[shell programming] - shell introductory learning

Shell Introduction learning

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summary

Shell What is it?

Shell It's a command line interpreter , It provides a way for users to Linux The kernel sends requests to run the program's interface system level program , Users can use Shell To start up , Hang up , stop it , Write some programs
Linux It needs to be written in operation and maintenance Shell Program to manage the server ; To write Shell Scripts can be used for program or server maintenance , Such as the script of regularly backing up the database

introduction

To write Shell Script

Create and save a .sh file (Shell Scripts are usually written in sh Suffixed name , But it's not necessary )
Script files must be in #!/bin/bash start
There is no need to add a semicolon at the end of each statement in the script file
'#' For single line comments ; use ":<<!“ as well as ”!" Include content for multi line comments , And these two symbols should occupy a separate line ：

:<<!
 The comment 
!

hello world Example ：

#!/bin/bash
echo "hello world"

perform Shell Script

The way 1： Enter the absolute or relative path of the script . First, give the script file executable permission
The way 2：sh Script file path , The script can be executed directly without giving executable permission . This is not recommended

Variable

Shell The variables in are divided into system variables and user-defined variables
System variables such as $HOME,$PWD,$SHELL,$USER,$PATH wait , Such as echo "PATH=$PATH"
Use set Command to view the current Shell All variables in
Defining variables ： Variable = value
Revoke variables ：unset Variable

A=100
echo "A=$A" # Output  A=100. use '$' To reference variables , Or add braces ：${ Variable name }
unset A
echo "A=$A" # Output  A=

Declare static variables ：readonly Variable ( Static variables cannot unset, There's... In the script unset An error will be reported when executing the script for the operation of static variables )

Rules for variable definition

1. Variable names can be made up of letters , Numbers , Underline composition , But don't start with a number ( Will report a mistake )
2. No spaces on both sides of equal sign
3. Variable names are usually capitalized

Assign the return value of the command to the variable

1.A=`ls - la`, The quotation marks , Run the command inside , And return the result to the variable A
2.A=$(ls - la), Equivalent to backquotes ;MYDATE=$(date), Get the date and assign a value

environment variable

In profile ( Such as /etc/profile file ) Set environment variables in , It can be quoted elsewhere
export Variable name = A variable's value , take shell Variable output is environment variable
source The configuration file , Let the modified configuration information take effect immediately
$ Variable name , Take the value of the environment variable

Position parameter variable

When executing a shell Script time , If you want to get the parameter information of the command line , You can use the position parameter variable . Such as execution ./myshell.sh 10 20 On command , Want to be in myshell.sh Get from the script file 10 or 20 These parameters in the command line
The specific method of use is in shell The following symbols are used in the script ：
$n,n Is the number ,$0 The command itself ,$1-$9 Represents the first to ninth parameters , Parameters above ten need to be enclosed in braces , Such as ${13}
$*, Represents all parameters in the command line , Treat all parameters as a whole
[email protected], It also represents all parameters in the command line , But it treats each parameter differently
$#, Represents the number of all parameters in the command line

Predefined variables

The predefined variable is shell Variables that have been defined by the designer in advance , Can be directly in shell Use in script
$$, The process number of the current process (PID)
$!, The process number of the last process running in the background (PID)
$?, Return status of last executed command . If its value is zero 0, It indicates that the last command was executed correctly ; If it is not right 0( The specific number depends on the command itself ), It means that the last command was executed incorrectly

Operator

Grammatical form ：
$(( Arithmetic expression )) or $[ Arithmetic expression ]( recommend )
`expr m + n`,expr There should be spaces between operators in ; Enclose in back quotation marks , And there must be no space between the backquote and the preceding equal sign
`expr m - n`
`expr \*` Multiplication ,`expr /` division ,`expr %` Remainder
Multiplication ’*' The backslash before is used to escape ,expr If there are left and right parentheses in, you also need to escape

conditional

[ condition ], Be careful condition Space before and after
Not empty return true, You can use $? verification ,0 by true,>1 by false;[] return false

Conditional judgment symbol

The following symbols should have spaces on the left and right

A comparison of two integers

-lt Less than ;-le Less than or equal to ;-eq be equal to ;-gt Greater than ;-ge Greater than or equal to ;-ne It's not equal to
= String comparison . The comparison result can be used ! Take the opposite

Judge according to the file authority

-r Have read permission ;-w Have the right to write ;-x Have the authority to execute
Put the file to be judged to the right of the symbol

Judge according to the document type

-f The file exists and is a regular file
-e File exists
-d The file exists and is a directory
Put the file to be judged to the right of the symbol

Process control

if sentence

if [  Conditional judgment  ];then
	 Other statements 
fi

perhaps

if [  Conditional judgment  ]
	then
		 Other statements 
	elif [  Conditional judgment  ]
		then
			 Other statements 
fi

There must be a space between the bracket and the conditional judgment in the conditional judgment ; Recommend the second way

case sentence

case $ Variable name  in
	" value 1")
		# If the variable value is a value 1, Execute the statement here 
	;;
	" value 2")
		# If the variable value is a value 2, Execute the statement here 
	;;
	......
	*)
		# If the value of the variable is not the value listed above , Execute the statement here 
	;;
esac

for sentence

for  Variable  in  value 1  value 2  value 3...
	do
		 Other statements 
	done

perhaps

for(( Initial value ; Cycle control conditions ; Variable change ))
	do
		 Other statements 
	done

Example 1： Print parameters in the command line

Script files test.sh：

#!/bin/bash

for i in "$*"
do
	echo "num is $i"
done

perform ./test.sh 10 20 30 After the command , The console will print out num is 10 20 30. because $* Treat the command line parameters as a whole . use [email protected] Instead of $* Words , Will be output ：

num is 10
num is 20
num is 30

This is it. $* and [email protected] The difference between

Example 2： from 1 Add up to 100 Output results

SUM=0
for((i=1;i<=100;i++))
do
	SUM=$[$SUM+$i]
done
echo "SUM=$SUM"

while sentence

while [  Conditional judgment  ]
do
	 Other statements 
done

Example ： Enter a number... From the command line n, Statistics from 1 Add to n Value

#!/bin/bash

SUM=0
i=0
while [ $i -le $1 ] # You can judge whether spaces are needed according to color changes , If it's here while If the space behind is removed while It changes color , Instructions need spaces 
do
	SUM=$[$SUM+$i]
	i=$[$i+1]
done
echo "sum= $SUM"

read Read console input

read [ Options ] [ Parameters ]
Options ：-p, Specify the prompt when reading the value ;-t, Specifies the time to wait while reading the value ( second ), If it is not entered in the specified time , I don't wait
Parameters ： Specifies the variable name of the read value

function

shell The functions in have system functions , You can also have custom functions

Examples of system functions

basename

This function is used to return the last of the full path / Part of , Often used to get file names
basename [path] [suffix]
basename [string] [suffix]
Options suffix For the suffix , If suffix Designated ,basename Will path or string Medium suffix Get rid of , Such as basename /home/test/test.txt .txt You'll get test

dirname

This function returns the full path last / The front part of , Commonly used in the return path part
dirname File absolute path
Such as dirname /home/test/test.txt You'll get /home/test

Custom function

[ function ] funName[()]
{
    
	Action;
	[return int;]
}

Write the function name directly when calling funName. Brackets here function as well as return A sentence means you can leave it unwritten

Example ： Calculate the sum of the two input parameters

#!/bin/bash

function getSum(){
    
	SUM=$[$n1+$n2]
	echo " And for $SUM"
}
read -p " Please enter the first number " n1
read -p " Please enter the second number " n2

getSum $n1 $n2

You can see that there are no formal parameters