How to Control Number of Parameters Entered into Function

Question

I am trying to make a simple menu-driven calculator script. I am trying to make it such that after selecting A (add) or B (subtract) from the menu, it'll call the function and display the corresponding message when:

The parameters entered when function called is greater than 3
No parameters are entered when the function is called
The operator entered when the function is called is neither "+" or "-"

Right now it is doing the parameter check when I call the script ./mycalc.sh executing the script

Am not sure how to make it check parameters after the function is called?

#!/bin/bash   
display() {
echo "Calculator Menu" 
echo "Please select an option between add, subtract or exit"
echo "A. Add"
echo "B. Subtract"
echo "C. Exit"
} 
#initialize choice n
choice=n 

if [[ $# -ne 3 ]]
then echo " You have not entered 3 parameters" 
exit 1
fi

if [ $# -eq 0 ]
then 
echo " You have not entered any parameters, please input 3. "
fi 

if  [[ $2 != [+-] ]]
then
echo " Please enter an add or subtract operator."
exit 1
fi


add() {
echo " The sum of $one + $three equals $(( $one $op $three ))"
}

subtract () {
echo " The difference of $one - $three equals $(( $one $op $three )) "
} 

while [ $choice != 'C' ] 
do display
read choice
if [ $choice = 'A' ] 
then 
read -p "Please enter two operands and the operator '+': " one op three
add $one $op $three

elif [ $choice = 'B' ] 
then
read -p " Please enter two operands and the operator '-': " one op three
subtract $one $op $three

elif [ $choice = 'C' ]
then
echo "Thank you for using this program. The program will now exit." 
fi 

done
 


sleep 3

exit 0

Where do you set the variables? Where do you call the functions? — Barmar, Mar 24 '22 at 21:09
Inside a function, `$#` is the number of arguments given to the function, not the arguments to the main script. — Barmar, Mar 24 '22 at 21:09
It's not clear how this script works. It displays a menu of operations, but never asks the user for input. — Barmar, Mar 24 '22 at 21:11
I am actually not sure, so should the if statements be inside the function? — hashslingingbasher, Mar 24 '22 at 21:11
BTW, `||` should be `&&`. See https://stackoverflow.com/questions/26337003/why-non-equality-check-of-one-variable-against-many-values-always-returns-true — Barmar, Mar 24 '22 at 21:11
You can check the input before calling the functions. Then you don't need to do it inside the functions. — Barmar, Mar 24 '22 at 21:12
What is the point of `choice=n`? You never use the `choice` variable. Why do you just set it to `n` instead of getting it from the user, like `read choice`? — Barmar, Mar 24 '22 at 21:15
Sorry my code was not complete, this is what I actually had. — hashslingingbasher, Mar 24 '22 at 21:18
I was checking if neither the '+' nor '-' is entered, there should be an error message — hashslingingbasher, Mar 24 '22 at 21:18
If the user has already selected `Add` from the menu, why do they need to enter the operator `+` again? Either get the operator from the input, or from the menu, but don't make them do it twice. — Barmar, Mar 24 '22 at 21:43
That part is part of the assignment requirements: have a display menu, check if parameters are entered correctly. I just want to find a way to make this code work — hashslingingbasher, Mar 24 '22 at 21:48

glenn jackman · Answer 1 · 2022-03-24T21:52:31.830

0

if [ $# > 3 ] 
then 
echo " You need to input 3 parameters. "
fi

Within [...], the > is simply redirection. You'll find an empty file named 3 in your current directory.

Since this is an error condition for your script, you'll want to exit:

if [[ $# -ne 3 ]]; then
    echo "usage: $0 operand1 operator operand2" >&2
    exit 1
fi

And to test the operator, there are many ways.

case, but it's a bit verbose

case $2 in 
    '+'|'-') : ;;
    *)  echo "Operator must be + or -" >&2
        exit 1
        ;;
esac

Within [[...]] the == and != operators are pattern matching operators

if [[ $2 != [+-] ]]; then  
    echo "Operator must be + or -" >&2
    exit 1
fi

edited Mar 24 '22 at 21:52

answered Mar 24 '22 at 21:47

glenn jackman

238,783
38
220
352

1

Or use math mode: `if (( $# > 3 )); then`. – Wiimm Mar 24 '22 at 21:50
Hi Glenn, Thank you for your answer. That did work for the parameters but it checked as soon as I executed the script. – hashslingingbasher Mar 24 '22 at 21:59
Like as soon as I entered ./mycalc.sh, it would display that I did not enter any parameters. Is there a way to make it work AFTER I call the functions ( enter A or B from the menu) – hashslingingbasher Mar 24 '22 at 21:59

How to Control Number of Parameters Entered into Function

1 Answers1