Division in script and floating-point

Question

I would like to do the following operation in my script:

1 - ((m - 20) / 34) 

I would like to assign the result of this operation to another variable. I want my script use floating point math. For example, for m = 34:

results = 1 - ((34 - 20) / 34) == 0.588 

Answer 1

You could use the bc calculator. It will do arbitrary precision math using decimals (not binary floating point) if you set increease scale from its default of 0:

$ m=34 $ bc <<< "scale = 10; 1 - (($m - 20) / 34)" .5882352942 

The -l option will load the standard math library and default the scale to 20:

$ bc -l <<< "1 - (($m - 20) / 34)" .58823529411764705883 

You can then use printf to format the output, if you so choose:

printf "%.3f\n" "$(bc -l ...)" 

Answer 2

Bash does not do floating point math. You can use awk or bc to handle this. Here is an awk example:

$ m=34; awk -v m=$m 'BEGIN { print 1 - ((m - 20) / 34) }' 0.588235 

To assign the output to a variable:

var=$(awk -v m=$m 'BEGIN { print 1 - ((m - 20) / 34) }')