Shell programming specifications and variables

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One 、Shell Script Overview

1. What is? Shell

Shell Is a command interpreter , It's on the outermost layer of the system , Responsible for direct dialogue with users , Explain user input to the operating system , And handle the output of various operating systems , Output to screen feedback to user . This kind of dialogue can be interactive or non interactive , The command we input is something else , Then we need a program to help us translate , Become a binary program that a computer can recognize , At the same time, the computer generated results are returned to us .

2.Shell The role of

Linux In the system Shell It's a special application , It lies between the operating system kernel and the user , Acting as a " command interpreter " Role , Be responsible for receiving and interpreting the operation instructions input by the user , Pass the operation to be performed to the kernel for execution , And output the execution result .
common Shell There are many kinds of interpreter programs , Use different Shell when , Its internal instructions 、 There are some differences in command line prompt and so on .
adopt /etc/shells The file can understand what the current system supports Shell Types of scripts

/bin/sh     # yes bash Soft links to commands 
/bin/bash   # Base on GNU Developed under the framework of Shell
/usr/bin/sh # Has been bash Replaced by 
/usr/bin/bash  #centos and redhat System default use bash shell（ Interpreter ）
/bin/txsh   #csh Enhanced Edition , And csh Fully compatible and integrated csh, More features 
/bin/csh    # Has been /bin/bash Replaced by （ Integrate C shell, More features ）

3.Shell What can a script do

• Automate the installation and deployment of software , Such as installation and deployment LAMP Architecture Services
• Complete the management of the system automatically , Such as adding users in batch
• Automate backup , Such as regular database backup
• Automated analytical processing , Such as website visits

4.Shell Script usage scenarios

• Repetitive operations
• Interactive tasks
• Batch transactions
• Service running status monitoring
• Scheduled task execution

Two 、Shell The composition of the script

1. Script declaration （ Interpreter ）： If the first act "#！/bin/bash", Indicates that the code statement below this line is passed /bin/bash Program to explain the execution of ,#!/bin/bash Is the default interpreter . There are other types of interpreters , such as :#!/usr/bin/python、#!/usr/bin/expect

2. Annotation information ： With "#" The opening statement is expressed as a comment message , The annotated statement will not be executed when the script is run again
3. Executable statement ： such as echo command , For output " " String between

3、 ... and 、Shell Script execution

1. Method 1 ： Specify the path

Command to specify the path , It is required that the document must have x jurisdiction

Specify the absolute path

Specify the relative path

2. Method 2 ： Appoint Shell

Appoint Shell To explain the script , It is not required that the document must have x jurisdiction
sh Script path

soucre Script path

bash Script path

Four 、 Redirection and pipeline operation

1. Pipeline operation "|"

The pipe symbol "|" The command output on the left , As input to the command on the right （ Deal with people ）, Multiple pipes can be used in the same command line

2. Redirect

2.1 Interactive hardware devices

type	Device file	Document description number	Default device
The standard input	/dev/stdin	0	keyboard
standard output	/dev/stdout	1	Monitor
Standard error output	/dev/stderr	2	Monitor

2.2 Redirection operation

Redirect output

Overwrite the contents of the original file

Redirect input


Error output

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