一、分支逻辑

1、if

if condition1
thencommand1
elif condition2 
then command2
elsecommandN
fi

echo please input first number
read a
echo please input second number
read bif [ $a -eq $b ]
thenecho "equal"
elif [ $a -gt $b ]
thenecho "the first is greater than the second"
elseecho "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 choicecase $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
docommands
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
docommands
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 Ni=1
sum=0
while(( i<=N ))
doecho "Now the number of i is $i"let sum+=ilet i++
doneecho "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=5while((i<10))
doj=3while((j>0))doecho "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#continueecho This is after continuedonelet i++
done