One 、Shell Script Overview

1.1.1 Shell Basic concepts

• 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

1.1.2 Shell Script application scenarios

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

1.1.3 Shell effect —— The interpreter

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 .

Two 、shell Programming specification

2.1 The user login shell

• After logging in, the default shell Program , It's usually /bin/bash
• Different shell The internal instructions of 、 The operating environment will be different
  

shell	explain
bash	$ Base on GNU Developed under the framework of shell
csh	The grammar is a bit similar to C Linguistic shell
tcsh	Integrated csh, More features （ Enhanced version ）
sh	Has been bash Replaced by （ Soft link ）
nologin	strange shell, This shell It can prevent users from logging in to the host

2.2 shell The composition of the script

• Script statement （ Interpreter ）： If the first act “#!/bin/bash”, Indicates that the code statement below this line is passed /bin/bash Program to execute ,#!/bin/bash Is the default interpreter , There are other types of interpreters , such as #!/usr/bin/python、#!/usr/bin/expect.
• Annotation information ： With “#” The opening statement is expressed as a comment message , The annotated statement will not be executed when the script is run .
• Executable statement ： such as echo command , For output “ ” String between .

2.3 shell How to execute the script

Method 1 ： Command to specify the path , It is required that the document must have x jurisdiction

• Specify the absolute path ：/root/first.sh
• Specify the relative path ：./first.sh

Method 2 ： Appoint shell To explain the script , It is not required that the document must have x jurisdiction

• sh Script path ：sh first.sh
• source Script path ：.first.sh perhaps source first.sh
  

Method 3 ：source Script path execution shell Script

2.4 Script error

Wrong command ： Command error will not affect the next command to continue

Grammar mistakes ： Will affect the next command to continue

Logic error ： You can only screen yourself

 Find the correct code ：

bash -n  Script name  （ Do not add an absolute path to the current directory ）    Check for grammatical errors 
bash -x  Script name  （ Do not add an absolute path to the current directory ）    Check for logic errors 

2.5 Redirect

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

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

Redirection operation

type	operation	purpose
Redirect input	<	Read data from the specified file
Redirect output	>	Save the standard output results to the specified file , And overwrite the original file
	>>	Appends the standard output to the end of the specified file , Don't cover the original content
Standard error output	2>	Save the error message to the specified file , And overwrite the original file
	2>>	Append the error message to the end of the specified file , Don't cover the original content
Mixed output	&> and 2>&1	Standard output , The standard error is saved to the same — In file

Case a
Put the standard output result “132456” Save to file passwd.txt; Then enter the standard password to niangao When , From the specified file passwd.txt Read data from ：

Case 2

“>”  It will cover the original file 
“>>”  The original file will not be overwritten 

Case three :& usage

2>/dev/null  ## Output the error to the black hole 
>/dev/null 2>&1  ## Redirect standard output to black hole , Error output 2 Redirect to standard output （ Both standard output and error output go to black hole ）
2>&1>/dev/null   ## Error output redirected to standard output , Standard output into black hole 

Add

2.6 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

Use format ：
cmd1 command 1 | cmd2 command 2[… | cmdn command n]

Pipeline operation

 awk -F:  '{print $1,$3}'   ## It means to force the output of information and print the first and third Columns 

3、 ... and 、Shell Script variables

Various Shell It is used in the environment “ Variable ” The concept of .Shell Variables are used to store specific parameter values that the system and users need to use .

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 environment

The type of variable

Custom variable ： Customized by user 、 Modify and use
Special variables ： environment variable , A read-only variable , Positional variable , Predefined variables

environment variable ： Maintained by system , Used to set up the work environment
A read-only variable ： Used when the variable value cannot be modified
Positional variable ： Pass parameters to the script through the command line
Predefined variables ：bash A class of variables built into , Cannot be modified directly

3.1 Custom variable

1. Define new variables ：

 Format ：  Variable name = A variable's value 
[root@pxw ~]# product=c++

2. View the value of the defined variable ：

 Format ：echo $ Variable name 
[root@pxw ~]# version=3.5.1

3. Special operation of variable assignment

quotes	explain
Double quotes	Allowed to pass through $ Symbols refer to other variables
Single quotation marks	Do not reference other variable values ,$ Treat as normal characters
Apostrophe	Command substitution , Extract the output of the command after execution

4.read command

-------- Method 1 ：read Command to get input ---------
read [-p " Prompt information "]  Variable name 
echo $ Variable name 
[root@pxw ~]# echo 192.168.61.128 > ip.txt
[root@pxw ~]# read -p "input your IP" < ip.txt
[root@pxw ~]# echo $IP
192.168.100.10

-------- Method 2 ： Write... In a script , Implement... On the command line read obtain ---------
read  Variable name 
echo $ Variable name 

Method 1 ：

Method 2 ：


5. Set the scope of the variable

export  Variable name = Variable 

3.1.1 Operation of numerical variables

Operator ：+ Add 、- Subtraction 、* Multiplication 、/ division 、% Remainder

expr  Variable 1  Operator   Variable 2 
var=$(expr  Variable 1  Operator   Variable 2)
var=$(( Variable 1  Operator   Variable 2))
var=$[ Variable 1  Operator   Variable 2]
let var= Variable 1  Operator   Variable 2
 
i++ amount to i=$[$i + 1],i++ Is the assignment first , Re operation 
i-- amount to i=$[$i - 1],++i Is to calculate first and then assign value 
i+=1 amount to i=$[$i + 1]

---- expand -----
[root@Kiro shell]# expr $[2 * 2] ## You can use the normal operator directly in brackets 

Case a ： The remainder only needs the remainder after division

# Law 1 
[root@localhost data]#expr 3 % 2 
1
 
# Law two ： Defining variables , Use the apostrophe 
[root@Kiro shell]# sum=`expr $X + $X + $Y`
[root@Kiro shell]# echo $sum
301

Case 2 ： Extension of multiplication sign

Case three ： Script

3.2 environment variable

Environment variable refers to the environment variable generated by Linux A type of variable created by the system in advance , It is mainly used to set the user's working environment , Include user host Directory 、 Command to find the path 、 User current directory 、 Login terminal, etc . The environment variable consists of Linux Automatic system maintenance , It will change with the change of user state ：

• If after modification ,（uname） , Restart to restore the original ;
• Or copy the previous one and assign the variable directly

 adopt env Command to view , It's all set ：
HOSTNAME=Kiro
SHELL=/bin/bash
HISTSIZE=1000
USER=root
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin
PWD=/root/shell

Case study ：

3.3 Positional variable

In order to use Shell Script program , It is convenient to provide parameters through the command behavior program ,bash The concept of position variable is introduced .

[root@Kiro shell]# cat vars.sh
#!/bin/bash

echo $1
echo $2
echo $3
[root@Kiro shell]# ./vars.sh 11 22 cc
11
22
cc

Case study ：

3.4 Predefined variables

And defining variables is bash A special class of pre-defined variables , Can only be used and cannot create new ：

$# : Indicates the number of positional parameters in the command line 
$*:  Represents the contents of all unknown parameters 
$?:  Indicates the return status after the execution of the previous command , by 0 The execution is correct , Not 0 Indicates an exception occurred during execution ;
$0:  Indicates the name of the currently executing script or program 
$$：  Indicates the process number of the current script 

Case study ：

3.5 Use awk Value extraction IP value

[root@Kiro shell]# ip=`ifconfig ens33|awk /netmask/'{print $2}'`
[root@Kiro shell]# echo $ip
192.168.61.100

3.6 Global and local variables

• Global variables

 For global variables  export 
[root@Kiro ~]# export Y=22
[root@Kiro ~]# echo $Y
22

• Local variables

 Local variables are only currently valid 
[root@Kiro ~]# a=100
[root@Kiro ~]# echo $a
100
[root@Kiro ~]# exit
exit
[root@Kiro ~]# echo $a

Can be found in pstree Look at the sub variables in

【 expand 】

• uname -p ## Query processor model
  
• /dev/null ## empty , Error output can be put in
• Command line input “bc”==Linux The computer

• permanent /etc/profile