How can I do division with variables in a Linux shell?

Question

When I run commands in my shell as below, it returns an expr: non-integer argument error. Can someone please explain this to me?

$ x=20
$ y=5
$ expr x / y 
expr: non-integer argument

Solution 1

Those variables are shell variables. To expand them as parameters to another program (ie expr), you need to use the $ prefix:

expr $x / $y

The reason it complained is because it thought you were trying to operate on alphabetic characters (ie non-integer)

If you are using the Bash shell, you can achieve the same result using expression syntax:

echo $((x / y))

Or:

z=$((x / y))
echo $z

Solution 2

I believe it was already mentioned in other threads:

calc(){ awk "BEGIN { print "$*" }"; }

then you can simply type :

calc 7.5/3.2
  2.34375

In your case it will be:

x=20; y=3;
calc $x/$y

or if you prefer, add this as a separate script and make it available in $PATH so you will always have it in your local shell:

#!/bin/bash
calc(){ awk "BEGIN { print $* }"; }

Solution 3

Why not use let; I find it much easier. Here's an example you may find useful:

start=`date +%s`
# ... do something that takes a while ...
sleep 71

end=`date +%s`
let deltatime=end-start
let hours=deltatime/3600
let minutes=(deltatime/60)%60
let seconds=deltatime%60
printf "Time spent: %d:%02d:%02d\n" $hours $minutes $seconds

Another simple example - calculate number of days since 1970:

let days=$(date +%s)/86400

Solution 4

Referencing Bash Variables Requires Parameter Expansion

The default shell on most Linux distributions is Bash. In Bash, variables must use a dollar sign prefix for parameter expansion. For example:

x=20
y=5
expr $x / $y

Of course, Bash also has arithmetic operators and a special arithmetic expansion syntax, so there's no need to invoke the expr binary as a separate process. You can let the shell do all the work like this:

x=20; y=5
echo $((x / y))

Solution 5

To get the numbers after decimal point, you can do this:-

read num1 num2
div=`echo $num1 / $num2 | bc -l`
echo $div

View more solutions

Comments

[Todd A. Jacobs]

@ShivanRaptor While one might argue that the question is an RTFM question, it is certainly a valid shell programming question. It is also a reasonable question for someone coming from languages that don't require dereferencing (e.g. Ruby or JavaScript). It should be left open.

[Brian Campbell]

@ShivanRaptor No, this is on topic here. It's about programming in Bash. Unix/Linux is primarily for using the system, not programming. Now, shell scripting does span the boundary between programming and using the system, so this could be on topic on either site. If there were a question about "how do I set up networking", that would definitely belong on Unix/Linux. If it were a question about interactive keybindings in Bash, that would also belong there. But a question about shell scripting is definitely on topic here as well as there.

[Victoria Stuart]

See my answer here, that illustrates subtraction and division of $BASH variables, using a call to Python from the shell (to convert int to float ...): stackoverflow.com/questions/8385627/…

[paddy]

You can find out a lot by reading through the man-page for bash. Type man bash at the prompt (q to exit)

[Todd A. Jacobs]

See Arithmetic Expansion and Shell Arithmetic in the Bash Reference Manual for all the gory details.

[Gilles Quenot]

This has nothing to do with dereferencing but interpolating and expr is discouraged in 2013.

[Todd A. Jacobs]

@sputnick You are clearly confused. Please feel free to consult a dictionary. See dereference and interpolate.

[Prashant Kumar]

I've heard "interpolate" being used to refer to what others might call parameter expansion. At first, I was going to say, keep dereferencing talk to C and C++ code, but no, they say it in Bash world too. So we're all talking about the same thing. +1 for the vote against expr though.

[Gilles Quenot]

A better word is expanding, but not dereferencing. dereferencing is used when we use pointers, that's not the case here, that's just simple variables.

[Gilles Quenot]

@Prashant: tldp is not known to be a good reference in the bash world.

[Skippy le Grand Gourou]

It has to be noted somewhere on this page that most (if not all) GNU/Linux shells only perform integer operations.

[Eugene]

You may also use echo '1 / 3' | bc -l

[dave_thompson_085]

This is wrong. / works fine as a shell-case label. What doesn't work is to use * for multiply without quoting it, which might be what you actually did; that causes it to effectively override all following items in the case, which in your example is 'devide' and 'modulo'