Shell Basic Learning

#! /bin/bash

#设置只读变量, read-only variables are not modifiable
Myurl= "Http://www.baidu.com"
ReadOnly Myurl

#删除变量, unset cannot delete read-only variables
myurl1= "Http://www.sina.com"
Unset MYURL1

#运行shell时会同时存在三种变量
#局部变量, environment variables, shell variables

#单引号: cannot have a variable in single quotation marks, cannot have single quotation marks
#双引号: You can have variables in double quotes, and escape characters can appear.
Your_name= "QINJX"
Str= "Hello,i know your name is \" $your _name\ "! \ n "

#拼接字符串
Your_name= "Cye"
Greeting= "Hello," $your _name "!"
Greeting_1= "Hello, $your _name"
echo $greeting $greeting _1

#获取字符串长度
String= "ABCDEFG"
echo ${#string} #输出 7

#提取子字符串, 4 characters are intercepted starting with the second character.
String= "Runoob is a great site"
Echo ${string:1:4} #输出unoo

#查找子字符串, find the position of I or S, "'" in the following script is the inverted quotation mark instead of the single quote "'",
String= "Runoob is a great company"
echo ' expr index ' $string ' is ' #输出8

#数组, defining the array
Array_name= (VALUE0,VALUE1,VALUE2,VALUE3)

#读取数组, ${array name [subscript]}
Valuen=${array_name[n]}

#使用 the @ symbol to get all the elements in the array
Echo ${array_name[@]}

#获取数组的长度
length=${#array_name [@]}
#或者
length=${#array_name [*]}
#取得数组单个元素的长度
lengthn=${#array_name [n]}

#shell传递参数, when executing a shell script to pass parameters to the script, the format of the parameters obtained in the script is: $n, n is a number, 1 represents the first argument to execute the script, and 2 is the second argument that executes the script.
echo "Shell pass argument instance"
echo "file name executed: $ A"
echo "First parameter: $ $"
echo "The second parameter is: $"
echo "The third parameter is: $ $"

#调用时为
#chmod +x test.sh
#./test.sh 1 2 3
#结果为:
#shell Passing Parameter instances
#执行的文件名:./test.sh
#第一个参数为: 1
#第二个参数为: 2
#第三个参数为: 3

#输出输入重定向
Command > file redirects the output to file.
Command < file redirects the input to file.
Command >> file redirects the output to file in an append manner.
n > file redirects the file descriptor n files to filename.
n >> files redirect files with file descriptor N as an append to file.
N >& m merges the output file m and N.
N <& m merges the input file m and N.
"<< Tag" will start to tag the contents of tag and end tag tag as the output

0 is standard input (STDIN), 1 is standard output (STDOUT), 2 is standard error output (STDERR)
/dev/null is a special file, and the content written to it is discarded, and if you try to read from the file, nothing is read. However, the/dev/null file is very useful and redirects the output of the command to it, which results in a "no output" effect.
$ command >/dev/null 2>&1

Shell Basic Learning