Introduction to Shell Script

concept

Shell script is a text file containing a series of commands
Shell reads this file and executes commands

Application scenarios

Repetitive operation
Bulk transaction processing
Automated operation and maintenance
Scheduled task execution

The first shell script

Write the first Shell script helloword.sh

#!/bin/bash
echo “Hello World”

#! is an agreed mark, which tells the system what interpreter the script needs to execute

Common execution methods of scripts

The first: use the relative path or absolute path of bash or sh+ script (do not give script +x permission)

sh helloworld.sh
bash helloworld.sh

The second type: use the absolute path or relative path of the input script to execute the script (must have executable permission +x)"

First, give the +x permission to the helloworld.sh script

chmod 777 helloworld.sh

Execute script

./helloworld.sh

Variables in Shell

System variable

Common system variables:

$HOME, $PWD, $SHELL, $USER, etc.

Note: Display all variables in the current shell: set

[root@cent02 ~]# echo $PWD
/root

Custom variable

Basic grammar

Define variable: variable = value
Undo variable: unset variable
Declare static variables: readonly variables, note: cannot be unset

Rules for defining variables

Variable names can consist of letters, numbers, and underscores, but they cannot start with numbers.
There can be no spaces on both sides of the equal sign
Variable names are generally used to uppercase

Assign the return value of the command to a variable

A=`ls -la` backquote, run the command inside, and return the result to the variable A
A=$(ls -la) is equivalent to backticks

       NN0[root@cent02 ~]# A=`ll`
[root@cent02 ~]# echo $A
total 48 -rw-------. 1 root root 1320 Aug 11 03:20 anaconda-ks.cfg -rw-r--r--. 1 root root 18430 Aug 18 03:57 ‪H:testschool.sql -rw-r--r--. 1 root root 10399 Aug 19 01:12

Set environment variables

export variable name=variable value (function description: output shell variables as environment variables)

source configuration file (function description: make the modified configuration information take effect immediately)

echo $ variable name (function description: query the value of environment variable)

Positional parameter variable

$n (Function description: n is a number, $0 represents the command itself, $1-$9 represents the first to ninth parameters, parameters above ten, parameters above ten need to be enclosed in braces, such as ${10})
$* (Function description: This variable represents all the parameters in the command line, $* regards all the parameters as a whole)
$@ (Function description: This variable also represents all the parameters in the command line, but $@ treats each parameter differently)
$# (Function description: This variable represents the number of all parameters in the command line)

Predefined variables

$$ (Function description: Process ID (PID) of the current process)
$! (Function description: Process ID (PID) of the last process running in the background)
$? (Function description: the return status of the last executed command. If the value of this variable is 0, it proves that the previous command was executed correctly; if the value of this variable is not 0 (which number is determined by the command itself), then Prove that the previous command was executed incorrectly.)

Operator

"$((Expression))" or "$[Expression]"
expr m + n Note that there must be spaces between expr operators
expr m-n
expr *, /,% multiplication, division, remainder

Conditional judgment

[condition] (note that there must be spaces before and after condition)

#Non-empty returns true, you can use $? to verify (0 is true, >1 is false)

Commonly used judgment conditions

Comparison of two integers

Options	Description
-eq	equal
-born	not equal to
-gt	more than the
-lt	Less than
-the	less than or equal to
-give	greater than or equal to

Judge according to file permissions

Options	Description
-d	Determine whether it is a directory
-e	Determine whether the directory or file exists
-f	Determine whether it is a file
-r	Determine whether a file or directory is readable by the current user
-w	Determine whether the file or directory is writable by the current user
-x	Determine whether the file or directory is executable for the current user

Process control

if judgment

if [ 条件判断式 ];then 

	程序 

fi

if [ 条件判断式 ] 

	then

		程序 

	elif [条件判断式]

	then

		程序 

fi

Note: (1) [Conditional judgment formula], there must be a space between the brackets and the conditional judgment formula (2) The second method is recommended

case statement

case $变量名 in 

	"值 1"） 

		如果变量的值等于值 1，则执行程序 1 

		;; 

	"值 2"） 

		如果变量的值等于值 2，则执行程序 2 

		;; 

…省略其他分支… 

	*）

		如果变量的值都不是以上的值，则执行此程序

		;; 

esac

for loop

for 变量 in 值 1 值 2 值 3… 

	do

		程序 

	done

for (( 初始值;循环控制条件;变量变化 )) 

	do

		程序 

	done

while loop

while [ 条件判断式 ] 

	do

		程序 

	done

read read console input

read(选项)(参数)

Options:

-p: Specify the prompt when reading the value;

-t: Specify the waiting time (seconds) when reading the value. If it is not entered within the specified time, it will not wait anymore. .

parameter

Variable: Specify the variable name to read the value

function

System function

basename basic syntax

Function: return to the last part of the full path, often used to get the file name

basename [pathname] [suffix]

basename [string] [suffix] (Function description: basename command will delete all prefixes including the last ('/')

Character, and then display the string.