The Simple Maths post seems to be the most popular article in the so-far short life of this blog.

It’s also something that I have received a few emails about recently, so I feel like posting a bit more on the subject.

I think that the code can speak for itself… We implement a loop, which calls the builtin `read`

function (I’m not sure the “-p” flag, to provide a prompt, is universal. It does work with the Bash builtin. If it doesn’t work on your *nix, it’s really only for show, so you can live without it.

Because `read`

works on standard input (aka “stdin”), it will work interactively from the keyboard, or direct from a file (one number per line).

We use two methods of doing maths in the shell:

`expr`

, because it’s a simple and easily-read way to do simple maths:`n=`expr $n + 1``

`bc`

, because it is more powerful. Do have a play with`bc`

interactively, it can do a lot... see below.

```
```So, we can write a fairly simple script (read down, it's only actually 11 lines of code without the comments), which is actually quite versatile - it can do running averages, it can be interactive or run from cron, called from another script, even used as a function.

So, here's the code. It should be fairly self-explanatory, but do have a look at the interactive `bc`

sample session below, to see what we are doing with `bc`

. Also, do play with `bc`

(some Linux distros have dropped it from the default install recently, so you'll have to `yast -i bc`

, or equivalent)

### The Script - Calculate Averages

#!/bin/sh
# Calculate mean (average) of integer data
# Initialise the variables
n=0 # n being the number of (valid) data provided
sum=0 # sum being the running total of all data
# Note that by using ^D (aka "EOF") to quit, this
# script will work just as well interactively, as
# when provided with a file containing the data.
while read -p "Enter a number (^D to quit): " x
do
# expr is useful for simple maths
sum=`expr $sum + $x`
# If this fails, it was non-numeric input
if [ "$?" -eq "0" ]; then
# Okay, it was valid input.
n=`expr $n + 1`
# We can provide a "running average" here;
# I'll comment it out for now.
# echo "Running Average:"
# echo "scale=2;$sum/$n" | bc
# echo
fi
done
# Okay, we've done the loop.
# Present the data.
echo "Overall Average:"
# bc is more useful than expr for
# more involved maths, though its
# syntax, particularly in a script,
# is possibly less obvious.
# Using bc interactively is easier
# than using it in a shell script
echo "scale=2;$sum/$n" | bc

### Interactive bc

The bold text is user input. The rest is from `bc`

:

$ **bc**

bc 1.06

Copyright 1991-1994, 1997, 1998, 2000 Free Software Foundation, Inc.

This is free software with ABSOLUTELY NO WARRANTY.

For details type `warranty'.
**ibase=2**
I'll be entering base2 (binary)
**01001001**
So, I enter 1001001 (73)
73
And it replies with the answer in base 10
**ibase=10**
Does this set the input base back to 10?
**10**
Let's input "10", it should reply "10"
2
No, we entered "10" in base 2, which is 2!
**ibase=1010**
So, 10 in binary is 1010 (8+2)
**10**
We say 10
10
And bc says 10. Good, we're back to normal
**11**
And the same for 11
11
Good, it works. Now for some maths..
**1 + 2**
(tricky stuff!)
3
Yes, that's good, 1+2=3
**23 + 34 + 45 + 56**
We're not limited to x+y
158
So we can build up our sums
**10/3**
10/3 = 3 and a third, right?
3
Not to 0 decimal places.
**scale=2**
Okay, let's have 2 decimal places
**10/3**
Now ask again
3.33
That's better
**scale=5**
Or to 5 points?
**10/3**
Ask again...
3.33333
And it works!
**scale=1**
One point:
**10/3**
And ask again
3.3
As we expected.
**scale=0**
So, scale=0 means 0 places
**10/3**
Should say 3
3
Yes, we're back to where we started.

### Back to the Script

That made a nice break. Now we'll go back to the script... it's only actually 11 lines long:

#!/bin/sh
n=0
sum=0
while read x
do
sum=`expr $sum + $x`
if [ "$?" -eq "0" ]; then
n=`expr $n + 1`
fi
done
echo "scale=2;$sum/$n" | bc

And as I said, we can use it interactively, or with a file of data:

$ cat data.txt
4
5
6
$ average.sh
5.00

Because, under *nix, *EVERYTHING IS A FILE*, even the keyboard!

for some reason on my system, expr did not work with floating point numbers. I had to change the script to use bc for the calculation:

#!/bin/sh

n=0

sum=0

while read x

do

sum=`echo $sum + $x|bc`

if [ “$?” -eq “0” ]; then

n=`expr $n + 1`

fi

done

echo “scale=2;$sum/$n” | bc

Unfortunately, expr can only deal with integers

I ran this script on 10’s of thousands of numbers and it takes a *long* time. That’s because you spawn an expr not once but twice for every number you average. Here is the script with your expr statement changed to native bash arithmetic expansion.

#!/bin/sh

n=0

sum=0

while read x

do

sum=$(( $sum + $x ))

if [ “$?” -eq “0” ]; then

(( n += 1 ))

fi

done

echo “$(echo “scale=2;$sum/$n” | bc) $n”

The last command, bc, is changed a bit to also show the number of elements in the average.

Time to average 10000 items:

OLD: 25.76s

NEW: 0.44s (440ms)

Thanks Bruce. There are lots of things that Bash can do that Bourne can’t do.

These days, most *nix boxes have Bash available, but /bin/sh still points to the Bourne shell. Indeed, Debian is going back from /bin/sh being bash, to dash. Ubuntu has already replaced /bin/sh with dash.

http://release.debian.org/lenny/goals.txt

Steve

/bin/sh in Linux is on most distros a symlink to bash (or dash) and when run it actually runs “Bash in POSIX compatibility mode”, which should be like Bourne Shell, but neither Bash nor Dash is good for actually testing if your script runs on real Bourne Shell.

For those wanting to make sure their script is Bourne Shell compatible but you don’t have actual Bourne Shell available, Heirloom Bourne Shell is something you may want to look into – comes in source package, no configure script, you just have to edit very simple modifications to very simple Makefile, compile and install and you have Bourne Shell with NO extensions whatsoever – with that I’ve learned a lot on how much imagination and crazy hacks you may have to achieve to do things that in Bash are quite normal things to do :) It may be fun, I for one like to do some hobby projects “just to see if I can” where I try to do something in Bourne Shell – like transforming script relying on recursive function calls into Bourne Shell which has no function local variables, only script global ones (hint: subprocess inside function can help, of course the things you can do are limited, subprocess can have it’s own variables, but you can’t change variables outside it, he-he).

[…] Link: http://www.howtoforge.com/apache2_mod_deflate Link: http://nixshell.wordpress.com/2007/03/26/calculating-averages/ […]

hi,

what about

$ awk ‘{ sum += $1} END {print sum/NR}’ /path/to/data.txt

Thanks for that, slevin. I’m sure that there are lots of ways of doing it with awk, perl, and many other languages.

[…] Calculating Averages March 20077 comments […]

#!/bin/sh

# Copied/pasted directly from: http://nixshell.wordpress.com/2007/03/26/calculating-averages/

# Then modified a little – mainly for taking numbers directly from the command line. – delt.

n=0 # n being the number of (valid) data provided

sum=0 # sum being the running total of all data

# “Note that by using ^D (aka “EOF”) to quit, this script will work just as well blah blah”

# — Thanks, so i can make a function out of this and pipe data directly into it =)

function calc_avg() {

while read -p “” x; do

sum=`expr $sum + $x` && n=$[n+1] # “expr” return status indicates if valid integer or not

done

# ok, finished adding, now calculate the average.

echo “scale=2;$sum/$n” | bc

}

# like said above, just pipe $* args through calc_avg with a newline between each one.

echo $* | tr ‘ ‘ ‘\n’ | calc_avg

### TODO: write a version that accepts floating point numbers as arguments ###

Nicely done, delt. Functions and scripts can easily replace each other too, which is a really nice feature: $1 $2 $#, etc all work just as well for a function as for a script.

PS. There’s no need for ‘-p “”‘, it should default to a blank prompt