Background Bash

Background bash

The built-in bash tool blocks the assistant turn until the command exits. bg_bash spawns the command off-turn, returns a job id immediately, and exposes one multi-action tool (start, list, status, logs, wait, signal, stdin, remove) you drive across later turns to poll, steer, and collect output. Reach for it when a command outlives the moment you launch it - or when you would rather keep working than sit on a blocking call.

When to use this skill

Use bg_bash start when at least one of these is true:

• The command does not terminate on its own. Dev servers, file watchers, tail -f, npm run dev, queue workers. These would hang a blocking bash call forever.
• The command is slow and you have other work. A full build, a long test suite, a training run, a large download. Background it, make progress inline, then wait for it.
• You want to watch progress over several turns. logs with a sinceCursor lets you resume the stream without re-reading from the top.
• The command may need steering. A REPL or installer that reads stdin - pass interactiveStdin: true and feed it with the stdin action.

Stay on the built-in bash tool when:

• The command is quick (a git status, a one-shot test file, an ls) and you need its output to decide the next step. Backgrounding it just adds a poll round-trip.
• You cannot proceed without the result. If the very next thing you do consumes the output, block on bash.
• The work is pure calculation or data shaping. That is the compute tool, not a shell at all - see compute-over-bash.

Workflow

1. Start the job. bg_bash start with command (and cwd / label / env if needed). It returns a job id and the job begins running detached. Record the id - you need it for every later action.
2. Do inline work. The turn is yours again. Draft code, read files, start other jobs.
3. Check cheaply when you want a pulse. bg_bash status with the id is a single-line snapshot (running / exited, duration, bytes). Do not tight-poll - one status between substantial inline work, not one per line of reasoning.
4. Read output incrementally. bg_bash logs with the id. Use tail: N for the last N lines, grep to filter, and sinceCursor (the opaque cursor from a prior logs call) to resume without re-reading. If the response sets droppedBefore: true, the ring buffer evicted that range - fall back to the on-disk logFile it names.
5. Block when you actually need the result. bg_bash wait with a generous timeoutMs (default 15000). It returns when the job exits, or timedOut: true if still running - then loop or keep working.
6. Stop or clean up. bg_bash signal (default SIGTERM) kills a live job and its process group. bg_bash remove drops a terminal job from the registry - it refuses live jobs, so signal first.

Steering interactive jobs

A job started with interactiveStdin: true gets a writable stdin. Feed it with bg_bash stdin (text, optional eof to close the stream after writing). Without interactiveStdin, stdin is /dev/null and any stdin action errors - this is the default so non-interactive commands that incidentally read stdin see EOF instead of hanging.

Do not orphan jobs

Every job id you start is a commitment. Jobs live for the pi session and survive turn boundaries; a forgotten dev server keeps running and burning resources. Before ending a turn, for each live job: wait on it (if you need it), note in scratchpad that you will check it next turn, or signal it (if obsolete). On session_shutdown pi sends every live job SIGTERM, waits PI_BG_BASH_KILL_GRACE_MS (default 3000), then SIGKILL - but that is a backstop, not your exit plan.

Common pitfalls

• Backgrounding a quick command, then immediately waiting. That is a blocking bash call with extra steps. If you will wait right away, use bash.
• Tight status polling. Each call burns a tool slot. Prefer one wait with a real timeoutMs over a stream of status checks.
• Forgetting the job id across a compaction. Stash id -> what it runs in scratchpad so you can re-attach.
• Expecting state to persist across pi restarts. Jobs are session-scoped. A restart leaves only exited / error history; live jobs from a dead runtime are pruned.
• Reaching for bg_bash to compute. No filesystem-free sandbox here - it is a real shell. Pure math / JSON reshape belongs in compute.

Related docs

• bg-bash.md - full tool reference: every action, log buffering, signals, env vars.
• compute-over-bash - route pure computation to compute instead of any shell.
• subagent-background - the analogous lifecycle for background subagents.