12.1 grep
egrep：扩展正则表达式
fgrep（快速的搜索）: 不支持正则表达式，直接搜索字符本身
grep: 根据模式搜索文本，并将符合模式的文本行显示出来。
Pattern（模式）: 文本字符和正则表达式的元字符组合而成的匹配条件
grep [options] PATTERN [FILE...]
-i：忽略字母大小写
--color：对于被选中的目标用颜色显示
-v: 显示没有被模式匹配到的行
-o：只显示被模式匹配到的字符串
12.2 正则表达式（REGular EXPerssion,REGEXP）
通常使用的匹配的字符
*: 匹配任意长度的任意字符
?: 任意单个字符
[]:指定范围内的内容
[^]:制定范围外的内容
12.2.1 正则表达式的元字符：
1、元字符
. : 匹配任意单个字符
[]: 匹配指定范围内的任意单个字符
[^]：匹配指定范围外的任意单个字符
字符集合：[:digit:], [:lower:], [:upper:], [:punct:], [:space:], [:alpha:], [:alnum:]
[root@Daniel-R480 ~]# grep 'r..t' /etc/passwd
root:x:0:0:root:/root:/bin/bash
operator:x:11:0:operator:/root:/sbin/nologin
ftp:x:14:50:FTP User:/var/ftp:/sbin/nologin
[root@Daniel-R480 ~]#
2、匹配次数（贪婪模式，尽可能多的匹配）：
*: 匹配其前面的字符任意次
a, b, ab, aab, acb, adb, amnb
a*b：a出现任意次之后跟着b
[root@Daniel-R480 ~]# grep 'a*b' test.txt
b
ab
asb
aab
abb
ammjdjfdjb
asdbsdabsfdab
a?b：匹配a任意次，然后加上b
[root@Daniel-R480 ~]# grep 'a\?b' test.txt
b
ab
asb
aab
abb
ammjdjfdjb
asdbsdabsfdab
a.*b：a开头，b结尾，中间跟任意字符
[root@Daniel-R480 ~]# grep 'a.*b' test.txt
ab
asb
aab
abb
ammjdjfdjb
asdbsdabsfdab
3、匹配规则
.*: 任意长度的任意字符
\?: 匹配其前面的字符1次或0次
\{m,n\}:匹配其前面的字符至少m次，至多n次
\{1,\}
\{0,3\}
[root@Daniel-R480 ~]# grep 'a\{1,3\}b' test.txt
ab
aab
abb
asdbsdabsfdab
[root@Daniel-R480 ~]# grep 'a.\{1,3\}b' test.txt
asb
aab
abb
asdbsdabsfdab
[root@Daniel-R480 ~]#
4、位置锚定：
^: 锚定行首，此字符后面的任意内容必须出现在行首
$: 锚定行尾，此字符前面的任意内容必须出现在行尾
^$: 空白行
\<或\b: 锚定词首，其后面的任意字符必须作为单词首部出现
\>或\b: 锚定词尾，其前面的任意字符必须作为单词的尾部出现
[root@Daniel-R480 ~]# grep 'r..t' /etc/passwd
root:x:0:0:root:/root:/bin/bash
operator:x:11:0:operator:/root:/sbin/nologin
ftp:x:14:50:FTP User:/var/ftp:/sbin/nologin
[root@Daniel-R480 ~]# grep '&r..t' /etc/passwd
[root@Daniel-R480 ~]# grep '^r..t' /etc/passwd
root:x:0:0:root:/root:/bin/bash
[root@Daniel-R480 tmp]# grep "root\>" ./test.txt
This is root
the user is mroot
chroot is a command
[root@Daniel-R480 tmp]# grep "\>root" ./test.txt
[root@Daniel-R480 tmp]# grep "root\>" ./test.txt
This is root
the user is mroot
chroot is a command
[root@Daniel-R480 tmp]# grep "\<root" ./test.txt
This is root
rooter is a dog's name
[root@Daniel-R480 tmp]# grep "\<root\>" ./test.txt
This is root
[root@Daniel-R480 tmp]#
5、分组：
\(\)：把内容分组
\(ab\)*：ab可以出现任意次
[root@Daniel-R480 ~]# cat test.txt
a
b
ab
asb
aab
abb
ammjdjfdjb
asdbsdabsfdab
[root@Daniel-R480 ~]# grep "\(ab\)*" test.txt
a
b
ab
asb
aab
abb
ammjdjfdjb
asdbsdabsfdab
[root@Daniel-R480 ~]# grep "\(ab\)\?" test.txt
a
b
ab
asb
aab
abb
ammjdjfdjb
asdbsdabsfdab
[root@Daniel-R480 ~]#
6、后向引用（被前面括号括起来的字符，可以在后面继续引用他）
\1: 引用第一个左括号以及与之对应的右括号所包括的所有内容
\2:引用第二个左括号以及与之对应的右括号所包括的所有内容
\3:引用第三个左括号以及与之对应的右括号所包括的所有内容
He love his lover.
She like her liker.
He like his lover.
[root@Daniel-R480 ~]# vim test2.txt
[root@Daniel-R480 ~]# grep 'l..e' test2.txt
He love his lover.
She like her liker.
He like his lover.
She love her liker.
[root@Daniel-R480 ~]# grep 'l..e.*l..e' test2.txt
He love his lover.
She like her liker.
He like his lover.
She love her liker.
[root@Daniel-R480 ~]# vim test2.txt
[root@Daniel-R480 ~]# grep 'l..e.*l..e' test2.txt
He love his lover.
She like her liker.
He like his lover.
She love her liker
[root@Daniel-R480 ~]# grep 'l..e.*\1' test2.txt
grep: Invalid back reference
[root@Daniel-R480 ~]# grep '\(l..e\).*\1' test2.txt
He love his lover.
She like her liker.
l..e：l开头，e结尾
练习：
1、显示/proc/meminfo文件中以不区分大小的s开头的行；
[root@Daniel-R480 ~]# grep -i '^s' /proc/meminfo
SwapCached: 0 kB
SwapTotal: 31018612 kB
SwapFree: 31018612 kB
Shmem: 17720 kB
Slab: 13868 kB
SReclaimable: 6744 kB
SUnreclaim: 7124 kB
[root@Daniel-R480 ~]# grep '^[sS]' /proc/meminfo
SwapCached: 0 kB
SwapTotal: 31018612 kB
SwapFree: 31018612 kB
Shmem: 17720 kB
Slab: 13868 kB
SReclaimable: 6744 kB
SUnreclaim: 7124 kB
[root@Daniel-R480 ~]#
2、显示/etc/passwd中以nologin结尾的行;
[root@Daniel-R480 ~]# grep 'nologin$' /etc/passwd
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
operator:x:11:0:operator:/root:/sbin/nologin
games:x:12:100:games:/usr/games:/sbin/nologin
ftp:x:14:50:FTP User:/var/ftp:/sbin/nologin
nobody:x:99:99:Nobody:/:/sbin/nologin
systemd-network:x:192:192:systemd Network Management:/:/sbin/nologin
dbus:x:81:81:System message bus:/:/sbin/nologin
polkitd:x:999:998:User for polkitd:/:/sbin/nologin
rpc:x:32:32:Rpcbind Daemon:/var/lib/rpcbind:/sbin/nologin
rpcuser:x:29:29:RPC Service User:/var/lib/nfs:/sbin/nologin
nfsnobody:x:65534:65534:Anonymous NFS User:/var/lib/nfs:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
postfix:x:89:89::/var/spool/postfix:/sbin/nologin
chrony:x:998:995::/var/lib/chrony:/sbin/nologin
test3:x:10014:10014::/home/test3:/sbin/nologin
取出默认shell为/sbin/nologin的用户列表
[root@Daniel-R480 ~]# grep 'nologin$' /etc/passwd | cut -d: -f1
bin
daemon
adm
lp
operator
games
ftp
nobody
systemd-network
dbus
polkitd
rpc
rpcuser
nfsnobody
sshd
postfix
chrony
test3
[root@Daniel-R480 ~]#
取出默认shell为bash，且其用户ID号最小的用户的用户名
[root@Daniel-R480 ~]# grep 'bash$' /etc/passwd | sort -n -t: -k3 | head -1 | cut -d: -f1
root
3、显示/etc/inittab中以#开头，且后面跟一个或多个空白字符，而后又跟了任意非空白字符的行；
[root@Daniel-R480 ~]# grep "^#[[:space:]]\{1,\}[^[:space:]]" /etc/inittab
# inittab is no longer used when using systemd.
# ADDING CONFIGURATION HERE WILL HAVE NO EFFECT ON YOUR SYSTEM.
# Ctrl-Alt-Delete is handled by /usr/lib/systemd/system/ctrl-alt-del.target
# systemd uses 'targets' instead of runlevels. By default, there are two main targets:
# multi-user.target: analogous to runlevel 3
# graphical.target: analogous to runlevel 5
# To view current default target, run:
# systemctl get-default
# To set a default target, run:
# systemctl set-default TARGET.target
[root@Daniel-R480 ~]#
4、显示/boot/grub/grub.conf文件中以一个或多个空白字符开头的行；
[root@Daniel-R480 ~]# grep '^[[:space:]]\{1,\}' /boot/grub/grub.conf
root (hd0)
kernel /boot/vmlinuz-3.10.0-957.1.3.el7.x86_64 ro root=UUID=f41e390f-835b-4223-a9bb-9b45984ddf8d console=hvc0 LANG=en_US.UTF-8
initrd /boot/initramfs-3.10.0-957.1.3.el7.x86_64.img
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