Bash trap: Unrecognized signal specification handling divergence between 4.x and 5.x series

Answer

Bash 5.0 and later validate symbolic signal names when trap runs, so an unrecognized name fails immediately with a non-zero exit status. Bash 4.4 and earlier accept the same name at definition time and only fail, if at all, when the signal would be delivered. This matters for scripts that build trap arguments from external configuration. Numeric signal specifications outside the valid range fail immediately in both series; only symbolic names show the divergence.

Confirmed facts and open questions

Question	Answer
Exact release where validation shifted	Reported as Bash 5.0; not marked as deliberate in the changelog, so treat the boundary as approximate.
trap -l as predictor of accepted names	Reliable in 5.x; in 4.x it lists recognized names while trap tolerates additional unknown names.
Detect validation mode without forking	Check BASH_VERSION: major 5 or higher implies immediate validation, 4.x implies deferred. Distribution backports can invalidate this check.

Steps for Bash 4.4–5.2 portability

1. Avoid symbolic signal names sourced from external configuration.
2. Use numeric signal values where possible; they behave identically across the range.
3. If symbolic names are required, validate each name against trap -l before use, or restrict names to the portable POSIX set.
4. Confirm the shell with bash --version; major version 5 or higher means immediate validation.

Verification

bash --version | head -n 1
trap ':' NOSUCHSIG 2>&1; echo "exit=$?"

Expect exit=0 on Bash 4.4 and a non-zero exit with an error message on Bash 5.0+. If you manage patched distribution builds, provide the full bash --version output for the oldest target host; a backport can change which validation mode applies.