Getting started with shell programming base

1、 What is? Shell

2、 What is? Shell Script

3、Shell Scripting language type

4、 Script creation and execution 、 stop it

5、 Basic operators of scripts

1、 What is? Shell

Shell Is a command interpreter , His role is to interpret and execute commands and programs entered by users , The user enters a command , It executes a , The way of dialogue that can get an immediate response is called " Interaction "

Shell It exists in the outermost layer of the operating system , Responsible for direct dialogue with users , Interpret the commands entered by the user to the operating system , And process the output of various operating systems , Then output to Mingming and return to the user ; for example , stay Linux System users enter user name and password and log in to Linux All operations are performed by Shell Explain and execute .

from ’Shell’ English definitions of words are available , command interpreter (Shell) It means shell , From the picture Shell Like a shell, it wraps the core of the system

2、 What is? Shell Script

When a command or program statement is not executed on the command line , It's executed through a program file , The program can become Shell Script , Generally speaking, it means writing a lot of commands into the script , Again by Shell perform , It's called Shell Script , This way of executing scripts through files is called non interactive , stay Windows Next , This is called ‘Dos Batch program ’

Example : eliminate /var/log/ Under the message Log files

Must be root User execution

#!/bin/bash
cd /var/log
cat /dev/null > messages
echo "Logs Cleaned up."

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3、Shell Scripting language type

1、Shell Script language in Unix/Linux There are two main categories Shell: One is Bourne Shell 、 One is C Shell

1.Bourne shell

Include Bourne shell(sh)、 Korn shell(ksh) 、Bourne Again Shell(bash) Three

2.C shell

Include csh 、 tcsh Two kinds of

Check... Supported by the system Shell

#[[email protected] ~] # cat /etc/shells

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2、 Other common scripting languages

①PHP Language

②Perl Language

③Python Language

3、Linux Check the system default Shell

 Law 1 
[[email protected] ~] # echo $SHELL
 Law two :
[[email protected] ~] # grep root /etc/passwd

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4、 Script creation and execution

1、 At the beginning of the script ( first line ) It will indicate which interpreter executes the content in the script in the following form , as follows

#!/bin/bash
 or 
#!/bin/sh

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1、 Write a simple script , The conditions are as follows

1、 stay /root/ Create the script under , Name it test.sh 2、 Turn off firewall (firewalld) 3、 Turn off firewall self startup 4、 Set up Selinux The security module is in allow mode 5、 Change the hostname to test 6、 View current time 7、 Delete the script after execution

#!/bin/bash
systemctl stop firewalld
systemctl disable firewalld
setenforce 0
hostnamectl set-hostname test
date
echo "Execution completed ！"
rm -rf /root/test.sh

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2、 utilize while Output

#!/bin/bash
while [ 1 -gt 0 ]
do
sleep 1
echo 'Welcome to China !'
done

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Here is the use of while Statement to output circularly , Be careful [] There must be a space between brackets and numbers , Can't be together , there -gt Means greater than , if 1>0 Conditions established , Then execute

2、 Script execution

1、 Use . Add the file name or absolute path , There is a space after dot

2、 Use the interpreter to execute , as follows ：

3、 Use source

3. Termination of script

CTRL + C

5、 Basic operators of scripts

Operator	explain
-eq	Check whether two numbers are equal , Equal return true.
-ne	Check if two numbers are not equal , Unequal return true.
-gt	Check whether the number on the left is greater than that on the right , If it is , Then return to true.
-lt	Check if the number on the left is less than the number on the right , If it is , Then return to true.
-ge	Check whether the number on the left is equal to or greater than the number on the right , If it is , Then return to true.
-le	Check whether the number on the left is less than or equal to the number on the right , If it is , Then return to true.