You could (ab)use the Nagios plugin
check_disk for this. The advantage is that this piece of software is lightweight, well-tested and probably available by your package management already (Debian/Ubuntu package
nagios-plugins-basic). Example usage:
check_disk -w 10% -c 3 -u GB -p / -p /mnt
In this example, it checks two mointpoints
/mnt. If it exits with a exit status of 2, your disk has less than 3GB available, if it exits with status 1, it has less than 10% available and if it exits with a status of 0, then neither of the values is reached. Checking for exit status in Bash is trivial, i.e.