Floating point comparison with variable in bash

Question

I want to compare a floating point variable to an integer. I know this is not the best to do with bash, but my whole script is already written in bash. $number can be any integer. If it below or equal 50, I want output1, for all others I want an output with the other variable k. This is what I have so far:

number=43
test=$(echo "scale=2; $number/50" | bc -l)
echo "$test"
for k in {1..5}
do
    if ["$test" -le 1]
    then echo "output"

    elif ["$test" -gt $k]
    then echo "output$k"
    fi
done

If I try with test=0.43, the first loop does not even work. I think it has to do with an integer and a floating point comparison but cannot make it work.

Anything I am missing?

PS:this [0.43: command not found is what the terminal outputs.

Answer 1

Bash can't handle floats. Pipe to bc instead:

if [ $(echo " $test > $k" | bc) -eq 1 ]

The error you see though is because the test command (i.e. the [) needs spaces before and after

It is even better to use (( ... )) since you compare numbers like this:

if (( $(bc <<< "$test > $k") ))

The part in the loop should look like this:

if (( $(bc <<< "$test <= 1") ))
then
    echo "output"
elif (( $(bc <<< "$test > $k") ))
then
    echo "output$k"
fi

Relational expressions evaluate to 0, if the relation is false, and 1 if the relation is true [source]. Note however that is a behavior of GNU bc, and it is not POSIX compiant.

Answer 2

Kind of an old question, but it bears an additional answer I think.

While piping to a higher precision calculator (bc or dc) works, it is at the cost of a fork and a an extra process, since those calculators are not built in to bash. One thing that IS built in, though, is printf. So if you can deal with your numbers being within a particular number of decimal places, you can "fake" floating point comparisons, with a function like this:

#!/usr/bin/env bash

function [[[ () {
  local LANG=C lhs rhs
  printf -v lhs '%07.3f' "$1"; lhs=${lhs/./}
  printf -v rhs '%07.3f' "$3"; rhs=${rhs/./}
  case "$2" in
    -lt) return $(( ! ( 10#$lhs < 10#$rhs ) )) ;;
    -le) return $(( ! ( 10#$lhs <= 10#$rhs ) )) ;;
    -eq) return $(( ! ( 10#$lhs == 10#$rhs ) )) ;;
    -ge) return $(( ! ( 10#$lhs >= 10#$rhs ) )) ;;
    -gt) return $(( ! ( 10#$lhs > 10#$rhs ) )) ;;
  esac
}

number=${1:-43}
test=$(dc -e "2k $number 50 / p")
echo "$test"

for k in {1..5}; do
    if [[[ "$test" -le 1 ]]]; then
      echo "output"
    elif [[[ "$test" -gt "$k" ]]]; then
      echo "output $k"
    fi
done

A few things to consider here.

• I've named the function [[[ to be cute. You can name it whatever you like. ntest or mynumericcomparison or even [[[.
• printf is an internal function within bash, so despite the fact that it's on your path, it doesn't cost a fork.
• As it stands, the function supports numbers up to 999.999. If you need higher numbers (or more precision), adjust the printf formats.
• The 10# at the beginning of each variable inside the case statement is to force the comparison to happen at base 10, since a zero-padded number might otherwise be interpreted as octal.

See also: http://mywiki.wooledge.org/BashFAQ/022