Shell Scripting Introduction to Shell Script Structure and Execution of Date Command Usage Variables in Shell Script

Introduction to shell scripting

The shell is a scripting language aming_linux blog.lishiming.net
You can use syntax such as logical judgment, loop, etc.
customizable function
A shell is a collection of system commands
Shell script can realize automatic operation and maintenance, which can greatly increase our operation and maintenance efficiency

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Shell script structure and execution

The beginning (first line) needs to be added: #!/bin/bash
here means that the file uses bash syntax and is executed through the /bin/bash interpreter.
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Lines beginning with # are used as explanations:
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The name of the script ends with .sh to distinguish it as a shell script

There are two ways to execute the .sh script:

1 First add x permissions to the .sh script:

chmod +x 1.sh

Absolute path to .sh script Enter:

/root/1.sh

2bash (bash=sh) executes the .sh script:

bash 1.sh

View the script execution process:

sh -x 1.sh

Detect syntax errors in shell scripts:

sh -n 1.sh

date command usage

Most of the date command in a shell script is to change the filename of the log, and to distinguish the date of some files.

View current time

[root@shuai-01 ~]# date
2018年 02月 25日 星期日 14:51:35 CST

[root@shuai-01 ~]# LANG=en 
[root@shuai-01 ~]# date
Sun Feb 25 14:52:25 CST 2018

date +%Y : print the four-digit year
[root@shuai-01 ~]# date +%Y
2018
date +%y : print the year with two digits
[root@shuai-01 ~]# date +%y
18
date +%m : print month
[root@shuai-01 ~]# date +%m
02
date +%d : indicates the date
[root@shuai-01 ~]# date +%d
25
date +%H : indicates the hour
[root@shuai-01 ~]# date +%H
14
date +%M : indicates minutes
[root@shuai-01 ~]# date +%M
58
date +%S : indicates seconds
[root@shuai-01 ~]# date +%S
35
date +%w : means week, 0 means Sunday
[root@shuai-01 ~]# date +%w
0
Indicates the year, month and day:
[root@shuai-01 ~]# date +%Y%m%d
20180225
[root@shuai-01 ~]# date +%F
2018-02-25

Indicate time:

[root@shuai-01 ~]# date +%T
18:44:10

Show calendar:

[root@shuai-01 ~]# cal
    February 2018   
Su Mo Tu We Th Fr Sa
             1  2  3
 4  5  6  7  8  9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28

date marks yesterday's date:

[root@shuai-01 ~]# date -d "-1 day"
Sat Feb 24 18:47:16 CST 2018

[root@shuai-01 ~]# date -d "-1 day" +%F
2018-02-24

Variables in Shell Scripts

1. 当脚本中使用某个字符串较频繁并且字符串长度很长时就应该使用变量代替
2. 使用条件语句时，常使用变量    if [ $a -gt 1 ]; then ... ; fi
3. 引用某个命令的结果时，用变量替代   n=`wc -l 1.txt`
4. 写和用户交互的脚本时，变量也是必不可少的  read -p "Input a number: " n; echo $n   如果没写这个n，可以直接使用$REPLY
5. 内置变量 $0, $1, $2…    $0表示脚本本身，$1 第一个参数，$2 第二个 ....       $#表示参数个数
6. 数学运算a=1;b=2; c=$(($a+$b))或者$[$a+$b]