bash - expr 显示表达式而不是求解它

Question

这是我的 shell 脚本，

#! /bin/sh
# basic calculator
echo "Please input your choice"
printf " 1.Addition \n 2.SUbstraction \n 3.Multiplication \n 4.Division\n"
read choice
case "$choice" in
    1) echo "Enter number 1:";read n1;echo "Enter number 2:";read n2;t=$(expr "$n1"+"$n2");echo "$n1+$n2=$t";;
    2) echo "Enter number 1:";read n1;echo "Enter number 2:";read n2;t='expr $n1-$n2';echo "$n1-$n2=$t";;       
    3) echo "Enter number 1:";read n1;echo "Enter number 2:";read n2;t='expr $n1\*$n2';echo "$n1*$n2=$t";;
    4) echo "Enter number 1:";read n1;echo "Enter number 2:";read n2;t='expr $n1/$n2';echo "$n1/$n2=$t";;
esac

这是我的输出，

Script started on Sunday 08 November 2015 12:05:21 PM IST
Please input your choice
 1.Addition 
 2.SUbstraction 
 3.Multiplication 
 4.Division
1
Enter number 1:
5
Enter number 2:
6
5+6=5+6

问题是我的 expr 实际上并没有解决表达式

Answer 1

空格很重要:

$ expr 5+6
5+6
$ expr 5 + 6       
11 

要进行算术运算，您需要给 expr 3 个不同的参数。