Shell programming specifications and variables

Shell Programming specification

Shell A script is a special application , It lies between the operating system kernel and the user , Acting as a “ command interpreter ” Role , Be responsible for receiving the operation instructions input by the user （ command ） And explain , Pass the operation to be performed to the kernel for execution , And output the execution result .

Shell The role of

command interpreter ," The interpreter "

Translator between kernel and user

Shell The concept of script

Save the commands to be executed in order to a text file

Give the file executable permissions

It can combine all kinds of Shell Control statements to perform more complex operations

Shell Script application scenarios

Repetitive operations
Sometimes you need to repeat the operation , You can write a script to implement the loop

Interactive tasks

Batch transactions

Service running status monitoring

Scheduled task execution
Write script to crotable -e Come in and do the scheduled task

Write script code

Use vim Text editor

One in each line Linux command , Write... In order of execution
Writing shell Script makes , Put... After the name .sh, To represent this is shell Script command

Give executable permission

Make the script executable
chomd +x file

Execute script file

Method 1 ： Script file path （ Absolute path and relative path ）
There must be x jurisdiction

Method 2 ：sh Script file path

Method 3 ：source Script file path    ( Execute the script in the system environment )
It can also be done through “.” To execute

 shell Script composition

Script declaration
Also called “ Interpreter ”： If the first act "#！/bin/bash", This line of code is represented by /bin/bash Program to explain the execution of ,#！/bin/bash Is the default interpreter , There are other types of interpreters

Annotation information

Executable statement

Redirection and pipeline operations

Interactive hardware devices

The standard input ： Receiving data input from the user from the device

standard output ： Output data to the user through the device

The standard error ： Report execution error information through this device

Redirection operation

  Pipe operation symbols “|”

Output the command on the left , As the processing object of the right command

Shell The role and type of variables

Role of variables

It is used to store specific parameters that the system and users need to use （ value ）
Variable name ： Use a fixed name , Preset by the system or defined by the user
A variable's value ： According to the user settings , The system changes with the change of the system environment

The type of variable

environment variable ： Maintained by system , Used to set up the work environment

Predefined variables ：Bash A class of variables built into , Cannot be modified directly

Custom variable ： Defined by the user 、 Modify and use

Positional variable ： Pass parameters to the script through the command line

Define a new variable

Variable names start with letters or underscores , Case sensitive , It is suggested to use all capitals .
Variable name = A variable's value （“=” Is the meaning of assignment ）

Look at the value of the variable
echo $ Variable name

  If you want to closely connect other content behind the output , Adding directly is not enough , We need to use it at this time “{}”, Use the symbol , You can realize the above statement . Or use " ", take $ Put all the codes in quotation marks , It can also be realized .

Let's set a variable to output normally . If we want to add 123 Add... Directly after , It cannot be output . Or the middle space is being added  

  At this time, we can use {}

  perhaps “”

Or will 123 Also assign values and output them together

Use quotation marks when assigning values  

Double quotes ： Allow to use $ The symbol references other variable values

  Single quotation marks ： Do not reference other variable values ,$ Treat as normal characters

  Apostrophe ： Command substitution , Extract the output of the command after execution   （`` Only quotation marks You can't use braces ）

notes ： Backhanded cannot be nested inside backhanded

Similar to backhand

Assign values to variables from keyboard input

read -p " Prompt information " Variable name

Action range of variable  

In order to make user-defined variables in all sub variables shell Can continue to be used in the environment , Reduce duplicate setup work , It can be done by internal command export Exports the specified variable as a global variable

Format 1：export Variable name
Format 2：export Variable name = A variable's value

 

 

The operation of integer variables

expr Variable 1 Operator Variable 2 [ Operator Variable 3]…

Common operators

• Addition operation ：+
• Subtraction ：-
• Multiplication ：\*
• Division operations ：/
• modulus （ Remainder ） operation ：%
  notes ： This method is not simple enough , Just for understanding

Variables and operators should be separated by spaces Otherwise, it will be recognized as a character and output directly, such as

Concise operation method

1 echo $[1+2]

2 echo $((1+2))

3 let i=1+2

  echo $i

n++ amount to  i=$[$i+1]
n-- amount to  i=$[$i-1]
n+=2 amount to i=$[$i+2]