- Debian based systems need to update the index before being able to report upgradable packages.
- Debian old-stable doesn't have `apt` yet and Ubuntu 14.04 emits the following warning when using `apt` in a script:
`WARNING: /usr/bin/apt does not have a stable CLI interface yet. Use with caution in scripts.`
By using `apt-get`, issuing a `update` call first and then counting the result of `grep 'Inst'`, this script now works on Debian 7, Debian 8, Ubuntu 14.04 and Ubuntu 16.04.
