Detailed explanation of code cases about grep and regular expressions in Linux

Grep Introduction

Grep is a powerful text search tool that can use regular expressions to search text and print out matching lines. 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 size of the characters Write

　-o: Display only matched strings

　-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 Pattern-matched lines and their preceding # lines

　-C #: Display pattern-matched lines and their preceding and following # lines

　Note: Required when using grep matching 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 method: egrep [OPTIONS] PATTERN [FILE...]

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

　-i: Ignore the case of characters
　-o: Only display the matched string itself
　-v: Display not matched by the pattern Reached line
　-q: Silent mode, that is, no information is output
　-A #: Display the line matched by the pattern and the following # lines
　-B #: Display the line matched by the pattern and its following Previous # lines
-C #: Display the lines matched by the pattern and the # lines before and after
-G: Support basic regular expressions

grep regular expression metacharacters

'^': Anchor at the beginning of the line

'$': Anchor at the end of the line

'.': Match any character

'*': Match Zero or more previous characters

'\?': Match the preceding character 0 or 1 times;

'\+': Match the preceding character 1 or more times ;

　'\{m\}': Matches the character before it m times (\ is an escape character)

　'\{m,n\}': Matches the character before it At least m times, at most n times

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

　'\' or '\b': anchor the end of the word (available \: match the complete word)

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

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

Grouping The content matched by the pattern in brackets may be automatically recorded in internal variables by the regular expression engine:

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

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

　Extended regular expressions are slightly different from regular expressions:

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

　[:digit:] matches any single digit

　　[:lower:] matches any single lowercase letter　　　　　

　　　[:upper:] matches any A single uppercase letter

[:alpha:] matches any single letter

[:alnum:] matches any single letter or number

[:punct:] matches any single symbol

[:space:] Matches a single space

Some places cancel the use of escape characters:

'?': Matches the preceding character 0 or 1 times ;

'+': Matches the preceding character 1 or more times;

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

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

　(): Bundle one or more characters together and process them as a whole, and vice versa References are used as usual.

　|': or (Note: 'C|cat' is C and cat, '(C|c)at is Cat and cat')

Exercise:

1. List the usernames of all logged-in users on the current system. Note: If the same user logs in multiple times, it will only be displayed once

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

　2. Take out the relevant information of the user who last logged in to the current system

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

　3. Take out the shell that is regarded as the default shell by the most users on the current system

[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' ' -f10192.168.0.133

　　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 -l21

　　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
disklpmem
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 0UUID=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

　　对于正则表达式的使用需要多联系加强记忆，否则是用不好正则表达式的，在学习过程中切记多写多背。

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