How to use if statement with "expr"

Tauatioti · 10-24-2013, 08:04 AM

Hi everyone

Im trying to write a simple script the can calculate the price of something, but when i run the script there is an error "expr: syntax error". here is my script

#ex1

shirt=\$15.00
black=\$13.50

echo "Please enter how many shirt you want"
read num1
echo
echo "Please choose"
echo "1 ---> Normal Shirt"
echo "2 ---> black Shirt"
read choice
echo

if [ $choice = "1" ]; then
answer=`expr $num1 * $shirt`
echo "$num1 * $shirt = $answer"
elif [ $choice = "2" ]; then
answer=`expr $num1 * $black`
echo "$num1 - $num2 = $answer"

else
echo "invalid selection"

fi

Please anyone can help tell me what was missing in the script?

sory for my poor english

Thanks

Tauatioti

druuna · 10-24-2013, 08:25 AM

You are running into 2 problems:

1 - The * is special when using `expr $num1 * $shirt`. This causes the expr: syntax error
You need to escape it: `expr $num1 \* $shirt`

2 - This is the biggest problem: Bash doesn't handle fractions that well. This may cause the following error:

Code:

num=3
shirt=13.5
total=`expr $num1 \* $shirt`
expr: non-integer argument

Using bc can solve all the above:

Code:

num=3
shirt=13.5
total=$( echo "$shirt * $amount" | bc )

BTW: Don't do this:

Code:

shirt=\$15.00
black=\$13.50

The dollar sign can be added to the amount/price individually. If you don't you need to remove the $ from the string before you do the arithmetic.

Tauatioti · 10-24-2013, 09:50 AM

Hello druuna

Thanks for your quick reply, my first problem is solved, and i try your step to solve the second one (fraction) but it not working, any suggestion?

Here is my script

#exercise 3

clear
shirt=15
black=13

echo "The price of the shirt is listed below"
echo "Normal shirt \$15"
echo "Black shirt \$13.5"

echo "Please enter how many shirt you want"
read num1
echo
echo "Please choose"
echo "1 ---> Normal Shirt"
echo "2 ---> black Shirt"
read choice
echo

if [ $choice == "1" ]; then
answer=`expr $num1 \* $shirt`
echo "$num1 * $shirt = $answer"
echo "The Price is $answer dollars for $num1 Shirt"
elif [ $choice = "2" ]; then
total=$( echo "$black * $num1" | bc )

else
echo "invalid selection"

fi

druuna · 10-24-2013, 10:05 AM

There are multiple errors/typo's in your code.

Have a look at this:

Code:

if [ $choice == "1" ]
then
  answer=$( echo "$num1 * $shirt" | bc )  # just in case, if the price goes up (15.00 -> 15.25) it will still work.
  echo "$num1 * $shirt = $answer"
  echo "The Price is $answer dollars for $num1 Shirt"
elif [ $choice == "2" ]   # == instead of =
then
  total=$( echo "$black * $num1" | bc )
  echo "The Price is $total dollars for $num1 Shirts"   # you never print the answer....
else
  echo "invalid selection"
fi

Tauatioti · 10-24-2013, 10:37 AM

you are right about my script.... i know that i solved my first problem using \* instead of *... but could you help on how i calculate decimal numbers

Thanks

druuna · 10-24-2013, 10:43 AM

Have a look at these links:

- Perform arithmetic operations
- Chapter 13. Arithmetic Expansion
- Arithmetic Expressions in BASH

Tauatioti · 10-24-2013, 10:55 AM

I'll look at now, anyway thank you for your time

God bless you