Multiplication with expr in shell script

Question

I am making a basic calculator to add, subtract, multiply, and divide.

Addition works, but not multiplication. When I attempt to multiply, I get the "You did not run the program correctly" response:

$ ./calculator 4 + 5
9
$ ./calculator 4 * 5
You did not run the program correctly
Example: calculator 4 + 5

I've searched around on google, where I found the \\* code, but still doesn't work. Can someone provide me with a solution or explanation?

Here is my code

#!/bin/bash

if [ $# != 3 ]; then  
  echo You did not run the program correctly   
  echo Example: calculator 4 + 5                
  exit 1 
fi

if [ $2 = "+" ]; then    
  ANSWER=`expr $1 + $3`    
 echo $ANSWER 
fi

if [ $2 = "*" ]; then
  ANSWER=`expr $1 \\* $3`  
  echo $ANSWER 
fi

exit 0

Answer 1

Your code has many problems. Here is a fix. * means "all files in the current directory". To instead mean a literal asterisk/multiplication character, you have to escape it:

./calculator 3 \* 2

or

./calculator 3 "*" 2

You also have to double quote "$2", otherwise * will start meaning "all files" again:

#!/bin/bash
#Calculator
#if [ `id -u` != 0 ]; then
#  echo "Only root may run this program." ; exit 1
#fi
if [ $# != 3 ]; then   
  echo "You did not run the program correctly"
  echo "Example:  calculator 4 + 5"
  exit 1
fi
# Now do the math (note quotes)
if [ "$2" = "+" ]; then echo `expr $1 + $3`
elif [ "$2" = "-" ]; then echo `expr $1 - $3`
elif [ "$2" = "*" ]; then echo `expr $1 \* $3`
elif [ "$2" = "/" ]; then echo `expr $1 / $3`
fi
exit 0

Answer 2

The * needs to get escaped since it is a special char in shell syntax. (If not escaped, it will get expanded to the list of all files in the current directory). But you need to use just a single backslash in order to escape it:

ANSWER=`expr $1 \* $3`