Make POSIX-compliant to increase portability and remove bash depend…

[nothingnesses]

…ency on Linux. Fix causes of shellcheck warnings.

This PR fixes #100144 by removing bashisms and replacing them with POSIX-compliant alternatives.

[ghost]

All CLA requirements met.

[nothingnesses]

Should the VS Code on lines 8 and 33 be replaced with @@PRODNAME@@?

[joaomoreno]

Does this fix an actual issue?

[nothingnesses]

I'm unable to start VS Code on my system which doesn't have bash, and this fixes that.
For what it's worth, this would still allow VS Code to work on systems with bash. It simply also allows it to work on systems that don't have bash but do have other POSIX-compliant shells.

[joaomoreno]

Can you give me an example of said system? I'm just looking for a specific practical example.

[nothingnesses]

Sure thing. There are certain Linux distributions that don't enforce the usage of, and don't come pre-packaged with bash. For example, there's the minimal version of the Void Linux distribution which opts for dash and Kiss Linux which opts for busybox instead. On these systems, bash would arguably be considered redundant and could (in the case of Kiss where all packages must be compiled by the user) cause other software that depend on it an unnecessarily longer amount of time to build (when considering that bash also has to be built even though all it might be needed for is to run a start up script for VS Code). As such, reducing dependencies would be beneficial for the end user.

[joaomoreno]

I'm actually surprised VS Code runs in said systems. 👌

[joaomoreno]

@aeschli @bpasero Please review whether your areas of the script still work as designed.