Shell script description, shell script structure and execution, date command usage, variables in shell scripts

Shell script introduction
The shell is a scripting language aming_linux (public account) blog.lishiming.net (blog)
Can use syntax such as logical judgment, loop, etc.
Can customize functions
shell is a collection of system commands
Shell script can realize automatic operation and maintenance, which can greatly increase our operation and maintenance efficiency. Shell script structure and execution
You need to add #! / Bin / bash at the beginning, which means that the next statement is parsed by this file, because with it we can execute ./1.sh like this, or only / bin / bash 1.sh
[[email protected] shell] # chmod a + x 1.sh
[[email protected] shell] # ./1.sh
123
22:37:09 up 4:02, 2 users, load average: 0.00, 0.01, 0.05
USER TTY FROM [email protected] IDLE JCPU PCPU WHAT
root pts / 0 192.168.21.1 22:29 5.00s 0.10s 0.00s / bin / bash ./1.sh
root pts / 1 192.168.21.1 19:00 3: 31m 0.08s 0.08s -bash
1.sh
[[email protected] shell] # / bin / bash 1.sh
123
22:37:27 up 4:03, 2 users, load average: 0.00, 0.01, 0.05
USER TTY FROM [email protected] IDLE JCPU PCPU WHAT
root pts / 0 192.168.21.1 22:29 7.00s 0.10s 0.00s / bin / bash 1.sh
root pts / 1 192.168.21.1 19:00 3: 31m 0.08s 0.08s -bash
1.sh
The lines starting with # are used as explanations, except for the first line of #! / Bin / bash and some special scripts
The name of the script ends with .sh to distinguish this is a shell script
There are two methods of execution
chmod + x 1.sh; ./1.sh
sh 1.sh
sh -x 1.sh shows the execution process

[[email protected] shell] # sh -x 1.sh
echo 123
123
w
22:43:05 up 4:08, 2 users, load average: 0.00, 0.01, 0.05
USER TTY FROM [email protected] IDLE JCPU PCPU WHAT
root pts / 0 192.168.21.1 22:29 1.00s 0.11s 0.00s sh -x 1.sh
root pts / 1 192.168.21.1 19:00 3: 37m 0.08s 0.08s -bash
ls
1.sh
The + sign is the executed statement
sh -n 1.sh can check the syntax of the script date command usage
year month day
[[email protected] shell] # date +% Y-% m-% d
2018-07-11
[[email protected] shell] # date +% y-% m-% d
18-07-11
[[email protected] shell] # date +% Y% m% d
20180711
[[email protected] shell] # date +% D
07/11/18
[[email protected] shell] # date +% F
2018-07-11
date +% s ?? timestamp, how many seconds have passed since 0: 0 on January 1, 1970, and conversion between timestamp and time
[[email protected] shell] # date +% s
1531320917
[[email protected] shell] # date -d @ 1531321618
Wednesday, July 11, 2018 23:06:58 CST
[[email protected] shell] # date +% s -d "2018-07-10 23:06:58"
1531235218
time
[[email protected] shell] # date +% H:% M:% S
22:57:20
[[email protected] shell] # date +% T
22:57:32
week
[[email protected] shell] # date +% w
3
[[email protected] shell] # date +% W
28
W is the first week of the year
How to log yesterday's date
[[email protected] shell] # date -d "-1 day" +% F
2018-07-10
[[email protected] shell] # date -d "-1 month" +% F
2018-06-11
[[email protected] shell] # date -d "-1 year" +% F
2017-07-11
[[email protected] shell] # date -d "-1 hour" +% T
22:05:55
[[email protected] shell] # date -d "+1 day" +% F
2018-07-12
View calendar
[[email protected] shell] # cal
  July 2018
Day one two three four five six
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
29 30 31
Variables in shell scripts
When a string is used frequently in a script and the length of the string is long, you should use variables instead
When using conditional statements, often use variables?? If [$ a -gt 1]; then ...; fi
When referring to the result of a command, use variables instead?? N = wc -l 1.txt
Variables are also essential when writing scripts that interact with users ?? read -p "Input a number:" n; echo $ n?? If you didn't write this n, you can use $ REPLY directly
Built-in variables $ 0, $ 1, $ 2 ...?? $ 0 represents the script itself, $ 1 is the first parameter, $ 2 is the second ....? ??? $ # Is the number of parameters
Mathematical operations a = 1; b = 2; c = $ (($ a + $ b)) or $ [$ a + $ b]
Introduction to shell script, shell script structure and execution, date command usage, variables in shell script