About grep Regular Expressions in Linux, grep Regular Expressions

About grep Regular Expressions in Linux, grep Regular Expressions

Wildcard

* Any character can be repeated multiple times

Any character, repeat once

[] Represents a character

For example, [a, B, c] indicates any one of abc.

Wildcard is used to match the file name

Regular Expression

Regular expressions are used to match matching strings in the file.

Ls find cp does not support regular expressions

However, grep awk sed supports regular expressions.

[Root @ Hadoop-bigdata01 test] # touch aa

[Root @ hadoop-bigdata01 test] # touch aab aabb

[Root @ hadoop-bigdata01 test] # ll

Total 0

-Rw-r -- 1 root 0 May 16 :47 aa

-Rw-r -- 1 root 0 May 16 :47 aab

-Rw-r -- 1 root 0 May 16 :47 aabb

[Root @ hadoop-bigdata01 test] # ls aa

Aa

[Root @ hadoop-bigdata01 test] # ls aa

Aab

[Root @ hadoop-bigdata01 test] # ls aa *

Aa aab aabb

Special characters in Regular Expression

Regular Expression matching range

Use Regular Expressions

Grep "1"/etc/passwd

For a row that contains the keyword 1, grep only needs to contain the line. If you do not want a wildcard, it must be completely consistent.

[Root @ hadoop-bigdata01 test] # grep "1"/etc/passwd

Bin: x: 1: 1: bin:/sbin/nologin

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

Messages: x: 81: 81: System message bus: // sbin/nologin

Usbmuxd: x: 113: 113: usbmuxd user: // sbin/nologin

Avahi-autoipd: x: 170: 170: Avahi IPv4LL Stack:/var/lib/avahi-autoipd:/sbin/nologin

Abrt: x: 173: 173:/etc/abrt:/sbin/nologin

Wang: x: 501: 501:/home/wang:/bin/bash

Grep 'root'/etc/passwd

Cat/etc/passwd | grep 'root'

The same is true, but pipeline operators are more resource-consuming.

So

1. Match rows containing numbers

Grep '[0-9]'/etc/passwd

2. Match rows that contain three numbers consecutively

Grep '[0-9] [0-9] [0-9]'/etc/passwd or grep ': [0-9] [0-9] [0-9]: '/etc/passwd

[Root @ hadoop-bigdata01 test] # grep '[0-9] [0-9] [0-9]'/etc/passwd

Games: x: 12: 100: games:/usr/games:/sbin/nologin

Usbmuxd: x: 113: 113: usbmuxd user: // sbin/nologin

Rtkit: x: 499: 497: RealtimeKit:/proc:/sbin/nologin

Avahi-autoipd: x: 170: 170: Avahi IPv4LL Stack:/var/lib/avahi-autoipd:/sbin/nologin

Abrt: x: 173: 173:/etc/abrt:/sbin/nologin

Nfsnobody: x: 65534: 65534: Anonymous NFS User:/var/lib/nfs:/sbin/nologin

Saslauth: x: 498: 76: "Saslauthd user":/var/empty/saslauth:/sbin/nologin

Pulse: x: 497: 496: PulseAudio System Daemon:/var/run/pulse:/sbin/nologin

Liucheng: x: 500: 500:/home/liucheng:/bin/bash

Wang: x: 501: 501:/home/wang:/bin/bas

3. Match rows starting with "r" and ending with "n"

Grep '^ r. * n $'/etc/passwd

. * Indicates all

[Root @ hadoop-bigdata01 test] # grep '^ r. * n $'/etc/passwd

Rpc: x: 32: 32: Rpcbind Daemon:/var/cache/rpcbind:/sbin/nologin

Rtkit: x: 499: 497: RealtimeKit:/proc:/sbin/nologin

Rpcuser: x: 29: 29: RPC Service User:/var/lib/nfs:/sbin/nologin

4. Filter ifconfig and intercept ip addresses

Grep-v indicates reverse truncation, which means to remove the line sed with a keyword.

[Root @ hadoop-bigdata01 test] # ifconfig | grep 'inet addr :'

Inet addr: 192.168.126.191 Bcast: 192.168.126.255 Mask: 255.255.255.0

Inet addr: 127.0.0.1 Mask: 255.0.0.0

[Root @ hadoop-bigdata01 test] #

[Root @ hadoop-bigdata01 test] # ifconfig | grep 'inet addr: '| grep-v '2017. 0.0.1'

Inet addr: 192.168.126.191 Bcast: 192.168.126.255 Mask: 255.255.255.0

[Root @ hadoop-bigdata01 test] # ifconfig | grep 'inet addr: '| grep-v '2017. 0.0.1' | sed's/inet addr: // G'

192.168.126.191 Bcast: 192.168.126.255 Mask: 255.255.255.0

[Root @ hadoop-bigdata01 test] # ifconfig | grep 'inet addr: '| grep-V' 127. 0.0.1 '| sed's/inet addr: // G' | sed's/Bcast. * // G'

192.168.126.191

Misunderstanding

There is a misunderstanding here. For a long time, it is the difference between regular expressions and wildcards.

We know that wildcard * refers to any character, and repeated Regular Expressions * refer to matching the previous character> = 0

The two are completely different. How do I know whether * is a wildcard or a regular expression?

At first I got into a misunderstanding. Let's look at the following commands:

[Root @ hadoop-bigdata01 test] # touch ac aac abc abbc

[Root @ hadoop-bigdata01 test] # ll

Total 0

-Rw-r -- 1 root 0 May 16 :55 aac

-Rw-r -- 1 root 0 May 16 :55 abbc

-Rw-r -- 1 root 0 May 16 19:55 abc

-Rw-r -- 1 root 0 May 16 19:55 ac

[Root @ hadoop-bigdata01 test] # ls | grep 'a * C'

Aac

Abbc

Abc

Ac

[Root @ hadoop-bigdata01 test] # ls | grep 'a. * C'

Aac

Abbc

Abc

Ac

[Root @ hadoop-bigdata01 test] # ls | grep '^ a. * C'

Aac

Abbc

Abc

Ac

[Root @ hadoop-bigdata01 test] # ls | grep '^ a * C'

Aac

Ac

Why is the result of grep 'a * C' and grep '^ a * c $' different? I think one is a wildcard and the other is a regular expression, because a * c displays four results, exactly

Isn't it a match for any number of characters?

Otherwise

Wildcard is used to match the file name

Regular expressions are used to match matching strings in the file.

After being handed over to the pipeline operator, grep is no longer used to match the file name. This is an operation on the file, so it is completely a regular expression.

Grep 'a * C' indicates matching a> = 0, so it is acceptable if c is contained.

Grep '^ a * c $' is also a regular expression that begins with a, and the second character matches a zero or multiple times, followed by c letters

Therefore, only aac and ac meet the conditions.

So let's look at this example.

[Root @ hadoop-bigdata01 test] # ls

A aac abb abbc abc ac B bb c cb

[Root @ hadoop-bigdata01 test] # ls | grep 'a * B'

Abb

Abbc

Abc

B

Bb

Cb

Here, grep 'a * B 'means that a does not contain a and B, but a repeats 0 times or multiple times and then contains B <