1,let var=算术表达式
let num1=1+2 echo $num1
2, var=$[算术表达式]
num2=$[2+3] echo $num2
3, var=$((算术表达式))
num3=$((2+3)) echo $num3
4, var=$(expr arg1 arg2 arg3 ...)
num4=$(expr 1 + 3) echo $num4 num5=$(expr $(expr 2 + 3) * 5) echo $num5