fix(interpreter): exec >file redirects the shell's own output

[chaliy]

Requested by Михайло · project thread

What changed

exec >log 2>&1, exec >>log and exec 2>/dev/null now redirect the script's own output. Before, the fd table recorded them but only >&N lookups used it, so everything still printed to the caller.

• After each top-level command, fd 1/2 entries route that command's output (route_exec_output). Streaming is paused for the redirected streams.
• exec 3>&1 >log; echo x >&3 still reaches the caller through a passthrough buffer. exec 1>&3 3>&- restores.
• $( ) and pipelines keep their own stdout. $(cmd 2>&1) still captures instead of writing to the log.
• A failed exec >/bad/path leaves fd 1 alone and returns status 1.
• exec {var}>f with var unset no longer clobbers fd 1. Allocating an fd into var is still not implemented and is noted in L-PROC-001.

Why

This is a roadmap step 1 fidelity gap from the Debian-oracle case exec-redirect-whole-script. exec > >(tee log) 2>&1-style logging at the top of a script is a common agent pattern. In bashkit it silently did nothing.

Before / After

Script	Before	After (= bash)
exec >/tmp/l 2>&1; echo one; ls /nope	prints both	prints nothing, both in /tmp/l
exec 2>/dev/null; ls /nope; echo rc=$?	error shown	rc=2 only
exec 3>&1 >/tmp/l; echo a; echo b >&3	a, b	b (and a in the log)
exec >/tmp/l; x=$(echo cap); exec >&3; echo $x	(n/a)	cap

Proof: new spec exec-redirect-shell-output.test.sh (5 cases, bash-verified, run through both the library and the CLI) and new streaming test exec_redirected_output_is_not_streamed. bash_comparison passes, and the oracle case now matches.

Risk

Medium-low. The change only applies while exec has redirected fd 1/2. Output written before the exec in the same top-level command, and not streamed yet, also goes to the log (documented in L-PROC-001). The depth-50 nested $( ) stack need is unchanged.

Checklist

• Failing spec first
• cargo test -p bashkit --lib / --test integration / bash_comparison
• clippy -D warnings, fmt, check-okf
• knowledge: limitations L-PROC-001, log

https://claude.ai/code/session_019aFikmptPc91Fj4N2iDXQA

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