bash

Verified on bash 5.2 (2026-07).

Readline and prompt

• Running the exec builtin from PROMPT_COMMAND - even a harmless exec {fd}</dev/null - silently disables readline for the whole session: the prompt never paints again, no error, commands still execute non-interactively. Use coproc fds directly; never exec-dup or exec-close from prompt context.
• PS1 must stay '$(_omp_get_primary)' (single-quoted). With promptvars on, literal prompt content executes $(...) - a directory named $(cmd) becomes command injection.
• "Expansion returns correct bytes" is NOT "prompt displays" - always capture a real typescript (script -qe -c ... FILE) when verifying prompt behavior. Note script(1)'s stdout relay does not carry bash prompt bytes (zsh's does); probe ${PS1@P} in-session instead.

coproc

• coproc NAME { cmd; } reports the pid of a wrapper subshell - exec in the body so the daemon replaces it.
• The coproc's ORIGINAL fds must be closed after duplication (eval "exec ${NAME[0]}<&- ${NAME[1]}>&-") or the child's stdin never EOFs when the dups close.
• Subshells close coproc fds in non-interactive bash - a (trap '' PIPE; ...) subshell guard does not work there; save/ignore/restore trap '' PIPE in the parent instead (nothing forks while ignored).
• bash prints failed >&fd redirection errors before later redirections apply - put 2>/dev/null BEFORE >&"$fd".

History

• A bash serve daemon was implemented and reverted (2026-07-07): no measurable speedup - native-Linux spawns are 11-16ms and sync-only wait-mode plus the display-time subshell cannot beat that. Do not re-propose without new evidence. The gotchas above came out of that work and remain valid.