Make script POSIX compliant (and switch shebang to sh)

[shadowspawn]

Pull Request

May not merge in absence of any user feedback, but wanted to check it was possible!

Problem

n is currently written as a bash script. Bash isn't always available by default, and is an old version on macOS.

Related: #344 #648 #673

Solution

Rewrite as a POSIX shell script so runs with whatever (POSIX) shell is installed.

All tests pass on Ubuntu using dash as the shell. Have addressed all shellcheck issues with flags passed to utilities.

Currently still using local (a lot) which is commonly available but not POSIX.

https://stackoverflow.com/a/18600920/1082434

It is normally done with the local keyword, which is, as you seem to know, not defined by POSIX. Here is an informative discussion about adding 'local' to POSIX.

However, even the most primitive POSIX-compliant shell I know of which is used by some GNU/Linux distributions as the /bin/sh default, dash (Debian Almquist Shell), supports it. FreeBSD and NetBSD use ash, the original Almquist Shell, which also supports it. OpenBSD uses a ksh implementation for /bin/sh which also supports it. So unless you're aiming to support non-GNU non-BSD systems like Solaris, or those using standard ksh, etc., you could get away with using local.

ChangeLog

[shadowspawn]

Issue with "super barebones" setup and only sh available: #782