Regular expressions are special text strings that used to search for and match patterns in text. To make the search expression more specific, it can work together with the grep command. The grep command is the General Regular Expression Parser; it searches a file for strings matching a given regular expression, and by default it the prints out any line containing a string that matches. There are many useful options which can be set for grep which affect it output. This examples will show how to use caret ^ and dollar sign $ to print more specific output. This examples has been tested on Redhat Enterprise Linux 6 server. It may works on CentOS as well.
The caret ^ is meta-characters that respectively match the empty string at the beginning of a line.
Anchor : line begins with...
Meanwhile, the dollar sign $ is a meta-characters that respectively match the empty string at the end of a line.
Anchor : line ends with...
1. Print all usernames that begin with the letter e :
[[email protected] ~]# grep '^e' /etc/passwd ehowstuff:x:503:503::/home/ehowstuff:/bin/bash
2. Print all usernames that begin with the letter g :
[[email protected] ~]# grep '^g' /etc/passwd games:x:12:100:games:/usr/games:/sbin/nologin gopher:x:13:30:gopher:/var/gopher:/sbin/nologin
3. Print all usernames that begin with the letter a :
[[email protected] ~]# grep '^a' /etc/passwd adm:x:3:4:adm:/var/adm:/sbin/nologin abrt:x:499:499::/etc/abrt:/sbin/nologin apache:x:48:48:Apache:/var/www:/sbin/nologin
4. Print all lines that end with the letter h :
[[email protected] ~]# grep 'h$' /etc/passwd root:x:0:0:root:/root:/bin/bash mysql:x:27:27:MySQL Server:/var/lib/mysql:/bin/bash test:x:500:500::/home/test:/bin/bash sambauser1:x:501:501::/home/sambauser1:/bin/bash ftpuser:x:502:502::/home/ftpuser:/bin/bash ehowstuff:x:503:503::/home/ehowstuff:/bin/bash testuser:x:504:504::/home/testuser:/bin/bash