Searching Files For Text with grep

Written by Chris Gregg, Prasanna Vasudevan and Rahul Agarwal, with modifications by Nick Troccoli

Click here for a walkthrough video.

grep works like "find in file" (Ctrl-F or Command-F) search in Microsoft Word, or a search box on your computer's files and folders. The general syntax for grep is

    grep <pattern> <file(s) to search> 

For example, if we were looking for all the lines where the binky function is called , we could use grep to search the source file as follows:

grep "binky()" program.c

There is more to grep than just normal text matching. grep especially shines is in matching complex patterns expressed as regular expressions (commonly shortened to regex). Here are some of the core metacharacters that you will often use:

 .  matches any character
    e.g, 'a..' matches 'abc' and also 'adf'

 *  matches zero or more repeats of char to left of *
    e.g., 'ab*'' matches 'abbbbb' and also 'a'

 ^  matches the beginning of the line

 $  matches the end of the line

Note that the * symbol does what we call greedy matching. This means that it tries to match as many characters as possible. So therefore, ab* tries to match as many bs as possible instead of not matching any bs at all.

However, naive greedy matching strategy will sometimes miss matches. For example, when using ab*b to match abb, if one matches the b* to bb, then there is no text left to match the final b to. Grep is smart enough to backtrack after this failed match, and tries to match b* to the next longest string, b so that the final b in the search string can match the final b in the text.

Building on the binky example from earlier, consider the case where binky() actually takes a number of arguments, so matching on "binky()" will not suffice. We can instead search

grep "binky(.*)" program.c

A dictionary word list is available on myth in the file /usr/share/dict/words. This file is a good one to grep for practice, e.g. try grep joy /usr/share/dict/words or grep 'b.b' /usr/share/dict/words and see which words matched. Here are some suggested exercises to use as practice in forming regular expressions:

• match all words that end with zy
• match all words that start with k and end with k
• match all words that are exactly 7 letters long

Note that certain punctuation characters such as * and $ have special meaning to the shell and may get transformed before passing these arguments along to the program. It is best to get into the habit of enclosing pattern argument in single-quotes when invoking grep to ensure the pattern is received as intended.

There are also some helpful flags you can use when executing grep:

• Adding the --color flag (grep --color) will highlight the matched portion of the line in red. The red highlight makes it possible to see exactly which part of the string is matched.
• The -i flag (grep -i) makes the search case-insensitive.
• The -v flag (grep -v) negates the search, so it will respond with everything that doesn't match the pattern.