Using grep

When I first started working in systems integration, I was primarily a PC support person. I spent a lot of time installing and supporting Windows applications in various PC LAN configurations, running various versions (and vendors) of TCP/IP transports. Since then, I have successfully ditched DOS and moved on. Now, after working on various versions of Unix for a few years, I porting some of our networking and data manipulation libraries to other platforms and environments, such as the AS/400 minicomputer and the Macintosh. This ongoing experience has given me a chance to appreciate just how powerful the tools we take for granted with Linux really are.

Searching for a word (or any other value) in a group of files is a very common task. Whether it's searching for a function in a group of source code modules, trying to find a parameter in a set of configuration files, or simply looking for a misplaced e-mail message, text searching and matching operations are common in all environments.

Unfortunately, this common task doesn't have an easy solution on all platforms. On most, the best solution available is to use the search function in an editor. But when it comes to Linux (and other Unix descendants), you have many solutions. One of them is grep.

grep is an acronym for “global regular expression print,” a reference to the command in the old ed line editor that prints all of the lines in a file containing a specified sequence of characters. grep does exactly that: it prints out lines in a file that contain a match for a regular expression. We'll gradually delve into what a regular expression is as we go on.

First, let's look at a quick example. We will search for a word in the Configure script provided with Linux for setting up the Linux kernel source, which is usually installed in the /usr/src/linux directory. Change to that directory and type (the $ character is the prompt, don't type it):

$ grep glob Configure

You should see:

# Disable filename globbing once and for all.

glob is in bold to illustrate what grep matched. grep does not actually print matches in bold.

grep looked for the sequence of characters glob and printed the line of the Configure file with that sequence. It did not look for the word glob. It looked for g followed by l followed by o followed by b. This points out one important aspect of regular expressions: they match sequences of characters, not words.

Before we dig any deeper into the specifics of pattern matching, let's look at grep's “user interface” with a few examples. Try the following two commands:

$ grep glob < Configure
$ cat Configure | grep glob

both of these two commands should print

# Disable filename globbing once and for all.

which probably looks familiar.

In all of these commands, we have specified the regular expression as the first argument to grep. With the exception of any command line switches, grep always expects the regular expression as the first argument.

However, we presented grep with three different situations and received the same response. In the first exercise, we provided grep with the name of a file, and it opened that file and searched it. grep can also take a list of filenames to search.

In the other two exercises we illustrated a feature that grep shares with many other utilities. If no files are specified on the command line, grep reads standard input. To further illustrate standard input let's try one more example:

$ grep foo

When you run that, grep appears to “hang” waiting for something. It is. It's waiting for input. Type:

tttt

and press return. Nothing happens. Now type:

foobar

and press enter. This time, grep sees the string foo in foobar and echos the line foobar back at you, which is why foobar appears twice. Now type ctrl-d, the “end-of-file” character, to tell grep that it has reached the end of the file, whereupon it exits.

You just gave grep an input file that consisted of tttt, a newline character, foobar, a newline character, and the end-of-file character.

Piping input into grep from standard input also has another frequent use: filtering the output of other commands. Sometimes cutting out the unnecessary lines with grep is more convenient than reading output page by page with more or less:

$ ps ax | grep cron

efficiently gives you the process information for crond.