正则表达式，就是一个字符串。有一定的规律。我们用指定的字符串匹配一个指定的行。指定的字符串就是正则表达式。
正则表达式有这几个工具：grep egrep sed awk
命令：gerep
说明：过滤出指定的行
选项：--color 关键字有颜色
-n 显示行号
-c 显示一共出现了多少行
-v 取反 不包含指定字符的行
-A n n指数字 例如A2在有指定字符的行下面再显示两行
-B n n指数字 例如B2 在有指定字符的行上面再显示两行
-C n n指数字 例如C2 在有指定字符的行上面和下面再显示各两行
-r 显示目录里的所以带指定字符的行
-rh 显示目录里的所以带指定字符的行并不显示文件路径和文件名
grep 过滤出有root的行
[[email protected] ~]# grep --color ‘root‘ /etc/passwd
root:x:0:0:root:/root:/bin/bash
operator:x:11:0:operator:/root:/sbin/nologin
grep -n
[[email protected] ~]# grep -n ‘root‘ /etc/passwd
1:root:x:0:0:root:/root:/bin/bash
11:operator:x:11:0:operator:/root:/sbin/nologin
给grep创建个别名，把/etc/passwd拷贝一下
[[email protected] ~]# vim .bashrc
alias cg=‘grep --color’
[[email protected] ~]# cp /etc/passwd 1.txt
cp：是否覆盖"1.txt"？ y
-c
[[email protected] ~]# cg -c ‘root‘ 1.txt
2
-v
[[email protected] ~]# cg -v ‘root‘ 1.txt
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
....
-A
[[email protected] ~]# cg -n -A 2 ‘root‘ 1.txt
1:root:x:0:0:root:/root:/bin/bash
2-bin:x:1:1:bin:/bin:/sbin/nologin
3-daemon:x:2:2:daemon:/sbin:/sbin/nologin
--
11:operator:x:11:0:operator:/root:/sbin/nologin
12-games:x:12:100:games:/usr/games:/sbin/nologin
13-gopher:x:13:30:gopher:/var/gopher:/sbin/nologin
-B
[[email protected] ~]# cg -n -B1 ‘root‘ 1.txt
1:root:x:0:0:root:/root:/bin/bash
--
10-uucp:x:10:14:uucp:/var/spool/uucp:/sbin/nologin
11:operator:x:11:0:operator:/root:/sbin/nologin
-C
[[email protected] ~]# cg -n -C1 ‘root‘ 1.txt
1:root:x:0:0:root:/root:/bin/bash
2-bin:x:1:1:bin:/bin:/sbin/nologin
--
10-uucp:x:10:14:uucp:/var/spool/uucp:/sbin/nologin
11:operator:x:11:0:operator:/root:/sbin/nologin
12-games:x:12:100:games:/usr/games:/sbin/nologin
-r
[[email protected] ~]# cg -r ‘root‘ ~
.....
/root/.bash_history:HOME=/root
/root/.bash_history:grep ‘root‘ /etc/passwd
/root/.bash_history:grep -n ‘root‘ 1.txt
/root/.bash_history:grep -c ‘root‘ 1.txt
/root/.bash_history:grep -v ‘root‘ 1.txt
/root/.bash_history:grep -A2 ‘root‘ 1.txt
/root/.bash_history:grep -n -A2 ‘root‘ 1.txt
/root/.bash_history:grep -n -b2 ‘root‘ 1.txt
/root/.bash_history:grep ‘root‘ /etc/passwd
/root/.bash_history:grep -n ‘root‘ /etc/passwd
/root/.bash_history:cg -c ‘root‘ 1.txt
.......
[[email protected] ~]# cg -rh ‘root‘ ~
....
HOME=/root
grep ‘root‘ /etc/passwd
grep -n ‘root‘ 1.txt
grep -c ‘root‘ 1.txt
grep -v ‘root‘ 1.txt
grep -A2 ‘root‘ 1.txt
grep -n -A2 ‘root‘ 1.txt
grep -n -b2 ‘root‘ 1.txt
grep ‘root‘ /etc/passwd
grep -n ‘root‘ /etc/passwd
cg -c ‘root‘ 1.txt
......
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总结：vim ~/.bashrc /// alias cg=‘grep --color’
cg -n -v -c -r -rh -An -Bn -Cn