15、Linux-Shell04：分支和循环

一、分支逻辑

1、if

if condition1
then
    command1
elif condition2 
then 
    command2
else
    commandN
fi

echo please input first number
read a
echo please input second number
read b

if [ $a -eq $b ]
then
    echo "equal"
elif [ $a -gt $b ]
then
    echo "the first is greater than the second"
else
    echo "the first is less than the second"
fi

2、case

匹配成功后执行相应命令，执行结束后即退出，没有匹配到则执行*对应的命令，相当于else。

case value in
value1)
    command1
    ;;
value2)
    command2
    ;;
*)
    command2
    ;;
esac

echo please input your choice:1~4
read choice

case $choice in
1)
    echo your choice is 1;;
2)
    echo your choice is 2;;
3)
    echo your choice is 3;;
4)
    echo your choice is 4;;
*)
    echo illegal choice;;
esac

---

二、循环逻辑

1、for

for value in value1 value2 ... valueN
do
    commands
done

for value in 1 2 3 4 5
do 
    echo "This value is $value"
    square=`expr $value \* $value`
    echo "It's square is $square"
done

2、while和until

while condition
do
    commands
done

①对于条件，如果使用中括号[]，应该用-eq、-gt这样的 

如果使用两对小括号，则可以使用>、>=这样的

②使用let命令操作变量时，无需使用$

如let sum+=i的等价表达是sum=`expr $sum + $i`

③去掉condition则是无限循环

④把while换成until即为until循环，区别在于：

while循环当condition为真时执行循环；until执行循环直到condition为假。

echo This program calculates the sum from 1 to N
echo Now please input the value of N
read N

i=1
sum=0
while(( i<=N ))
do
    echo "Now the number of i is $i"
    let sum+=i
    let i++
done

echo "the sum from 1 to N is $sum"

3、break和continue

break跳出循环，continue结束当前循环，进入下一次循环。

i=5

while((i<10))
do
   j=3
   while((j>0))
   do
       echo "i is $i, j is $j"
       let j--
       #break
       #break 2 # 2表示跳出两层循环，在这个例子中会直接跳出最外层循环
       #continue
       echo This is after continue
   done
   let i++
done