Rainer Gerhards c791e0e232 shell: clean up ShellCheck findings
Why: make shell lint results actionable and deterministic.

Impact: no intended behavior change beyond small portability and lint
cleanup fixes.

Before/After: ShellCheck error-level findings are clear for changed
scripts, and CodeFactor reports fewer legacy shell lint findings.

Technical Overview:
- add explicit shebangs to scripts that started with commands
- mark sourced packaging config fragments as bash for ShellCheck
- replace ash-incompatible source and == usage in Alpine helpers
- clean up CodeFactor-reported ShellCheck findings in diag.sh and
  selected test helpers
- fix SC2035, SC2006, SC2028, SC2268, SC2319, and SC2145 buckets
- fix remaining small one-offs for SC2003, SC2103, SC2219, SC1001,
  SC2166, SC2062, SC2216, and SC2005
- preserve complex command status explicitly where direct if-command
  checks would be less clear

With the help of AI-Agents: Codex
2026-05-22 17:51:19 +02:00
..
2020-08-11 15:51:34 +02:00

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Use this with care. If in doubt, please have a look at the official
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