fish

Verified on fish 4.1.2 in WSL (2026-07).

Process and job model

• fish does NOT fork a backgrounded pipeline stage that is a function - it blocks the main shell. Run readers as an external process: fish --no-config -c $script args &.
• fish 4.x jobs --last --pid prints a "Process" header plus one pid per pipeline stage - filter with string match --regex '^\d+$' and treat the result as a list.
• kill -0 0 always succeeds (signals the caller's own process group) - guard zero pids everywhere before liveness checks.

Fifos and lifecycle

• A fifo write with no reader blocks forever - liveness-check the reader before every fifo write.
• The serve daemon opens its request fifo O_RDWR so fish's open-write-close pattern never EOFs it; consequence: SIGKILL of fish orphans daemon and reader permanently (no EOF, no SIGPIPE ever). Normal teardown runs via the fish_exit event handler (quit + kill the pipeline).
• --on-signal SIGUSR1 handlers fire between commands in non-interactive scripts (verified with a 200ms poll loop).

Testing quirks

• fish under script(1) discards/paste-buffers piped typeahead, so in-session probes never execute - a harness limitation, not a bug (zsh under the same setup is fine). Assert via files written by event handlers instead.
• The streaming transient prompt is cached in a tempfile ($_omp_streaming_tempfile.transient), not a variable, because _omp_cleanup_stream runs before the transient repaint.