Ion is a modern system shell that features a simple, yet powerful, syntax. It is written entirely in Rust, which greatly increases the overall quality and security of the shell, eliminating the possibilities of a ShellShock-like vulnerability, and making development easier. It also offers a level of performance that exceeds that of Dash, when taking advantage of Ion's features. While it is developed alongside, and primarily for, RedoxOS, it is a fully capable on other *nix platforms, and we are currently searching for a Windows developer to port it to Windows.
Syntax and feature decisions for Ion are made based upon three measurements: is the feature useful, is it simple to use, and will it's implementation be efficient to parse and execute? A feature is considered useful if there's a valid use case for it, in the concept of a shell language. The syntax for the feature should be simple for a human to read and write, with extra emphasis on readability, given that most time is spent reading scripts than writing them. The implementation should require minimal to zero heap allocations, and be implemented in a manner that requires minimal CPU cycles (so long as it's also fully documented and easy to maintain!).
It should also be taken into consideration that shells operate entirely upon strings, and therefore should be fully equipped for all manner of string manipulation capabilities. That means that users of a shell should not immediately need to grasp for tools like cut, sed, and awk. Ion offers a great deal of control over slicing and manipulating text. Arrays are treated as first class variables with their own unique @ sigil. Strings are also treated as first class variables with their own unique $ sigil. Both support being sliced with [range], and they each have their own supply of methods.
If Ion had to follow POSIX specifications, it wouldn't be half the shell that it is today, and there'd be no solid reason to use Ion over any other existing shell, given that it'd basically be the same as every other POSIX shell. Redox OS itself doesn't follow POSIX specifications, and neither does it require a POSIX shell for developing Redox's userspace. It's therefore not meant to be used as a drop-in replacement for Dash or Bash. You should retain Dash/Bash on your system for execution of Dash/Bash scripts, but you're free to write new scripts for Ion, or use Ion as the interactive shell for your user session. Redox OS, for example, also contains Dash for compatibility with software that depends on POSIX scripts.
That said, Ion's foundations are heavily inspired by POSIX shell syntax. If you have experience with POSIX shells, then you already have a good idea of how most of Ion's core features operate. A quick sprint through this documentation will bring you up to speed on the differences between our shell and POSIX shells. Namely, we carry a lot of the same operators: $, |, ||, &, &&, >, <, <<, <<<, $(), $(()). Yet we also offer some functionality of our own, such as @, @(), $method(), @method(), ^|, ^>, &>, &|. Essentially, we have taken the best components of the POSIX shell specifications, removed the bad parts, and implemented even better features on top of the best parts. That's how open source software evolves: iterate, deploy, study, repeat.