Ten Steps to Linux Survival

Chapter 5. Step 5: Search Me

In the preceding chapter, you learned to search for files by their attributes, such as name, last modified time, and the like. In this chapter, I show how to search inside a file, perhaps to find a specific error message.

Getting a grep

The grep command (whose name comes from globally search a regular expression and print) searches within files. It uses regular expressions (regex) to match patterns inside the files. It can be used to search within binary files, but is most useful for finding things inside text files. There are lots of uses for this command in our crisis scenario, such as searching for certain error messages within log files, or finding every mention of a certain resource inside the source files for an entire website.

There is an old joke by Jamie Zawinski:

Some people, when confronted with a problem, think, “I know, I’ll use regular expressions.” Now they have two problems.

Some regular expressions are simple—for example, *, which you should recognize as a valid wildcard in Windows Command Prompt. Others can be mind-blowingly complex. For example:

^\(*\d{3}\)*( |-)*\d{3}( |-)*\d{4}$

This regular expression is an (incomplete) approach to matching US phone numbers.

Because regexes are so inscrutable, sometimes I write a regex in a program or a script, come back to it six months later, and have no idea what it is doing. (Now I have two problems.) In this chapter, you’re just going to look at a few simple examples.

Here are some samples ...