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3.3.3.4. The grep command

3.3.3.4.1. General line filtering

A simple but powerful program, grep is used for filtering input lines and returning certain patterns to the output. There are literally thousands of applications for the grep program. In the example below, jerry uses grep to see how he did the thing with find:


jerry:~> grep -a find .bash_history
find . -name userinfo
man find
find ../ -name common.cfg

Tip Search history
 

Also useful in these cases is the search function in bash, activated by pressing Ctrl+R at once, such as in the example where we want to check how we did that last find again:


thomas ~> ^R 
(reverse-i-search)`find': find `/home/thomas` -name *.xml

Type your search string at the search prompt. The more characters you type, the more restricted the search gets. This reads the command history for this shell session (which is written to .bash_history in your home directory when you quit that session). The most recent occurrence of your search string is shown. If you want to see previous commands containing the same string, type Ctrl+R again.

See the Info pages on bash for more.

All UNIXes with just a little bit of decency have an online dictionary. So does Linux. The dictionary is a list of known words in a file named words, located in /usr/share/dict. To quickly check the correct spelling of a word, no graphical application is needed:


william:~> grep pinguin /usr/share/dict/words

william:~> grep penguin /usr/share/dict/words
penguin
penguins

Tip Dictionary vs. word list
 

Some distributions offer the dict command, which offers more features than simply searching words in a list.

Who is the owner of that home directory next to mine? Hey, there's his telephone number!


lisa:~> grep gdbruyne /etc/passwd
gdbruyne:x:981:981:Guy Debruyne, tel 203234:/home/gdbruyne:/bin/bash

And what was the E-mail address of Arno again?


serge:~/mail> grep -i arno *
sent-mail: To: <[email protected]>
sent-mail: On Mon, 24 Dec 2001, [email protected] wrote:

find and locate are often used in combination with grep to define some serious queries. For more information, see Chapter 5 on I/O redirection.

3.3.3.4.2. Special characters

Characters that have a special meaning to the shell have to be escaped. The escape character in Bash is backslash, as in most shells; this takes away the special meaning of the following character. The shell knows about quite some special characters, among the most common /, ., ? and *. A full list can be found in the Info pages and documentation for your shell.

For instance, say that you want to display the file "*" instead of all the files in a directory, you would have to use

less \*

The same goes for filenames containing a space:

cat This\ File

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