[shell programming] getting started with basic shell

[Station B-Chao Ge said shell notes]
The content is incomplete, only what I think is important is recorded! ! !
Refer to Acridine as appropriate

common basis

shell scripting – escaping and quoting
Shell backticks, $The\; difference\; between\; ()and$ {}

1. Special variables

$0	#当前文件名
$?	#上一条命令的执行结果
# 关于传参
$#	#参数个数
$1,$2,...$n	#取输入参数值
$!	#获取上一次后台执行的程序的PID
$$	#获取当前脚本的PID
$_	#获取上一条命令的最后一个参数

Interview: How to let the program execute in the background?

ans：

# 以某shell脚本为例
nohup bash ***.sh & 1> /dev/null

Two, shell substring

2.1 Basic built-in commands

echo	#不可以自动识别特殊字符，需要加参数
printf	#可以自动识别特殊字符，但是默认不换行
eval	#执行多个命令；
exec	#不创建子进程，执行后续命令，且执行完毕后，自动exit
export	#取环境变量
read	#接收命令行输入
shift	#参数左移
test	#

For details, please refer to:
shell programming | shift command usage notes, author: Wen Jiasange
rookie tutorial | shell -test usage

---

The following is a detailed introduction of the above built-in commands, if you know it, you can skip it.

echo

-n	#不换行输出
-e	#解析字符串中的特殊字符

\n	#换行
\r	#回车
\t	#制表符
\b	#退格

eval
executes multiple commands

eval ls ; cd /home	#会先执行 ls 命令；再切换至/home目录

exec

Do not create a child process, execute subsequent commands, and automatically exit after execution

exec date

2.2 Playing tricks of shell substring

#不一定常用，记录最常用的几个
${var}	#返回变量var的值
${
    
    #var}	#返回变量var的长度，有多少个字符
${var:start}	#返回变量start数值之后的字符
${var:start:length}	#提取start之后的length限制的字符
${var#word}	#从变量开头,删除最短匹配的word字串
${var##word}	#从变量开头删除最长匹配的word字串
${var%word}	#从变量结尾删除最短的word
${var%%word}	#从变量结尾删除最长匹配的word
${var/pattern/string}	#用string代替第一个匹配的pattern
${var//pattern/string}	#用string代替所有的pattern

---

Explain a few commonly used

# 0,1,2,3...
name="helloworld"
${
    
    #name}  # 返回值为10
${name:4}	# 返回 world

${name#hello}	#返回 world
${name%world}	#返回 hello

${name/hello/fuck}	#返回 fuckworld

2.3 Calculation of variable length

1. ${#name}
2. expr
3. wc -L
4. awk

forround robin rule

# 多行
for num in {
    
    1..100}
do 
	echo ${name}
done

# 一行
for num in {
    
    1..100};do echo ${name};done

seqOrder

seq 3	
# 1
# 2
# 3 
seq -s ":" 3	# 1:2:3

for n in {
    
    1..3}
do 
	str=`seq -s ":" 10`
	echo $str
done

timeCommand
to be continued