I want to divide a sum of numbers by a value

Question

So here is my code for Unix shell scripting

#! /bin/ksh

echo "Enter your first number:"    
read first

echo "Enter your second number:"    
read second

echo "Enter your third number:"    
read third

SUM=`expr $first + $second + $third`    
AVERAGE="$SUM/3"

echo "The average is $AVERAGE"

Basically I want to find the average of the 3 values that a user enters. The result I get once I enter all of the values, for example all values add up to 12; is "The average is 12/3".

How is it that you knew you had to use expr to do the sums, but didn't realize you also needed it for the division? — Barmar, Oct 02 '14 at 14:37
possible duplicate of How to do integer & float calculations, in bash or other languages/frameworks? — G-Man Says 'Reinstate Monica', Oct 06 '14 at 20:40

score 5 · Answer 1 · answered Oct 04 '14 at 22:04

slm's answer here hasn't taken into account that you asked about the Korn shell, not about the Bourne Again shell. The (93) Korn shell has no built-in expr command, so when using expr in Korn shell scripts you are using an external expr command. This is not a problem per se. After all, it's how one did things with the Bourne shell, which also had no expr built-in command. But as M. Kohen points out, one might prefer to use shell built-ins when a shell has them. And the Korn shell has.

M. Kohen points to the arithmetic substitution available in the Korn shell. It's important to remember that this is a substitution, so you must do something with the substituted result if you don't want to just run it as a command. The more complete form of M. Kohen's answer (fixing the operator precedence error along the way) is thus:

AVERAGE="$(( (first + second + third) / 3))"

But there are two other ways of doing this in the Korn shell. The Korn shell has a built-in command named let that does arithmetic evaluation on each of its arguments:

let "AVERAGE = (first + second + third) / 3"

Every argument to the command is a single expression, so whitespace needs to be quoted as here.

It also has a piece of syntax that is described in one ksh clone's manual as "syntactic sugar for let" with the expression turned into a single argument to that command:

(( AVERAGE = (first + second + third) / 3 ))

Thanks for the correction, I've added the tag and amended my A so that it's clear that it pertains to Bash and not Korn. If you see something egregiously wrong like this feel free to do a suggested edit on the Q to get it retagged and to leave the answered a comment alerting them to this. I only found this by accident. — slm, Oct 06 '14 at 00:29

score 3 · Answer 2 · edited Feb 08 '18 at 11:58

3

You can use the built-in arithmetic support in the shell that is used by a double parenthesis preceded by a dollar sign:

$(( (first + second + third) / 3))

This is more efficient than expr since it does not require a separate process (and hence fork, exec, pipe management and return).

edited Feb 08 '18 at 11:58

Jeff Schaller

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answered Oct 02 '14 at 15:21

Didi Kohen

1,841

slm · Answer 3 · 2014-10-06T00:27:16.317

Your question originally did not have the Korn Shell tag so I've since added it. If you were using Bash the following would apply.

The command expr can perform a variety of mathematical calculations. Simply make use of it to perform the averaging as well.

$ AVG=$(expr $SUM / 3)

Also you'll typically want to make use of this form of sub commands, $(...cmd...), instead of backticks. So change your SUM= line to this instead:

SUM=$(expr $first + $second + $third)

If you're unsure what a particular command can do for you be sure to check out the man pages. The command expr has all the operations that it's able to carry out, man expr.

I want to divide a sum of numbers by a value

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