Fixes issue 21059: installer fails when piped on Posix.

[veelo]

The assumption that $SHLVL increases when the script is run does not always hold. $SHLVL does not increase if the script is piped into bash, thereby failing to detect Posix. Also, if the default terminal shell is not bash, $SHLVL might be 1 on Posix.

[dlang-bot]

Thanks for your pull request and interest in making D better, @veelo! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21059	normal	install.sh: posix_terminal returns false on Linux Mint 20

[jmh530]

Thanks for working on this.

Is this tested with Windows Subsystem for Linux (WSL)?

[veelo]

Is this tested with Windows Subsystem for Linux (WSL)?

No it is not tested with WSL. The installer is expected to install the Linux version of dmd on WSL, producing Linux executables (that require WSL to run on Windows).

[nexor]

I've checked it locally and it seems that issue with docker is resolved.
Thank you!

[veelo]

Thanks for confirming, and sorry for the breakage!

[wilzbach]

there are no booleans in bash!

Suggested change

	false
	return 0

(I'm aware that it works, but it's still a bad style)

[veelo]

But you got it backwards. return 0 == true in bash, unlike D. https://stackoverflow.com/questions/23320150/bash-boolean-testing/23324368#23324368. I believe I had it this way in my first iteration and you commented rightfully that it was confusing. Good or bad style, at least this cannot be misunderstood.

[wilzbach]

(deployed)

[veelo]

Thanks.