if statement returning [[: not found

Question

I'm writing a dinky little script and I need to do a check on values.

size is going to be a float, or number with decimal point for example like 28.0.

For some reason whenever I run, I get this error:

   while read -r line;
   do
           sample=$(echo $line | awk 'BEGIN{FS="/"}; {print $5}')
           size=$(aws s3 ls --summarize --human-readable $line | grep "Total Size:" | awk 'BEGIN{FS=" "}; {print $3}')
   #       aws s3 cp $line ./crams/
           if [[ $( echo "${size} < 20" | bc ) -eq 1 ]]; then
                   echo "${sample}\t${size}"
           fi
   done < $1

download-aws.sh: 6: download-aws.sh: [[: not found 

Any thoughts???

Also if you know of a better way to do the comparison of a float to a int... that'd be great!

Thanks

EDIT:

So I found a solution, I changed the if statement to:

if [ $( echo "${size} < 20" | bc ) -eq 1 ]; then

If anyone could point me in the direction of some awesome explanations of the logic behind the brackets etc... that'd be amazing!