shellcheck

[AndreasLMeg]

There are many functionally important bash files in the system.
Therefore, special attention should be paid to syntax and coding guidelines.
I'm not very familiar with the bash specifics, so it would be particularly important to me to check it with external tools (experts).

For this I found bash -h and shellcheck.

Shellcheck is very precise, so I excluded all existing warnings at the beginning.
Please check, there may be potential errors

(please use squash to merge the PR)

[EricClaeys]

@AndreasLMeg variables.sh has "set -a" which automatically exports all variables.
ON_TTY is used by some scripts that "source variables.sh"

[AndreasLMeg]

@EricClaeys I am aware that some anomalies probably cannot be fixed either.

#SC2034: ON_TTY appears unused. Verify use (or export if used externally).
#SC2046: Quote this to prevent word splitting.
#SC2086: Double quote to prevent globbing and word splitting.
#SC1090: Can't follow non-constant source. Use a directive to specify location.

That's why I defined the exceptions.
It is important to me that a second tool checks the bash scripts to check for possible errors in future changes.
It's nice if the known exceptions (-e) also help to clean up existing anomalies when the opportunity arises.

#SC2164: Use 'cd ... || exit' or 'cd ... || return' in case cd fails
#SC2166: Prefer [ p ] || [ q ] as [ p -o q ] is not well defined.
#SC2006: Use $(...) notation instead of legacy backticked `...`.
#SC2196: egrep is non-standard and deprecated. Use grep -E instead.

[EricClaeys]

Looks good.