Shell basic programming and trap commands, user online examples

write a script first

先写一个Shell脚本文件
  1 #!/bin/bash    #脚本默认开头                                                                              
  2 
  3 echo "hello!"
默认没有执行权限x，可通过chmod添加
$ sudo chmod +x T01.sh    #添加执行权限
$ bash T01.sh             #通过bash执行脚本
$ ./T01.sh                #直接执行脚本

1. Variables

Local variables: limited to the command line, or Shell script files.
Global variables: Shell process and all its child processes
You can use the export built-in command to set local variables as global variables, usage: export local variable name
There are three types of variables: ordinary variables (user-defined variables), system predefined environment variables (PATH) and command line variables ( $KaTeX parse error: Expected 'EOF', got '#' at position 1: #̲,$ *)
When assigning a value to a variable, there must be no spaces on both sides of the equal sign, and you need to add $ in front of the reference variable.
Setting of environment variables (each environment variable is denoted by a capital letter)

$ myname="hello I am xiaona!"    #普通变量赋值
$ echo $myname                   #引用普通变量
$ env                            #查看系统环境变量
$ export PATH=dir/               #修改PATH的变量为dir/
$ export PATH=$PATH:dir/         #PATH再增加一个目录（增加新的值需要使用:分隔开，不该便原有的值）
$ source ~/.bashrc               #使刚添加的环境变量生效

command line variables

#       命令行变量
$ ./T01.sh  abcd 1234            #以这行命令为例子
#  $0   保存的当前脚本名字（文件名字）
#  $#   代表命令行的参数个数，即2
#  $*   代表所有的参数，即abcd、1234
#  $@   代表所有的参数，即abcd、1234
#  $n   代表第n个参数，如$1表示abcd,$2表示1234
# 两个特殊变量 $?和$$
#  $?   代表最后一个命令的返回值
#  $$   代表当前Shell的进程号PID
#  $!   后台运行的最后一个进程的PID号
#  $-   使用set及其执行时传递给Shell的标志位

Special symbols quotes, vertical bars (pipes), and redirection

There are 3 kinds of quotation marks, backtick `` single quotation mark '' double quotation mark ""
- Single quotes: Any variables, metacharacters, wildcards, and escape characters are not allowed to be parsed by the shell within single quotes, and are output as they are.
- Double quotes: Protect special metacharacters and wildcards from being parsed by the shell, but allow parsing of variables and commands, as well as parsing of escape characters.
- Backticks: The function of backticks is command substitution. The content in backticks (``) is usually a command line. The program will execute the content in backticks first, and replace the content in backticks with the running results.
Vertical bar (pipeline): The output of the previous command is used as the input of the next command (completed in sequence)
Redirection (>, <, >>, <<)
- Three devices of Linux
  - Standard input: stdin, represented by 0 , is accepted as keyboard input by default
  - Standard output: stdout, represented by 1 , the default output is the terminal window
  - Standard error: stderr, represented by 2 , the default output is the terminal window

$ ls notexist 2> a.txt  #（notexist是一个不存在的文件，所以将错误信息重定向到a.txt中）
$ echo 0< a.txt         #echo读取a.txt的内容
$ echo "hello world" 1>&2  #重定向到标准错误设备
# 在脚本中，在重定向的右边，标准输入、标准设备文件描述符要写成&0、&1和&2

字符串的处理
$ echo "${
     
     #var}"           #计算var变量的字符数
$ echo "${var#apple}"    #删除var变量的左边字符串apple
$ lev=${var%%[a-zA-Z]*}  #删除var变量的右边字符串
#  从右到左 %%贪恋匹配（尽可能多的匹配） %无贪恋原则（尽可能少的匹配）
#  从左到右 ##贪恋匹配                   #无贪恋原则

2. Test statement

The test statement is completely equivalent to the square brackets [ ]
Argument substitution: "Argument can be either a positional argument or another variable"

grammar	Function
variable=${parameter:-word}	If the parameter is set, replace the value of the variable with the value of the parameter, otherwise replace it with word
variable=${parameter:=word}	If the parameter is set, the value of the variable is replaced with the value of the parameter, otherwise the word is assigned to the parameter, and then the word is replaced with the variable
variable=${parameter:?word}	If the parameter is set, the value of the variable is replaced with the value of the parameter, otherwise the word is displayed and the shell exits
variable=${parameter:+word}	If the parameter is set, the value of the variable is replaced with the value of the parameter, otherwise no replacement is performed

Arithmetic operation format: $ [expression]
- Expressions are meaningful formulas composed of integers, variables, and operators.
- The three logical operators of bash!, && and ||; their priority is ! > && > ||
- Bash allows the use of parentheses to make the expression a whole, and the parentheses have the highest priority.

3. Grammatical unit

1. Branch control

single branch statement

# 单分支语句
if [ -e file ] && [ -r file ] then  #file文件是否存在，是否可读
	cat file  #查看该文件
fi

Multi-branch writing of if-else

## if-else的多分支写法
if [ -e file ] && [ -r file ] then #方括号内的左右两边都要空格隔开
	cat file
elif [ -e file ] then
	chmod u+r file   #授予读权限
	cat file
else           #只有else不需要then，其他的if-else的分支都需要then
	touch file
fi

How to write case branch

## case分支的写法
read VAR             #从键盘接收一个用户输入，付给变量VAR
case $VAR in         #判断VAR变量的值
	1) echo "one";;  #若值为1，则输出one
	2) echo "two";;  #每个分支都必须以双分号作为结束(最后一个分支除外)
	*) echo "unknown"  #星号*匹配任意字符
esac   #必须以esac结尾，1可以是'1'

2. Loop control (three types)

while loop

##while的循环
declare -i n=0      #declare -i 表示该变量为数值
while [ $n -le 100 ] #条件n小于等于100循环
do                   #循环提用do和done包含起来
	echo "$n"
	n=$n+1           #变量n的自加1
done

until loop

##until的循环
declare -i n=0      #declare -i 表示该变量为数值
until [ $n -gt 100 ] #条件n小于等于100循环 until是条件不成立执行循环体
do                   #循环提用do和done包含起来
	echo "$n"
	n=$n+1           #变量n的自加1
done

for loop

##for的循环
files=`ls`     #当前目录下的文件名放在变量files中 每个值以空格分开
for i in $files   #遍历files
do
	if [ -f $i ] then   #若是普通文件
		wc -l $i        #计算$i 该文件的行数
	fi
done 

#或者for的另一种写法
##for的循环
for((i=1;i<=10;i++))   #双括号的for循环
do
	sum=$(expr $sum + $i) #expr支持算术运算
	echo $sum             #求和输出sum
done

3. Function (judging whether the user is online)

##定义函数和函数调用
check_user()  #定义函数 ()中无空格
{
    
    
	if [ $1 = "quit" ] then #若位置参数第一个为quit,则立即退出
		exit                # $0表示文件名，位置参数0，exit表示退出命令
	fi
	
	USER=`who | grep $1 | wc -l`
	if [ $USER -eq 0 ] then
		return 0    #return 该函数结束
	else
		return 1    #用户在先返回1，不在线返回0
	fi
}

while true #死循环
do
	echo -n "input a user name:"   # echo -n 表示不换行
	read USER                      #接收键盘输入
	#上面的的等效写法
	# read -p "input a user name:" USER
	
	check_user $USER      #函数调用 并传递参数$USER
	
	if [ $? -eq 1 ] then  # $?是上一步命令执行的结果接收变量；这里值函数调用后传来的参数值
		echo "[$USER] online."
	else
		echo "[$USER] offline."   
	fi
	
done

4. The trap command

use of trap

####     trap的使用

$ trap "" INT                     #当脚本收到信号SIGINT时，忽略该信号
$ trap do_something INT QUIT HUP   #当脚本收到信号INT、QUIT或HUP时执行函数do_something 
$ trap on_exit  EXIT             #当脚本正常退出时，执行on_exit函数

#当脚本正常退出时执行函数on_exit，当脚本文件收到某个信号INT或HUP时执行空指令
#此时冒号代表一个空指令，如果没有冒号，脚本将完全忽略该信号，不做响应，不能立即退出
#执行顺序时由下而上,即先执行冒号空语句，再执行EXIT的执行函数on_exit函数
$ trap on_exit  EXIT
$ trap ":"  INT HUP

The classic use of trap

$ trap -l    #产看系统默认支持的信号类型前缀SIG可以忽略

Linux shell programming learning

Shell basic programming and trap commands, user online examples

write a script first

1. Variables

Special symbols quotes, vertical bars (pipes), and redirection

2. Test statement

3. Grammatical unit

1. Branch control

2. Loop control (three types)

3. Function (judging whether the user is online)

4. The trap command

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