Dealing with Real Numbers in Bash

devUnix · 08-11-2011, 09:50 AM

Hi,

let, expr, and bc - all can be used for Mathematical calculations but the first two commands fail when there is a real number or fractional number to deal with. bc does not help when we need multiplication and division operations.

Let's take this example:

Code:

$ echo "10000 * 100 / 15000" | bc
$ 66

That is not what we expect.

But this one works fine:

Code:

$ echo "10000 - 10.99" | bc
$ 9989.01

But I need to do multiplication and division operations and the resultant values / numbers can be real ones.

How to achieve it in bash?

Well, I do know that Shell Script or Bash can't help us much with Mathematical calculations that require (some or large) precision.

So, wherever in my script I need to do some calculations, what tool or approach should I employ?

catkin · 08-11-2011, 10:00 AM

This will get you what you want, if you adjust the value of scale as required

Code:

echo "scale=2; 10000 * 100 / 15000" | bc

I wonder how bc chooses the scale to use in each of the OP cases.

devUnix · 08-11-2011, 10:23 AM

Quote:

Originally Posted by catkin

This will get you what you want, if you adjust the value of scale as required

Code:

echo "scale=2; 10000 * 100 / 15000" | bc

Thanks!

I just did read about the scale thing on a manual online but did not give it a try.

Well, another problem is born now. "if [ $result -ge 10 ]" (for example) fails because of the obvious reason that it expects an integer expression.

So, 66.66 will not be compared with 10 in the above case.

Converting a decimal value into a string and then making a comparison is also not logical and correct every time.

However, I found some ways here:

http://www.unix.com/shell-programmin...l-numbers.html

catkin · 08-11-2011, 10:29 AM

A solution is to use bc to do such comparisons. From the bc man page: "Relational expressions are a special kind of expression that always evaluate to 0 or 1, 0 if the relation is false and 1 if the relation is true".

Unfortunately bash's usage of 0 and 1 (not 0) for true and false results in some counter-intuitive bits of code like:

Code:

[[ $( echo '1 > 2' | bc ) ]] && echo false || echo true

lyle_s · 08-11-2011, 10:39 AM

Quote:

Originally Posted by devUnix

Code:

$ echo "10000 * 100 / 15000" | bc
$ 66

Is this what you're looking for?:

Code:

$ echo "10000 * 100 / 15000" | bc -l
$ 66.66666666666666666666

Lyle.

ta0kira · 08-11-2011, 10:42 AM

Quote:

Originally Posted by catkin

Code:

[[ $( echo '1 > 2' | bc ) ]]

I don't know about you, but $? is 0 whether or not '1 < 2' or '1 > 2' for me.

Code:

[[ $( echo '1 > 2' | bc ) -ne 0 ]] && echo true || echo false

Kevin Barry

PS I believe what you had is shorthand for -n.

MTK358 · 08-11-2011, 12:18 PM

Quote:

Originally Posted by ta0kira

I don't know about you, but $? is 0 whether or not '1 < 2' or '1 > 2' for me.

That checks bc's output, not its exit code.

EDIT: After trying this, I'm not sure:

Code:

$ if [[ 0 ]]; then echo true; fi
true
$ if [[ 1 ]]; then echo true; fi
true

EDIT2: I tried this, too:

Code:

$ if [[ $(false) ]]; then echo true; fi
$ if [[ $(true) ]]; then echo true; fi
$ if [[ $(echo 1) ]]; then echo true; fi
true
$ if [[ $(echo 0) ]]; then echo true; fi
true
$ if [[ $(echo fgdfgsdf) ]]; then echo true; fi
true

It looks like it's checking whether the string is not empty, not the exit code or numeric value.

H_TeXMeX_H · 08-11-2011, 01:14 PM

I recommend you try awk. It can do a lot of math, and works more like C.

SigTerm · 08-11-2011, 01:28 PM

Quote:

Originally Posted by devUnix

Let's take this example:

Code:

$ echo "10000 * 100 / 15000" | bc
$ 66

That is not what we expect.

from man bc:

Quote:

There are four special variables, scale, ibase, obase, and last. scale defines how some operations use digits after the decimal point. The default value of scale is 0.
...
If bc is invoked with the -l option, a math library is preloaded and the default scale is set to 20. The math functions will calculate their results to the scale set at the time of their call.

I.e. use

Code:

echo "10000 * 100 / 15000" | bc -l

catkin · 08-11-2011, 02:15 PM

Quote:

Originally Posted by MTK358

It looks like it's checking whether the string is not empty, not the exit code or numeric value.

Thanks for the investigation and determining that I was wrong MTK358

Apologies to everyone else for peddling a bum steer