc' myFile. For your more complex match, you need to use .* -- grep 'abc.*def' myFile will match a string that contains abc followed by def with something optionally in between.

Update based on a comment:

* in a regular expression is not exactly the same as * in the console. In the console, * is part of a glob construct, and just acts as a wildcard (for instance ls *.log will list all files that end in .log). However, in regular expressions, * is a modifier, meaning that it only applies to the character or group preceding it. If you want * in regular expressions to act as a wildcard, you need to use .* as previously mentioned -- the dot is a wildcard character, and the star, when modifying the dot, means find one or more dot; ie. find one or more of any character.

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