Shell Introduction, Date command, Shell variable

What is Shellshell?

Shell is a scripting language
can use logical judgment, circular judgment and other statements;
functions can be customized;
A shell is a collection of system commands, such as a batch command of Windows;
Shell script can realize automatic operation and maintenance, can play to increase our operational efficiency;

Shell script Structure

#! must be added at the beginning /bin/bash
That is, the kernel that calls/bin/bash runs
The name of the script ends with. SH and is used to differentiate between a shell script;

There are two ways to do this: 1. As an executable program

chmod +x 1.sh       //是脚本具有执行权限./1.sh                       //执行脚本

2. As an interpreter parameter

bash 1.sh                              //也可以sh 1.sh

To view the script execution process: Bash-x 1.sh
See if the script is syntactically incorrect: Bash-n 1.sh

Date Day Command Month Date:

Command: Date +%y-%m-%d
Output: 2018-04-17

Command: Date +%f
Output: 2018-04-17

Command: Date +%y-%m-%d
Output: 18-04-17

Command: Date +%y%m%d
Output: 20180417

Time:

Command: Date +%h:%m:%s
Output: 20:23:16

Command: Date +%t
Output: 20:23:16

Week:

%w: The first few weeks of the month
%W: The first few weeks of this year

Time stamp:

Date +%s
Date-d @1504620492

Increase reduction Date:

Date-d "+1day" a day later
Date-d "-1 days" a day ago
Date-d "-1 month" month ago
Date-d "-1 min" A minute ago

Variable naming rules:

Names can only use letters, numbers, and underscores, and the first character cannot begin with a number.
You can use an underscore (_) without spaces in the middle.
Punctuation cannot be used.
You can't use the keywords in bash (you can see the reserved keywords using the help command).

Shell value:

a=1;b=2;        //给变量a赋予数值1，变量b赋予数值2c=$(($a+$b))    //c=a+b，使用运算必须前面加$

Shell string:

string when the shell becomes the most commonly used most useful data type (numbers and strings), strings can be in single quotes, or double quotes;

Single quotes:

str=‘this is a string’

Escape characters and variables cannot be used;

Double quotes:

your_name=‘aaaa‘str="Hello,I know your are \"$your_name\"! \n"

Advantages of double quotes: can have variables, can appear escape character;

The call variable value must precede with the $ symbol
When referencing the result of a command, replace n= with a variablewc -l 1.txt
When writing and user interaction scripts, variables are also essential for read-p "Input a number:" N; Echo $n If you don't write this n, you can use $reply directly.
Built-in variables $, $ $, $ ... $ $ represents the script itself, the first parameter, the $ A second ... $ #表示参数个数
Mathematical Operation a=1;b=2; c=$ (($a + $b)) or $[$a + $b]

Shell Introduction, Date command, Shell variable