The loop statement and escape character of shell script

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for loop statement

The for statement needs to define a variable and a list of values, execute the same command according to different values, and know that the variable value is used up.
The value list contains multiple objects with the same attributes, such as IP address, address book, etc.

#for语句结构
for 变量名 in 取值列表
do
  命令序列
done

Example: Find the sum of 1~100.

vim he.sh
#!/bin/bash
sum=0
for i in {1..100}
do
   sum=$[$i+$sum]
done
echo "0-100的和为：" $sum

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Test Results:

Example: Prompt the user to enter an integer less than 100, and calculate the sum of all integers from 1 to this number

vim 3.sh
#!/bin/bash
read -p "请输入一个小于100的整数: " n
sum=0
for ((i=1;i<=n;i++))
do
  sum=$[$i+$sum]
done
  echo "从1到到该数之间所有整数的和为: " $sum

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while loop statement

A sequence of commands is repeatedly executed according to a specific condition until the condition is not met.
There may be an infinite loop when writing, to avoid this from happening
true (true) and false (false) are special condition test operations, and can also be used in the condition test of the if statement.
When true is used as a condition, it means that the condition is always established and executed infinitely, that is, an endless loop. When
false is required to be forced to terminate as a condition, it means that the condition is not established and the command sequence is not executed.

#while语句结构
while 条件测试操作
do
  命令序列
done

Example: Calculate the sum of all integers from 1 to 100

vim w1.sh
#!/bin/bash
sum=0
shu=1

while [ $shu -le 100 ]
do
  sum=$[$sum+$shu]
  let shu++
done
  echo "和为: " $sum

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Example: Guess the commodity price game; obtain random numbers through the variable RANDOM; prompt the user to guess and record the number of times, and exit the loop after the guess is successful

vim c.sh
price=$[$RANDOM % 1000]
a=0
num=0
echo "猜猜商品价格是多少"
while [ $a -eq 0 ]
do
let num++
read -p "请输入你猜的价格：" b
if [ $b -eq $price ];then
echo "恭喜，你猜对了！"
let a++

elif [ $b -gt $price ];then
 echo "你猜大了！"

elif [ $b -lt $price ];then
 echo "你猜小了！"
fi
done
echo "你总共猜了 $num 次。"

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until loop statement

Contrary to while, test a certain condition repeatedly, and execute it repeatedly as long as the condition is not established.
Generally you can use while skillfully, this is rarely used.

Example: Calculate the sum of all integers from 1 to 100

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End the loop----break and continue

break out of a single loop

vim break.sh
#!/bin/bash
for i in $(seq 1 10)
do
   if [[ $i == 3 ]];then
    echo "yes"
    break
else
    echo "no"
    sleep 1
fi
done

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The continue command will not jump out of all loops, just jump out of the current loop.

vim con.sh
#!/bin/bash
for i in $(seq 1 10)
do
  if [[ $i == 3 ]];then
    echo "yes"
    continue
  else

    sleep 1
  fi
  echo $i
done

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IFS field separator

默认包含 空格，制表符，换行符
查看命令：set | grep IFS
IFS=$' \t\n'

修改成只换行
IFS=$'\n'

IFS=:
IFS=','

IFS.OLD=$IFS
IFS=$'\n'
...
IFS=$IFS.OLD

输出环境变量PATH所包含的所有目录以及其中的所有可执行文件

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Escape character

echo -n 表示不换行输出

echo -e 输出转义字符，将转义后的内容输出到屏幕上

The commonly used escape characters are as follows:

\b 转义后相当于按退格键（backspace），但前提是"\b"后面存在字符；"\b"表示删除前一个字符，"\b\b"表示删除前两个字符。

\c 不换行输出，在"\c"后面不存在字符的情况下，作用相当于 echo -n； 但是当"\c"后面仍然存在字符时，"\c"后面的字符将不会被输出。

\n 换行，被输出的字符从"\n"处开始另起一行。

\f 换行，但是换行后的新行的开头位置连接着上一行的行尾；

\v 与\f相同；

\t 转以后表示插入tab，即横向制表符；

\r 光标移至行首，但不换行，相当于使用"\r"以后的字符覆盖"\r"之前同等长度的字符；但是当"\r"后面不存在任何字符时，"\r"前面的字符不会被覆盖

\\ 表示插入"\"本身；

Example:

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