Grep this

The grep utility, which allows files to be searched for strings of words, uses a syntax similar to the regular expression syntax of the vi, ex, ed, and sed editors. grep comes in three flavors, grep, fgrep, and egrep, all of which I'll cover in this article.

The name grep is derived from the editor command g/re/p, which literally translates to "globally search for a regular wxpression and print what you find." Regular expressions are at the core of grep, and I'll cover them after a brief description of some of the utility's command options.

The simplest grep command is grep (search pattern) (files list), as in:

 grep hello *

The output of this command might be something like this:

 $ grep hello *
 story.txt: so I said hello and she smiled back
 intro.txt: use the hello.c program as an example of C programming
 $

grep is case sensitive, so in order to change the search to include "hello," "Hello," or "HELLO," use the -y or -i option. Earlier versions of grep used -y, and later versions use -i. -y is now considered obsolete, although some versions of grep do support both. In the following example, more hellos show up because the search is case independent.

 $ grep -i hello *
 story.txt: so I said hello and she smiled back
 story.txt: I could hear my echo, "HELLO."
 intro.txt: use the hello.c program as an example of C programming
 hello.c:     printf("Hello, world. \n");
 $

This command searches all files in the current directory and prints the file name and the line containing the string "hello" for any files that contain that string.

The output of grep varies depending on whether you're searching one or several files. If only one file is named on the command line, the output doesn't include the file name, as in the following example:

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