Linux-Beginner Series 7_shell programming

When managing server clusters, it is necessary to write shell programs for server management.

The shell is a command line interpreter that provides users with an interface system-level program that sends requests to the Linux kernel to run programs. Users use the shell to start, suspend, stop, and write some programs.

1. Execution method of shell script

1. Script format requirements

Scripts start with #!/bin/bash

#!/bin/bash

The script needs to have executable permission

2. Common execution methods of scripts

01-Enter the absolute path or relative path of the script

To give the +x permission to the script, execute the script.

02-sh+script

Execute directly without giving the script +x permission.

Two, shell variables

1. Variable introduction

The variables in the Linux shell are divided into: system variables and user-defined variables.

01-system variable

[root@bogon shcode]# echo $HOME
/root
[root@bogon shcode]# echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin
[root@bogon shcode]# echo $PWD
/root/shcode

Display all variables in the current shell

set

02-shell variable definition

1) Basic grammar

define variable

变量名=值

[root@bogon shcode]# vim var.sh
#!/bin/bash
#定义变量A
A=100
#输出变量需要加上$
echo A=$A
[root@bogon shcode]# chmod u+x var.sh
[root@bogon shcode]# ll
总用量 8
-rw-r--r--. 1 root root 30 5月   7 10:52 hello.sh
-rwxr--r--. 1 root root 70 5月   7 11:17 var.sh
[root@bogon shcode]# ./var.sh
A=100

undo variable

unset 变量

declare static variable

readonly变量

Note: Cannot unset.

tips:

If you want to find line 15, show the line number

:nu

2) Rules for defining variables

• Variable names can be composed of letters, numbers, and underscores, but cannot start with a number

  For example: 5A=200(x)

• There cannot be spaces on both sides of the equal sign

• Variable names are generally uppercase

Assign the command's return value to a variable

反引号：运行里面的命令，并把结果返回给变量A
A=`zhang`
A=$(zhang)

3. Set environment variables

1. Basic grammar

export 变量名=变量值
//将shell变量输出为环境变量/全局变量

source 配置文件
//修改后的配置信息立即生效

echo $变量名
//查询环境变量的值

Multi-line comments for shell scripts

:<<! 内容！

Fourth, the position parameter variable

When executing a shell script, if you want to obtain the parameter information of the command line, you can use the positional parameter variable.

./myshell.sh 100 200

This is a command line to execute the shell, and the parameter information can be obtained in the myshell script.

basic grammar

grammar	describe
$n	n is a number $0 stands for the command itself, $1-9represents\; the\; first\; to\; ninth\; parameters,\; and\; more\; than\; ten\; parameters\; need\; to\; be\; enclosed\; in\; braces:$ {10}
$*	For all parameters in the command line, $* regards all parameters as a whole
$@	Represents all parameters in the command line, but $@ treats each parameter differently
$#	An individual representing all arguments on the command line

5. Predefined variables

The variables defined in advance by the shell designer can be used directly in shell scripts.

basic grammar

grammar	describe
$$	The process number PID of the current process
$!	The process number PID of the last process running in the background
$?	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 non-zero (the specific number is determined by the command itself), it proves that the previous command was executed incorrectly.

6. Operators

Perform various operations in the shell.

1. Basic grammar

01-First way

"$((运算式))"

02-Second way

"$[运算式]"

03-The third way

expr m + n

There must be a space between expr operators.

If you want the result of expr to be assigned to a variable, use backticks ``.

expr \*,/,%
乘，除，取余

practise

Seven, conditional judgment

1. Basic grammar

#condition前后要有空格
[ condition ] 
#非空返回true,可以使用$?验证（0为true,>1为false）

2. Judgment statement

Common Judgment Conditions

01-string comparison

=

02- Comparison of two integers

sentence	describe
-lt	less than
- the	less than or equal to
-eq	equal
-gt	more than the
-ge	greater or equal to
- is	not equal to

03- Judgment according to file permissions

grammar	describe
-r	read permission
-w	write permission
-x	permission to execute

practise

3. Process control

01 if judgment

basic grammar

if [ 条件判断式 ]
then
  代码
fi

if [ 条件判断式 ]
then
  代码
elif [ 条件判断式 ]
then
  代码
fi

Note: [Conditional Judgment], there must be a space between the square brackets and the conditional judgment.

02 case statement

basic grammar

case $变量名 in
"值1"）
如果变量的值等于值1，则会执行程序1
;;
"值2"）
如果变量的值等于值2，则会执行程序2
;;
...省略其他分支...
*)
如果变量的值都不是以上的值，则会执行此程序
;;
esac

03 for loop

Basic Grammar 1

for 变量 in 值1 值2 值3...
do
程序
done

Basic Grammar 2

for ((初始值;循环控制条件;变量变化))
do
程序
done

04 while loop

basic grammar

while [ 条件判断式 ]
do
程序
done

Eight, read read console input

basic grammar

read(选项)(参数)

options

options	describe
-p	Specifies the prompt when reading values
-t	Specify the time (in seconds) to wait when reading the value, if it is not entered within the specified time, it will not wait anymore

parameter

变量：指定读取值的变量名

Example:

Nine, function

Shell programming, like other programming languages, has system functions and can also customize functions.

1. System functions

01 basename basic syntax

Returns the last / part of the full path, often used to get the file name.

basename [pathname] [suffix]

Used to get the filename (not including the path). where, pathnamedenotes the file path to process, and suffixdenotes the suffix to remove from the filename.

For example:

The following command to get the filename of the file:

If we want to remove .txtthe suffix, we can use the following command

02 basic grammar of dirname

Used to get the file path (not including the file name).

dirname [pathname]

pathnameIndicates the file path to process.

2. Custom functions

basic grammar

[function] funname[()]
{
  Action;
  [return int]
}
调用函数名：funname [值]

functionUsed to declare a function, the function name can be named arbitrarily.