A brief introduction to Linux grep and regular expressions

grep is a powerful text search tool that uses regular expressions to search text and prints matching lines. This article will share with you knowledge about Linux grep and regular expressions. Friends who are interested should take a look at it

grep introduction

grep is a powerful text search tool that uses regular expressions to search text and prints 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 case of characters

-o: Only display 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 the following # lines

　-B #: Display the pattern matched Matched lines and the # lines before and after

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

Note: When using grep matching, you need to use double quotes (single quotes) Quotation marks 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 lines that are not matched by the pattern
　-q: Silent mode, that is, no information is output
-A #: Display the line matched by the pattern and # 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 it #Row
　-G: Support basic regular expressions

grep regular expression metacharacter

'^': Anchor line Heading

'$': Anchor line end

'.': 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\}': Match the character before it m times (\ is an escape character)

'\{m,n\}': Match the character before it at least m times and 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 one Processed as a whole

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

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

\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 Starting 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                                                                                                                                                                    

　[: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;

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

'{m}': Match the character before it 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. Backreferences are used as usual.

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

Exercise:

1. List the user names 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|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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