Apply all current ShellCheck suggestions and add lint-shell.sh

[kristapsk]

Based on #1175, but also covers install.sh changes since then, all other shell scripts and adds ShellCheck linter script. Not adding this to CI for now. I'm thinking we could split off all the linting to separate job, don't see a point in running them under all OS / Python version combinations.

[kristapsk]

@dmp1ce You were author of original PR, would be nice if you could take a look.

[PulpCattel]

@kristapsk Is there a reason we want to write the linters (or other scripts) in shell rather than Python? Asking because I'm personally much more comfortable with Python and, well, the entire project is written in Python already. Some other reasons can be found at bitcoin/bitcoin#24783

It's not a big deal, I'm just curious if there are specific reasons.

[kristapsk]

No specific reason. I based old lint-python.sh on one from Core, before they ported them to Python. Now for shellcheck also copied old one written in bash, to not use different languages here. Bet I'm not against somebody changing our linters to Python. Just don't care about that too much myself, haven't run into issues because of that.

[dmp1ce]

LGTM. My only question is about commenting out old code that should probably be removed.

[dmp1ce]

@PulpCattel @kristapsk Unless issues exist with the shell scripts they should stay as is because converting them to Python will likely cause a bug or two. Shellcheck does a good job of finding potential issues with shell scripts.

[kristapsk]

@PulpCattel @kristapsk Unless issues exist with the shell scripts they should stay as is because converting them to Python will likely cause a bug or two. Shellcheck does a good job of finding potential issues with shell scripts.

Agree there is no much need to change what already works. But if we do convert to Python, likely we can just copy and modify little bit existing converted scripts from Bitcoin Core. That would lower possibility of introducing new bugs. Their current lint-shell.py also uses ShellCheck, just calls it not from a bash script, but from a Python script.