Basic editing specifications and variables of shell script

One 、Shell

●Linux A special program running in a system

1、Shell The role of

●shell： Between the system kernel and the user , Responsible for interpreting the command line

2、 User login Shell

● The user login Linux System time , Automatically load a Shell Program
●bash yes Linux It is used by default in the system Shell Program
●bash The file is located in /bin/bash

Two 、Shell Script Overview

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

2、 Give the file executable permissions

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

1、Shell Script application scenarios

​ Repetitive operations

Interactive tasks

Batch transactions

Service running status monitoring

Scheduled task execution

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3、 ... and 、 To write Shell Script

1、 Write script code

1、 Use vim Text editor

2、 One in each line Linux command , Write... In order of execution

2、 Execute script file （ Three ways ）

① Method 1 ： Script file path （ Absolute path and relative path , need x jurisdiction ）

Relative paths ：

Absolute path

② Method 2 ：sh Script file path （ Don't x jurisdiction ）

Relative paths

Absolute path

③ Method 3 ：source or . Script file path （ Unwanted x jurisdiction ）

source Commands are also called “ Click command ”, It's a dot symbol （.）, yes bash The internal order of

Unwanted x jurisdiction , You also need to use relative or absolute paths

3、 Better script composition

● Script declaration

● Annotation information

● Executable statement

1. Script statement （ 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 , such as #!/usr/bin/python、#!/usr/bin/expect.

2. Annotation information ： With “#” The opening statement is expressed as a comment message , The commented statement will not be executed while the script is running .

3. Executable statement ： such as echo command , For output " " String between

Writing process ：

Execution results ：

Four 、 Redirection and pipeline operation

1、 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

2、 Redirection operation

example ：
setenforce 0
useradd zhangsan
echo “123123” > 1.txt
passwd --stdin zhangsan < 1.txt

example ：
cat 1.txt
echo “123123” >> 1.txt
cat 1.txt

example ：

ls -lh > log.txt 2>&1 Equate to ls -lh &> log.txt

>( Save the standard output results to the specified file )

2>&1（ Redirect standard error output to standard output ）

&>( Standard output 、 Standard error output is saved to the same file )

2、 Pipe symbol operation “|”

● Output the command on the left , As input to the command on the right （ Deal with people ）, Multiple pipes can be used in the same command line .

example ：ps aux | wc -l Show all processes

example ： Pipe symbols match xargs usage

xargs Commands can accept strings through pipes , And the received string is divided into many parameters by space ( By default, it is separated by spaces ) Then pass the parameter to the following command , As the command line parameter of the following command

5、 ... and 、Shell Role of variables 、 type

Variables are values that change , What doesn't change is constants

1、 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

2、 The type of variable

● Custom variable ： Defined by the user 、 Modify and use

● Special variables ： environment variable 、 A read-only variable 、 Positional variable 、 Predefined variables

6、 ... and 、 Custom variable

Define a new variable

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        Variable name = A variable's value       # Variable names start with letters or underscores , Case sensitive , It is suggested to use all capitals 
       
       

       
       
echo $ Variable name       # Look at the value of the variable 
      
      
      
      

       
       
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Equal sign （=） Used to assign values to variables

Equal sign （=） To the left of the operator is a variable name , Equal sign （=） To the right of the operator is the value stored in the variable

1、 Use quotation marks when assigning values

Double quotes ： Allowed to pass through $ 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 ,​​``​​​ and $(…) The same effect

2、 Assign values to variables from keyboard input

read Command to get input
Method 1 ：

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       read -p  Prompt information   Variable name 
       
       
echo $ Variable name 
      
      
      
      

       
       
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​

Method 2 ：

Write... In a script , Implement... On the command line read obtain

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       vim name.sh
       
       
#！/bin/bash
       
       
read -p “ Please enter your name ” NAME
       
       
echo "--------"
       
       
read -p “ Please enter his name ” NAME2
       
       
echo "--------"
       
       
echo " Your name is "
       
       
echo $NAME
       
       
echo " His name is "
       
       
echo $NAME2
      
      
      
      

       
       
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​ Execution results ：

3、 Set the scope of the variable

have access to pstree Command view Shell Environmental Science , Input bash The command enters the sub Shell Environmental Science ,
Press Ctrl＋D Key combination or enter exit Command exit Shell Environmental Science

Use bash The command enters the sub Shell Environmental Science

By default , The newly defined variable is only in the current Shell Effective in the environment , So it's called a local variable . When entering a subroutine or a new subroutine Shell Environmental time , Local variables will no longer be used .

It can be done by internal command export Exports the specified variable as a global variable , Enables user-defined variables in all child variables Shell Can continue to be used in the environment .

​

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

       
       
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4、 The operation of integer variables

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

Common operational expressions ：

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       i=$(expr 12 \* 5)
       
       
i=$((10 * 5))
       
       
i=$[10 * 4]
       
       
let i=10*3
       
       

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

       
       
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7、 ... and 、 special Shell Variable

1、 environment variable

Created in advance by the system , Used to set up the user's work environment

The configuration file ：/etc/profile（ Global effect ）、~/.bash_profile（ Current user environment ）

2、 Common environment variables

PWD、PATH

USER、SHELL、HOME

Use env The command can view the environment variables in the current working environment

Variable USER Represents the user name ,HOME Represents the user's Host Directory ,LANG Represents the language and character set ,PWD Indicates the current working directory , Variable PATH Represents the default search path for executable programs

①、PATH（ Path environment variable ）

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       echo $PATH          # View the current search path 
       
       
PATH="$PATH:/root"      # take /root Add directory to search path 
       
       
export PATH="$PATH:/root" # The output is a global environment variable 
       
       
first.sh
      
      
      
      

       
       
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The directory has been added to the path environment variable , All files with execution permission in the directory , Can be executed in any directory in the current environment

②、 A read-only variable

Used when the variable value cannot be modified

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       readonly  Command to set read-only variables 
       
       
readonly PRODUCT        # Set to read-only variable 
       
       
echo $PRODUCT
       
       
PRODUCT=Python              # Read only variables cannot be reassigned 
       
       
unset PRODUCT         # Read only variables cannot be deleted ,unset  The command is used to delete variables , But you cannot delete read-only variables , It can only be solved by restarting the system 
      
      
      
      

       
       
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③、 Positional variable

When performing command line operations , The first field represents the command name or script program name , The remaining string parameters are assigned to the position variable in order from left to right .

$n：n Is the number ,$0 For the order itself ,1~9 Represents the first to ninth parameters , More than ten parameters need to be represented by braces , For example, the tenth parameter is ${10}

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       vim lic.sh
       
       
#!/bin/bash
       
       
echo $1
       
       
echo $2
       
       
echo $1 + $2
      
      
      
      

       
       
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④、 Predefined variables

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       $*、[email protected]： Represents the parameters to be processed by a command or script .
       
       
  $*： Think of all parameters as a whole string separated by spaces , representative "$1 $2 $3 $4".
       
       
  [email protected]： Separate each parameter with double quotation marks into n A list of the parameters of the , Each parameter is independent    Of , representative "$1" "$2" "$3" "$4".
       
       

       
       
$0： Represents the name of the currently executing script or command .
       
       
$#： Represents the number of parameters to be processed by a command or script .  
       
       
$?： Indicates the return status code after the execution of the previous command or script , The return value is 0 The execution is correct , Return any non 0 Values indicate an exception in execution .
       
       
 It is also often used for Shell Script return Exit function and return the exit value .
      
      
      
      

       
       
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       vim mybak.sh
       
       
#!/bin/bash
       
       
time=backup-`date +%F`.tar.gz
       
       
tar zcf $time $* &> /dev/null #/dev/null Represents a black hole file , Usually used to discard unwanted data output 
       
       
echo " Has been carried out  $0  Script ,"
       
       
echo " To complete  $#  Backup of objects "
       
       
echo " Specific contents include : $*"
      
      
      
      

       
       
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       chmod +x mybak.sh
       
       
./mybak.sh /etc/passwd /etc/shadow
      
      
      
      

       
       
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summary

shell The role and application scenarios of

shell Script writing specification and execution method

The role and usage of redirection and pipeline

Single quotation marks when assigning custom variables 、 Double quotes 、 How to use the apostrophe

Common operators for numeric variables ：+、-、\*、/、%

environment variable 、 A read-only variable 、 Positional variable 、 Purpose of predefined variables