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
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