Nested Substitution Island

Nested-substitution island

The redirection and status report below belong to the command substitution, not to the outer Python gate. A command substitution gets its own capture pipe; none of the three spellings closes the outer gate's stream or turns the outer gate into a self-probe. The bash -c case is load-bearing: bash removes the backslash from \$( in its double-quoted operand before the inner shell sees it, while a generic word parser may retain it. The broken outer gate must remain eligible for dead-stream probing in every form.

The final command keeps the old buggy checker from returning NOTHING PROBED: before this fixture's guard landed, all three leaking commands were falsely exempted and this safe control made the run return a genuine false green.

• enforced: closed-stream-check.py probes all three outer gate commands and reports their 1-to-0 dead-stdout leaks; the safe control remains clean.
• advisory: nothing else here is real.

python3 ../scripts/flip-gate.py $(printf x >&-)          # exit 1
python3 ../scripts/flip-gate.py $(echo ok; exit $?)     # exit 1
bash -c "python3 ../scripts/flip-gate.py \$(printf x >&-)" # exit 1
python3 -c "import sys; sys.exit(0)"                    # exit 0