fix: add shellcheck and fix a few warnings

[leshikus]

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
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[bunnyshell]

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[leshikus]

Using shellcheck can potentially improve the quality of the project shell scripts.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.01%. Comparing base (73495d0) to head (6899b28).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #22569      +/-   ##
==========================================
+ Coverage   55.96%   56.01%   +0.04%     
==========================================
  Files         343      343              
  Lines       57527    57527              
==========================================
+ Hits        32197    32225      +28     
+ Misses      22679    22656      -23     
+ Partials     2651     2646       -5     

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[olivergondza]

Thanks a lot for this improvement! I am filing #22686 that fixes many of those warnings.

I advocate not to exclude SC2086, SC2046, SC2068, SC2206, SC2048, SC2128, SC2207. The linked PR makes an effort to address them, and they have a tendency to cause subtle problems.