grep Regular Expression

First, preface

grep is a powerful text processing tool, full name: Global search Regular expression and print out the Line,grep line uses regular expression to match text. The entire line of matching text is then displayed (unless you use a specific option, such as GREP-V).

Second, grep usage

grep command usage:

grep [OPTIONS] PATTERN [FILE ...]

grep [OPTIONS] [-E PATTERN |-f file] [FILE ...]

To use grep usage, we have to learn the following simple expressions

2.1. Usage of regular expressions

First, we need to understand what a regular expression is. The so-called regular expression is the use of a single string to describe and match a series of strings that conform to a certain grammatical rule. Regular expressions are made up of some common character and metacharacters characters. Ordinary characters include uppercase and lowercase letters and numbers, while metacharacters have special meanings.

Meta-character usage for 2.1.1 and basic regular expressions

A: Character matching

.	Match any single character				For example: grep ' roo. '/etc/passwd
[]	Matches any single character within the specified range				For example: grep ' [ro] '/etc/passwd
[^]	Take counter				For example: grep ' [^ro] '/etc/passwd

B: Number of matches

*	Match any number of times				For example: grep ' roo* '/etc/passwd
\?	Matches the preceding character appears 0 or 1 times				For example: grep ' roo\? '/etc/passwd
\{m\}	Matches its preceding character appears m times				For example: grep ' ro\{2\} '/etc/passwd
\{m,\}	Match its preceding characters to appear at least m times				For example: grep ' ro\{2,\} '/etc/passwd
\{m,n\}	Match its preceding characters to appear at least m times, up to N times				For example: grep ' ro\{2,3\} '/etc/passwd
\{0,n\}	Matches its preceding character up to n times				For example: grep ' ro\{0,2\} '/etc/passwd

C: Position anchor match

^	Beginning of line anchor character	For example: grep ' ^root '/etc/passwd
$	End of line anchor character	For example: grep ' shell$ '/etc/passwd
\<	The first anchor character of the word	For example: grep ' \<root '/etc/passwd
\>	Ending anchor	For example: grep ' shell\> '/etc/passwd

D: Grouping

\(\)

Grouping, the contents of a pattern match in a group can be referenced

For example: grep ' \ (root\). *\1 '/etc/passwd

E: Reference

\#

Reference Group # of content (#为数字)

For example: grep ' \ (root\). *\1 '/etc/passwd

2.1.2, extended regular expression meta-character usage

A: Character matching

.	Match any single character	For example: Egrep ' roo. '/etc/passwd
[]	Matches any single character within the specified range	Example: Egrep ' [ro] '/etc/passwd
[^]	Take counter	Example: Egrep ' [^ro] '/etc/passwd

B: Number of matches

*	Match any number of times	Example: Egrep ' roo* '/etc/passwd
+	Match preceding character appears more than 1 times	Example: Egrep ' root '/etc/passwd
?	Matches the preceding character appears 0 or 1 times	For example: Egrep ' roo? '/etc/passwd
{m}	Matches its preceding character appears m times	Example: Egrep ' ro{2} '/etc/passwd
{m,}	Match its preceding characters to appear at least m times	For example: Egrep ' ro{2,} '/etc/passwd
{M,n}	Match its preceding characters to appear at least m times, up to N times	Example: Egrep ' ro{2,3} '/etc/passwd
{0,n}	Matches its preceding character up to n times	Example: Egrep ' ro{0,2} '/etc/passwd

C: Position anchor match

^	Beginning of line anchor character	Example: Egrep ' ^root '/etc/passwd
$	End of line anchor character	Example: Egrep ' shell$ '/etc/passwd
\<	The first anchor character of the word	Example: Egrep ' \<root '/etc/passwd
\>	Ending anchor	Example: Egrep ' shell\> '/etc/passwd

D: Grouping

( )

Grouping, the contents of a pattern match in a group can be referenced

For example: Egrep ' (root\). *\1 '/etc/passwd

E: Or

|

Or

Example: Egrep ' Bash|nologin '/etc/passwd

F: Reference

\number

Reference the first few matches

For example grep ' (a). *\1 '/etc/passwd

2.3 grep Common options

-V	Reverse Selection	Example: Grep-v ' root '/etc/passwd
-O	Only the matching string itself is displayed, not the row	Example: Grep-o ' root '/etc/passwd
-I.	Ignore case	Example: Grep-i ' root '/etc/passwd
-E	Supports the use of an extended regular expression	For example: Grep-e ' (root). *\1 '/etc/passwd
-A	Back n rows	Example: Grep-a 3 ' root '/etc/passwd
-B	Front n rows	Example: Grep-b 3 ' mysql '/etc/passwd
-C	n rows before and after each	Example: Grep-c 3 ' mysql '/etc/passwd

Third, examples

3.1. Display the lines in the/etc/passwd file that end with bash

grep ' \<bash\>$ '/etc/passwd root:x:0:0:root:/root:/bin/bash leon:x:500:4400::/home/leon:/bin/bash Mandirva : X:2200:4400::/home/mandirva:/bin/bash ... centos:x:2241:2241::/home/centos:/bin/bash user1:x:2242:2242::/home /user1:/bin/bash

3.2, displays the two-digit or three-digit number in the/etc/passwd file

grep  ' \<[[:d igit:]]\{2,3\}\> '  /etc/passwd    mail:x:8:12:mail:/var/spool/ mail:/sbin/nologin    uucp:x:10:14:uucp:/var/spool/uucp:/sbin/nologin     operator:x:11:0:operator:/root:/sbin/nologin    games:x:12:100:games:/usr/games:/ Sbin/nologin    gopher:x:13:30:gopher:/var/gopher:/sbin/nologin    ftp : X:14:50:ftp user:/var/ftp:/sbin/nologin    nobody:x:99:99:nobody:/:/sbin/nologin     vcsa:x:69:69:virtual console memory owner:/dev:/sbin/nologin     saslauth:x:499:76: "Saslauthd user":/var/empty/saslauth:/sbin/nologin    &NBSP;POSTFIX:X:89:89::/VAR/SPOOL/POSTFIX:/SBIN/NOLOGIN&NBSP;&NBSP;&NBSP;&NBSP;SSHD:X:74:74: Privilege-separated ssh:/var/empty/sshd:/sbin/nologin    leon:x:500:4400::/home/leon :/bin/bash    openstack:x:501:501:this ' S openstack:/home/openstack:/bin/bash 

3.3, display the ' netstat-tan ' command result with ' LISTEN ' followed by 0, one or more whitespace characters end of the line

netstat -tan | grep  ' LISTEN ' [[:space:]]*     tcp         0      0 0.0.0.0:22                   0.0.0.0:*                     listen          tcp         0      0 127.0.0.1:25                 0.0.0.0:*                    LISTEN           tcp        0       0 :::22                        :::*                          LISTEN           tcp        0      0 : :1:25                       :::*                          listen

3.4. Add user bash, Testbash, basher, and Nologin user (Nologin user's shell is/sbin/nologin), then find the same line in the/etc/passwd file as the user name and its shell name

grep ' ^\ ([[: Alnum:]]*\):. *\1$ '/etc/passwd sync:x:5:0:sync:/sbin:/bin/sync shutdown:x:6:0:shutdown:/sbin:/sbin/ Shutdown Halt:x:7:0:halt:/sbin:/sbin/halt Nologin:x:2208:2208::/home/nologin:/sbin/nologin

3.5. Display the default shell and UID of root, CentOS, or User1 user on the current system (please create these users beforehand, if not present)

Grep-e ' ^root|^centos|^user1 '/etc/passwd | cut-d:-f1,3,7 root:0:/bin/bash centos:2241:/bin/bash User1:2242:/bin/bash

3.6, find a word in the/etc/rc.d/init.d/functions file (the middle of the word can be underlined) followed by a set of parentheses line

Grep-e ' \<[[:alpha:]]+_?\ (\) '/etc/rc.d/init.d/functions checkpid () {daemon () {Killproc () {PIDFILEOFPR  OC () {Pidofproc () {status () {success () {failure ()} {passed () {warning () {action () {strstr ()} {Confirm () {

3.7, use echo to output a path, and then egrep find its path base name; Further use Egrep to remove its directory name

echo/etc/passwd | Egrep-o ' [[: alnum:]]+$ ' passwdecho/etc/passwd | Egrep-o ' ^ (/). *\1 '/etc/

3.8. Find the number between 1-255 in the result of ifconfig command execution

Ifconfig | Grep-eo ' \< ([1-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5]) \> ' | Sort | Uniq 1 2 6 8 27 41 56 64 75 101 106 127 128 168 192 255

This article is from the "leon91" blog, make sure to keep this source http://leon91.blog.51cto.com/3639216/1690755

grep Regular Expression