Add .shellcheckrc adapted for ReaR

[antonvoznia]

• enable/disable SC2168 (local is only valid in functions) because scripts under lib/ are always sourced from functions, so "local" is valid inside them.
• set shell=bash because ReaR uses bash but the scripts don't have shebangs.

Relax-and-Recover (ReaR) Pull Request Template

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• Type: Bug Fix

• Impact: Low

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As a part of my master's thesis, I made a "research" about the Differential shellcheck usage.

I solved 2 problems in this PR:

• unnecessary output of shellcheck "Tips depend on target shell and yours is unknown. Add a shebang or a 'shell' directive."
  Since the ReaR is written in bash and that is specified in the following line:
  https://github.com/antonvoznia/rear/blob/master/usr/sbin/rear#L1

  Example of such notice:
  https://github.com/antonvoznia/rear/pull/100/files

• Scripts outside the lib directory are handled from functions, so it's a valid usage of local variables in them. However, shellcheck "incorrectly" marks local variable usage in these scripts.
  Example:
  https://github.com/antonvoznia/rear/pull/100/files

These issues I described in my master's thesis, chapter 6.2
https://dspace.cvut.cz/bitstream/handle/10467/107238/F3-DP-2023-Voznia-Anton-dip_thesis-2.pdf?sequence=-1&isAllowed=y#toc%3A22

2 test examples with the fix:

• The example with a newly added file without shebang and local variable usage without described above warning.
  https://github.com/antonvoznia/rear/pull/357/files
  https://github.com/antonvoznia/rear/pull/349/files

• Another example with local variable usage in functions, and there is expected behaviour of the shellcheck/Differential shellcheck:
  https://github.com/antonvoznia/rear/pull/352/files
  https://github.com/antonvoznia/rear/pull/341/files

[pcahyna]

@antonvoznia thanks - I have one comment - you show two example PRs, both are generated with severity: style. But that's not the shellcheck severity we are using - we are using severity: error : 966379e#diff-eace62a8ebcdafd0f6975200d68cbf851300952aae47db07e71f383038a368b3R29 . Can you please update the example PR links to PRs generated with the same severity level?

[jamacku]

I think .shellcheckrc in usr/share/rear/lib/ is redundant since one level above is .shellcheckrc with the same content.

EDIT: Sorry, I didn't notice enable/disable

[antonvoznia]

@antonvoznia thanks - I have one comment - you show two example PRs, both are generated with severity: style. But that's not the shellcheck severity we are using - we are using severity: error : 966379e#diff-eace62a8ebcdafd0f6975200d68cbf851300952aae47db07e71f383038a368b3R29 . Can you please update the example PR links to PRs generated with the same severity level?

Hi @pcahyna , I'm sorry for the mistake.
I've updated it.

Thanks,
Anton

[pcahyna]

thank you, merging!