Shell script - definition, assignment and deletion of variables

Tradition ： What is? Shell

• Shell It's an application , It connects the user and Linux kernel , Let users be more efficient 、 Security 、 Low cost local use Linux kernel , This is it. Shell The essence of .
• Shell It's also a programming language , Its compiler （ Interpreter ） yes Shell This procedure .
• Shell Is a scripting language , We do not need to compile the source code , Run the source code directly .

Shell Script format

With #!/bin/bash start

#!/bin/bash
···

Shell Variable ：Shell Definition of variables 、 Assignment and deletion

Recommended values value Surround with quotation marks

variable='value'
variable="value"

The difference between single quotation mark and double quotation mark

• Single quote ’ ' When enclosing the value of a variable , Output what is in single quotation mark
• Double quotation marks " " When enclosing the value of a variable , The variables and commands in the output will be parsed first , Instead of outputting the variable names and commands in double quotes as they are

#!/bin/bash

author='jack'

name=' The writer's name is ${author}'
name2=" The writer's name is ${author}"

echo ${name}
echo ${name2}

Run the script output ：

 The writer's name is ${author}
 The writer's name is jack

Assign the execution result of the command to the variable

It supports assigning the execution result of a command to a variable , Surround the command variable=$(command)

#!/bin/bash

out=$(sh test.sh)
echo ${out}

Output

 The writer's name is ${author}  The writer's name is jack

A read-only variable

Use readonly Commands can define variables as read-only variables , The value of a read-only variable cannot be changed .
The following example attempts to change a read-only variable , The result is wrong ：

#!/bin/bash

name='jack'
readonly name
name='rose'
echo ${name}

Output

test3.sh: line 5: name: readonly variable

Delete variables

Use unset Command to delete variables . grammar unset variable_name
Variable cannot be used again after being deleted ;unset Command cannot delete read-only variables .

#!/bin/bash

name='jack'
unset name
echo ${name}

Output