check for how many bytes are left on a device where a particular folder exists

available_bytes.sh

#!/usr/bin/env bash

## Usage: ./available_bytes.sh         # checks device where current directory exists
##        ./available_bytes.sh /boot   # checks device where /boot exists

if [ $# -lt 1 ]; then
  TARGET=$PWD
else
  TARGET=$1
fi

DF_ENTRIES=$(df $TARGET | tail -n 1)
for entry in $DF_ENTRIES; do
  [[ $entry =~ "%" ]] && break;
  AVAILABLE=$entry
done
echo $AVAILABLE