How to subtract two floating point numbers derived from a string

Question

The floating point numbers come from an output string. Here is my sample code:

#!/bin/bash

    output="0.15 0.11"
    cputime=${output[0]}
    gputime=${output[1]}
    echo $cputime $gputime

    diff=`echo "$cputime - $gputime" | bc`
    echo $diff

Answer 1

Your code to separate the values can be better written with:

#!/bin/bash
output="0.15 0.11"
cputime=${output%% *}
gputime=${output##* }
diff=$(echo "$cputime - $gputime" | bc)
echo $diff

## and %% are, respectively, the removal-of-longest-prefix and removal-of-longest-suffix operators, deleting either everything from the first space onwards ( <space>* ) or everything up to the last space ( *<space> ).

The reason your current code doesn't work is because $output isn't an array for which the two values are in separate indexes, as it would be had you done something like:

output=("0.15" "0.11")

Alternatively, since you're already calling an external executable bc anyway, you could shorten the code with some added awk magic:

#!/bin/bash
output="0.15 0.11"
diff=$(awk '{print $1 - $2}' <<< "$output" | bc)
echo "$diff"

Answer 2

Assuming that the values are separated by a single space:

$ echo "$output"
0.15 0.11
$ tr ' ' - <<< "$output" | bc -l
.04