Shell programming specifications and variables

One 、Shell Script Overview

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

 Shell The role of

shell The role of —— command interpreter ,“ The interpreter ”

• Between the system kernel and the user , Responsible for interpreting the command line

  Two 、 Write the first Shell Script

Write script code

• Use vim Text editor
• One in each line Linux command , Write in order

#vim first.sh
cd /boot/
pwd
ls -lh vml*

Give executable permission

• Make the script executable

# chmod +x first.sh

Shell Script execution

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

chmod +x lroot/ first.sh

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

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 .

ps auxl wc -l

echo "abc123" l passwd --stdin zhangsan

3、 ... and 、 Redirect

1、 Interactive hardware devices

• The standard input ： Slave device ( Character device : Keyboard and mouse ) Receiving data from users
• standard output ： Through the equipment ( Monitor ) Output data to users
• The standard error ： Through the equipment ( Monitor ) Report execution error message

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、 Redirection operation

type	The operator	purpose
Redirect input	<	Read data from the specified file
Redirect output	>	Save the standard output results to the specified file , And cover the original content
	>>	Appends the standard output result 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 cover the original content
	2>>	Append the error message to the end of the specified file , Don't cover the original content
Mixed output	&>	Standard output 、 Standard errors are saved to the same file
	2>&1	Redirect standard error output to standard output

Four 、shell Script variables

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

    Custom variable ： Defined by the user , Modify and use
    environment variable ： Maintained by system , Used to set up the work environment
    A read-only variable ： Can only be read and cannot be changed
    Positional variable ： Pass parameters to the script through the command line
    Predefined variables ：Bash A class of variables built into , Do not modify
 

 3. Custom variable

Format : Variable name = A variable's value

• Variable name : A place where data is temporarily stored
• A variable's value : Temporary town change data
• There is no space on either side of the equal sign . Variable names should start with letters or underscores , Do not include special characters in the name （ Such as +、-、*、/、.、?、%、&、# etc. )

  Use echo View and reference the values of variables

By adding a leading symbol before the variable name "$", You can reference the value of a variable , Use echo Command can view variables , Can be in one echo View multiple variable values at the same time in the command

echo $ Variable name # Look at the value of the variable ,$ Replace the representative symbol for the variable

unset A variable's value # Cancel variables

echo Options ：
echo -n Indicates no line feed output
Use echo -e Output escape characters , Output the escaped content to the screen

The commonly used escape characters are as follows :

• \c Don't wrap output , stay "\c" If there are no characters after , The effect is equivalent to echo -n
• \n Line break
• \t After escape, it means to insert tab, That's the tab

 

 4. Define variables interactively （read command ）

You can also use Bash Built-in commands read To assign a value to a variable . Used to prompt the user to enter information , So as to realize a simple interactive process . It will be executed from the standard input device （ keyboard )7 Read in a line , And use a space as a separator , Assign the read fields to the specified variable in turn ( The extra content is assigned to the last variable ）. If only one variable is specified , Assign the entire line to this variable .

  Define variables interactively (read)

• -p Information to prompt users
• -n Define the number of characters
• -s Don't show user input , Commonly used to enter passwords read -s -p "input your password:" pass
• -t Define timeout , If you haven't lost for more than a long time, you will automatically quit

 

 

 stty echo // Turn on screen echo 

Operation content : Add (+)、 reduce (-)、 ride (*)、 except (/、 Remainder (%)
Operation symbol :$(o） and $[]
Operation command :expr and let
Computing tools :bc( System comes with )

expr command ( It can not only calculate , It also supports output to the screen )

Several commonly used operators are as follows .

• +: Addition operation .
• -: Subtraction .
• \*: Multiplication , Be careful not to use only "*" Symbol , Otherwise, it will be treated as a file wildcard .
• /: Division operations .
• %: Modular operation , Also known as remainder operation , Used to calculate the remainder after the division of numerical values .

 

  Use bc Carry out operations , Support decimal operations , However, it cannot be used directly in the script, otherwise it will enter the interactive interface , It can be used echo Use in combination with piping

  Boolean value ： really （true） Or false （false）

 

Common operational expressions :

• i=s (expr 12 \*5)
• i=$((12* 5))
• i=[12 * 5]
• let i=12*5

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

 echo $RANDOM Random generation 0-32767 The random number

 5. Predefined variables

The predefined variables are created by Bash A class of special variables pre-defined by a program , Users can only use predefined variables , You cannot create a new predefined variable , You can't assign values to predefined variables directly . Predefined variables use "$" A combination of a symbol and another symbol indicates

• $#: Indicates the number of positional parameters in the command line J
• $*: Represents the contents of all positional parameters , These contents as a whole
• [email protected]: Indicates that all position parameters are listed , But it is listed in a single form
• $?: Indicates the return status after the execution of the previous command , The return value is О The execution is correct , Return any non О Values indicate an exception in execution .
• $0: Indicates the name of the currently executing script or program
• $$: Indicates that the process number of the current process is returned
• $!: Returns the process number of the last background process

 $* Print out a whole

 

 [email protected] Print out individual