Linux basic command text filtering grep

It is often necessary to filter text or output content in Linux. The most commonly used filtering command is grep

grep [OPTIONS] PATTERN [FILE...]

grep retrieves each input line line by line. If the input line contains the pattern PATTERN, this line is output. The PATTERN here is a regular expression (refer to the previous article, this article will use grep as an example).

Output the line containing root in the file /etc/passwd:

[root@centos7 temp]# grep root /etc/passwd
root:x:0:0:root:/root:/bin/bash
operator:x:11:0:operator:/root:/sbin/nologin

Or get it from the standard input:

[root@centos7 temp]# cat /etc/passwd | grep root
root:x:0:0:root:/root:/bin/bash
operator:x:11:0:operator:/root:/sbin/nologin

What needs to be noted is that when grep’s input comes from both a file and standard input, grep will ignore the standard input The content will not be processed unless the symbol - is used to represent the standard input:

[root@centos7 temp]# cat /etc/passwd | grep root
root:x:0:0:root:/root:/bin/bash
operator:x:11:0:operator:/root:/sbin/nologin

At this time, grep will indicate which results come from the file and which come from the standard input.

Output the lines starting with root in the file /etc/passwd and the file /etc/group:

[root@centos7 temp]# cat /etc/passwd | grep root /etc/passwd -
/etc/passwd:root:x:0:0:root:/root:/bin/bash
/etc/passwd:operator:x:11:0:operator:/root:/sbin/nologin
(标准输入):root:x:0:0:root:/root:/bin/bash
(标准输入):operator:x:11:0:operator:/root:/sbin/nologin

At this time, grep will indicate which results come from the file and which come from the standard input.

Output the lines starting with root in the file /etc/passwd and the file /etc/group:

[root@centos7 temp]# grep "^root" /etc/passwd /etc/group
/etc/passwd:root:x:0:0:root:/root:/bin/bash
/etc/group:root:x:0:

Output the lines ending with /bin/bash in the file /etc/passwd:

[root@centos7 temp]# grep "/bin/bash$" /etc/passwd
root:x:0:0:root:/root:/bin/bash
learner:x:1000:1000::/home/learner:/bin/bash

Note the PATTERN in the above two examples Quoted in double quotes to prevent parsing by the shell.

Output the lines in the file /etc/passwd that do not start with any of the letters in a-s:

[root@centos7 temp]# grep "^[^a-s]" /etc/passwd 
tss:x:59:59:Account used by the trousers package to sandbox the tcsd daemon:/dev/null:/sbin/nologin
tcpdump:x:72:72::/:/sbin/nologin

You need to understand the different meanings between the two ^ here. The first ^ represents the beginning of the line, and the second one is inside [] The first character ^ means negation.

Output the lines in the file /etc/passwd where the character 0 appears three times or more in a row (note the escape character ''):

[root@centos7 temp]# grep "0\{3,\}" /etc/passwd
learner:x:1000:1000::/home/learner:/bin/bash

For example, output the lines starting with the character r or l in the file /etc/passwd:

[root@centos7 temp]# grep "^[r,l]" /etc/passwd
root:x:0:0:root:/root:/bin/bash
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
learner:x:1000:1000::/home/learner:/bin/bash

Option -i causes grep to ignore case when matching patterns:

[root@centos7 temp]# grep -i abcd file 
ABCD
function abcd() {
[root@centos7 temp]#

Option -o means to output only matching characters, not the entire line:

[root@centos7 temp]# grep -oi abcd file 
ABCD
abcd
[root@centos7 temp]#

Option -c counts the number of matching lines:

[root@centos7 temp]# grep -oic abcd file 
2
[root@centos7 temp]#

Option -v Indicates inverse matching, such as outputting lines in /etc/passwd that do not end with /sbin/nologin:

[root@centos7 temp]# grep -v "/sbin/nologin$" /etc/passwd
root:x:0:0:root:/root:/bin/bash
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
learner:x:1000:1000::/home/learner:/bin/bash

Option -f FILE means using each line in the file FILE as a pattern match:

[root@centos7 temp]# cat test
abcd
ABCD
[root@centos7 temp]# grep -f test file 
ABCD
function abcd() {
[root@centos7 temp]#

Option -x means the entire line Match:

[root@centos7 temp]# grep -xf test file 
ABCD
[root@centos7 temp]#

Option-w means to match the entire word:

[root@centos7 temp]# grep here file
here
there
[root@centos7 temp]# grep -w here file
here
[root@centos7 temp]#

Option-h means not to output the file name when there are multiple files:

[root@centos7 temp]# cat /etc/passwd|grep ^root - /etc/passwd -h
root:x:0:0:root:/root:/bin/bash
root:x:0:0:root:/root:/bin/bash

Option-n means to display line numbers:

[root@centos7 temp]# grep -n "^[r,l]" /etc/passwd
1:root:x:0:0:root:/root:/bin/bash
5:lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
24:learner:x:1000:1000::/home/learner:/bin/bash

Option-A N, -B N , -C N means output matching lines and their 'surrounding lines'

-A N 表示输出匹配行和其之后(after)的N行
-B N 表示输出匹配行和其之前(before)的N行
-C N 表示输出匹配行和其之前之后各N行
[root@centos7 temp]# grep -A 2 ^operator /etc/passwd
operator:x:11:0:operator:/root:/sbin/nologin
games:x:12:100:games:/usr/games:/sbin/nologin
ftp:x:14:50:FTP User:/var/ftp:/sbin/nologin
[root@centos7 temp]# grep -B2 ^operator /etc/passwd   
halt:x:7:0:halt:/sbin:/sbin/halt
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
operator:x:11:0:operator:/root:/sbin/nologin
[root@centos7 temp]# grep -C1 ^operator /etc/passwd  
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
operator:x:11:0:operator:/root:/sbin/nologin
games:x:12:100:games:/usr/games:/sbin/nologin

option -F treats PATTERN as its literal match (ignoring the special meaning of the characters), which is equivalent to executing the command fgrep:

[root@centos7 temp]# grep -F ^root /etc/passwd
[root@centos7 temp]#

The command has no output

option -E can use extended regular expressions, just like executing the egrep command:

[root@centos7 temp]# egrep "^root|^learner" /etc/passwd
root:x:0:0:root:/root:/bin/bash
learner:x:1000:1000::/home/learner:/bin/bash

Using extended regular expressions means that special meanings of characters can be expressed without escaping, including ?, +, {, |, (and).

The option -P means to use perl's regular expression for matching
For example:

[root@centos7 ~]# echo "helloworld123456"| grep -oP "\d+"
123456
[root@centos7 ~]#

"d" in perl regular expression means a number, and + means matching one to multiple times (same as vim).

Option -a treats binary files as text files:

[root@centos7 ~]# grep -a online /usr/bin/ls
%s online help: <%s>
[root@centos7 ~]#

Options --exclude=GLOB and --include=GLOB mean to exclude and include files matching GLOB respectively, GLOB means wildcard (see the basic command introduction for find and xargs usage) 3):

[root@centos7 temp]# find . -type f | xargs grep --exclude=*.txt --include=test* bash
./test.sh:#!/bin/bash
[root@centos7 temp]#

grep’s powerful filtering capabilities come from the combination of various options and regular expressions