The use of Linux grep and regular expressions

This time I will bring you the use of Linux grep and regular expressions. What are the notes when using Linux grep and regular expressions? The following is a practical case. Let’s take a look. take a look.

Introduction to grep

Grep is a powerful text search tool that can use regular expressions to search text and match the lines print it out. Usually there are three versions of grep: grep, egrep (equivalent to grep -E) and fgrep. egrep is extended grep, and fgrep is fast grep (fixed string to search text, does not support regular expression references but the query is extremely fast). grep is one of the three musketeers of Linux text processing.

How to use grep

How to use:

grep [OPTIONS] PATTERN [FILE...]

grep [OPTIONS] [-e PATTERN | -f FILE] [FILE...]

Common options:

--color= auto: Color the matched text and highlight it;

-i: Ignore the case of characters

-o: Display only the matched string

-v: Display lines that cannot be matched by the pattern

-E: Support the use of extended regular expressions

-q: Silent mode, that is, no information is output

-A #: Display the lines matched by the pattern and # lines after it

-B #: Display the lines matched by the pattern and the # lines before it

-C #: Display the lines matched by the pattern The line and the # lines before and after

Note: When using grep to match, you need to use double quotes (single quotes are strong quotes) to prevent the system from mistaking it for parameters or special commands and reporting an error.

Extended grep usage

Usage:

egrep [OPTIONS] PATTERN [FILE...]
grep -E [OPTIONS] PATTERN [FILE...]

　-i: Ignore the case of characters
　- o: Only display the matched string itself
　-v: Display the lines that are not matched by the pattern
　-q: Silent mode, that is, do not output any information
　-A #: Display the lines that are matched by the pattern Line and the # lines after it
-B #: Display the line matched by the pattern and the # lines before it
- C #: Display the line matched by the pattern and the # lines before and after
- G: Support Basic regular expressions

grep regular expression metacharacters

##　'^': anchor the beginning of the line

　'$ ': Anchor at the end of the line

'.': Match any one character

'*': Match zero or more previous characters

'\?' : Matches the character before it 0 or 1 times;

'\+': Matches the character before it 1 or more times;

'\{m\}': Matches it The preceding character m times (\ is an escape character)

'\{m,n\}': Match the preceding character at least m times and at most n times

'[]' : Matches a character within a specified range | '[^]' matches any single character outside the specified range

'\<' or '\b': anchors the beginning of the word, '\>' or '\b': anchor word ending (available\: match complete words)

'\(\)': treat multiple characters as a whole

Back reference: Reference the characters matched by the pattern in the previous grouping brackets

The content matched by the pattern in the grouping brackets or the internal variables automatically recorded by the regular expression engine Medium:

\1: The pattern starts from the left, the content matched by the pattern between the first left bracket and the matching right bracket

\2: The pattern starts from the left , the content matched by the pattern between the second left bracket and the matching right bracket...

Extended regular expressions are slightly different from regular expressions:

　[] ': Still matches any single character within the specified range; but there are many special matching methods.

　[:digit:] matches any single digit

　　[:lower:] matches any single lowercase letter     

　　　　[:upper:] 匹配任意单个大写字母

　　　　[:alpha:] 匹配任意单个字母

　　　　[:alnum:] 匹配任意单个字母或数字

　　　　[:punct:] 匹配任意单个符号

　　　　[:space:] 匹配单个空格

　　一些地方取消了转义字符的使用：

　　‘?‘：匹配其前面的字符0次或者1次；

　　‘+'：匹配其前面的字符1次或者多次；

　　‘{m}‘：匹配其前面的字符m次（\为转义字符）

　　‘{m,n}'：匹配其前面的字符至少m次，至多n次

　　()：将一个或多个字符捆绑在一起，当做一个整体进行处理，反向引用照常使用。

　　‘|'：或（注：‘C|cat'为C与cat，‘（C|c）at才是Cat与cat'）

练习题：

　　1、列出当前系统上所有已经登录的用户的用户名，注意：同一个用户登录多次，则只显示一次

[root@localhost ~]# who | cut -d' ' -f1|uniq
root

　　2、取出最后登录到当前系统的用户的相关信息

[root@localhost ~]# id `last | head -1 | cut -d' ' -f1`
uid=0(root) gid=0(root) groups=0(root)

　　3.取出当前系统上被用户当做其默认shell最多的那个shell

[root@localhost ~]# cut -d':' -f7 /etc/passwd|uniq -c|sort -n|tail -1|cut -d' ' -f7
/sbin/nologin

　　4.将/etc/passd中的第三个字段设置最大的后10个用户的信息全部改为大写保存至/tmp/maxuser.txt文件中

[root@localhost ~]# sort -t':' -k3 -n /etc/passwd|tail -10|tr 'a-z' 'A-Z' >/tmp/maxusers.txt
[root@localhost ~]# cat /tmp/maxusers.txt 
NOBODY:X:99:99:NOBODY:/:/SBIN/NOLOGIN
SYSTEMD-NETWORK:X:192:192:SYSTEMD NETWORK MANAGEMENT:/:/SBIN/NOLOGIN
NGINX:X:996:994:NGINX WEB SERVER:/VAR/LIB/NGINX:/SBIN/NOLOGIN
CHRONY:X:997:995::/VAR/LIB/CHRONY:/SBIN/NOLOGIN
POLKITD:X:998:996:USER FOR POLKITD:/:/SBIN/NOLOGIN
SYSTEMD-BUS-PROXY:X:999:997:SYSTEMD BUS PROXY:/:/SBIN/NOLOGIN
DINGJIE:X:1000:1000:DINGJIE:/HOME/DINGJIE:/BIN/BASH
JEFF:X:1001:1024:WOSHIDASHUAIBI:/HOME/JEFF:/BIN/BASH
EGON:X:1002:1002::/HOME/EGON:/BIN/BASH
NFSNOBODY:X:65534:65534:ANONYMOUS NFS USER:/VAR/LIB/NFS:/SBIN/NOLOGIN

　　5.取出当前主机的IP地址

[root@localhost ~]# ifconfig | egrep "inet.*broadcast.*"|cut -d' ' -f10
192.168.0.133

　　6.列出/etc目录下所有已.conf结尾的文件的文件名，并将其名字转换为大写后保存至/tmp/etc.conf文件中

[root@localhost ~]# find /etc -name '*.conf' | egrep -o "[^/]*(\.conf)$"|tr 'a-z' 'A-Z' >/tmp/etc.conf
[root@localhost ~]# cat /tmp/etc.conf 
RESOLV.CONF
CA-LEGACY.CONF
FASTESTMIRROR.CONF
LANGPACKS.CONF
SYSTEMD.CONF
VERSION-GROUPS.CONF
LVM.CONF
LVMLOCAL.CONF
ASOUND.CONF
LDAP.CONF
MLX4.CONF
RDMA.CONF
SMTPD.CONF

　　7.显示/var目录下一级子目录或文件的总数

[root@localhost ~]# ls /var | wc -l

　　8.取出/etc/group第三个字段数值最小的10个组的名字

[root@localhost ~]# sort -t: -k3 -n /etc/group|head -10 |cut -d':' -f1
root
bin
daemon
sys
adm
tty
disk
lp
mem
kmem

　　9.将/etc/fstab和/etc/issue文件的内容合并为同一个内容后保存至/tmp/etc.test文件中

[root@localhost ~]# cat /etc/fstab /etc/issue > /tmp/etc.test
[root@localhost ~]# cat /tmp/etc.test 
#
# /etc/fstab
# Created by anaconda on Sat May 13 10:12:58 2017
#
# Accessible filesystems, by reference, are maintained under '/dev/disk'
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info
#
/dev/mapper/cl-root   /            xfs   defaults    0 0
UUID=2789d01a-4e2b-47a5-9c3c-537641648663 /boot          xfs   defaults    0 0
/dev/mapper/cl-swap   swap          swap  defaults    0 0
\S
Kernel \r on an \m

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