shellingham | Tool to Detect Surrounding Shell

The question is about looking for a way to walk the list of ancestor processes and find the first whose executable matches an item in a hardcoded list of names, hoping that it will be the user’s ‘shell’. Before I answer it, here are some reasons why this is an absolutely stupid and useless thing to do:

• The executable you are looking for may be named something else from what you expect. In a comment to the question I gave an example of naming the Bash executable shab.exe, which the asker dismissed with the punchline of the well-known doctor joke (‘well, don’t do that then’) as the user trying to ‘hide’ their shell. But this can happen even without any ‘spiteful’ intent to ‘hide’ something on the user’s part. For example, the user may have several versions of Bash installed with different executable names (bash-4.00.exe, gitbash.exe, etc.), in order to test whether their scripts are compatible with all of them. Are you going to enumerate every single possible name in your code? Or are you going to match all executables with sh in their name and cross fingers that there aren’t any false positives?
• Conversely, just because an executable has the name you expect, it doesn’t mean it will have the behaviour you expect. This is even more true on Windows, where there isn’t really a standardised set of executable names that should always have specific, prescribed behaviour. And this is especially true if you indiscriminately lump programs like cmd, bash, fish and xonsh together under the name of ‘shell’: these programs accept different syntaxes on their own command lines and within their respective scripting languages. Unless all you want to do is launch the shell for the user to interact with, you’re going to be looking for the specific kind of the shell – whether it’s a POSIX-compatible shell, a DOS-derived shell like cmd or something else completely – in order to take advantage of its particular behaviours. Merely knowing that it’s ‘a shell’ doesn’t actually tell you anything useful. And let’s not forget that not all shells are even command lines – Windows Explorer is a shell, after all.
• Even if executable names did line up perfectly, there is no way a hardcoded list is going to be exhaustive. The asker’s list already omits tcmd or tclsh. I hear some crazy people use Python itself as a shell – who are you to stop them? If a new shell appears, it will have to be added as another entry in the list; let’s hope there is still someone who remembers where it is then.
• Though let’s say for the sake of the argument that we’re only interested in command-line shells and ignore everything else. What if the script is launched from an Explorer process that was itself started from Bash? The script is going to ignore the Explorer process and pick Bash as the ‘shell’, even though it’s clearly Explorer and not Bash which launched the script. Is this correct or desired? I think the answer is far from obvious.

But if the above doesn’t deter you from this futile task (or perhaps you want to do something similar for a more sensible purpose), here’s how you can accomplish this nonsensical thing anyway: