fux

fux is a minimal persistent terminal multiplexer. Workspaces group related work, tabs switch layouts, and splits show terminals together. One session server per user keeps pane processes and their bounded history alive while viewers attach, detach and reconnect. The authoritative model lives in a standalone bevy_ecs World; the viewer is a small terminal compositor. fux builds, installs and runs with no other program present, opens no network listeners and needs no keys.

Use

cargo build --release --locked -p fux
./target/release/fux

A fresh fux starts the session server on demand, creates the workspace default with the tab main and one pane running your shell, and attaches. Detaching or closing the terminal leaves the server and its panes running. Persistence means surviving detach: nothing is resurrected after the server or the machine restarts.

• fux attaches to the workspace most recently attached by any viewer; on a fresh server it creates default. fux NAME opens or creates a named workspace. There is no startup picker.
• fux workspace list, fux workspace new [NAME], fux workspace kill NAME.
• fux bindings prints the configured prefix and bindings by group.
• fux serve --name NAME runs the server in the foreground (SIGINT/SIGTERM shut it down).
• fux attach --socket PATH attaches to an explicit private attachment socket, for example one a koh gateway exposes.
• fux [NAME] new|split|focus|kill|resize|send-keys|capture|list|tab|subscribe … and fux [NAME] ctl JSON drive the workspace's control socket from scripts. fux --help lists them.

Configuration is $XDG_CONFIG_HOME/fux/config.toml (default ~/.config/fux/config.toml); a missing file means defaults. Every key is optional:

prefix = "C-a"                                  # one byte: a printable key, C-x, Esc, Space, DEL, 0xHH
default-command = { argv = ["/bin/zsh", "-l"] } # default: $SHELL -l, else /bin/sh -l
clipboard = "disabled"                          # or "write-only": OSC 52 copies reach the terminal
[bindings]                                      # key = action, merged over the defaults
"|" = "split-side"

[history]
scrollback-lines = 10000                        # per pane, 1-100000

[limits]
max-panes = 128                                 # per workspace
max-tabs = 32
max-workspaces = 64

[style]                                         # sixteen ANSI names, "default" (terminal foreground) or "none" (keep the cell's colour)
bar = "white"                                   # workspace name, inactive tabs, pane id: title
bar-background = "bright-black"                 # background of the bar row
tab-active = "default"                          # current tab (drawn reversed)
separator = "bright-black"                      # separators away from the focused pane
separator-focused = "default"                   # separators touching the focused pane (bold)
notice = "yellow"                               # transient notices; errors are always red

Private sockets live under $XDG_RUNTIME_DIR/fux (macOS fallback ~/Library/Caches/fux-runtime); daemon diagnostics go to $XDG_STATE_HOME/fux/daemon.log (default ~/.local/state/fux).

The protocols are not versioned: every fux, koh and zor build is expected to match the tree it was built from. A server older than the viewer shows up as a rejected frame or reply, reported as an error that names the session server; save your work in it and restart it (fux workspace kill per workspace, or SIGTERM to its pid from the descriptor). fux never stops a server on its own.

Keys

Ordinary keys are byte-exact pane input. The prefix (Ctrl-A by default) enters command mode and immediately shows the command column: a box in the bottom-right corner, directly above the bar, one row per binding under its group heading, only as wide as its widest line and only as tall as its content. Commands run at once; a burst such as prefix-| is applied before the next repaint, so nothing flashes. Prefix twice sends one literal prefix. Unknown keys stay in command mode and keep the column open; Esc leaves without sending anything. Dim rows are unavailable in the current context and say why when pressed. Keys are matched without Shift: x and X, | and \, - and _ are the same key, so no two bindings may differ only by Shift. When the terminal is too short for every row the column scrolls one row per ↑/↓ and a screenful per PgUp/PgDn, with ▲ n more / ▼ n more rows marking what is hidden (on one or two rows it scrolls without them). The tab and workspace choosers, the rename and new-workspace prompts and the close confirmations use the same corner box. There is no command-mode timeout.

The bottom row is always the bar, on its own background: the workspace name, the tabs with the current one reversed, and the focused pane as id: title on the right (or its exit status once it has exited). Transient notices such as copy results, errors and workspace switches appear in that right zone for two seconds or until the next key. Panes have no frame: adjacent panes share one thin separator, drawn bold next to the focused pane, and a single pane fills everything above the bar. Colours are muted by default and configurable through [style]. Pane sizes are negotiated over the smallest viewer showing the tab, so two viewers with different terminals see the same pane contents; the larger viewer leaves unused margins. On a tiny terminal the column first scrolls and then truncates labels with …; with a single row only the bar is shown. A pane that exits while unfocused keeps a dim exit N marker in its last row; the focused pane's status is in the bar.

Copy mode ([) browses the pane's private history: arrows or h j k l move, u/d and PgUp/PgDn scroll, Space starts a selection, y or Enter copies it and returns to live output, g jumps back to live output, q leaves, Esc clears the selection first and then backs out. New output never moves another viewer's viewport or changes a selection silently: if eviction or a resize invalidates the selected rows, the selection is cleared with a visible notice.

Mouse: when the application under the pointer does not request mouse input, the wheel browses that pane's history and dragging selects text inside the pane. When it does, events reach the application with pane-relative coordinates. Hold Shift to force fux's own history/selection handling; the keyboard path above always works for terminals that reserve gestures. Copies use the configured clipboard policy (disabled by default; write-only emits one bounded OSC 52 sequence of at most 1 MiB encoded) and report success or the reason for failure.

Escape is both a key and the start of many sequences. A lone Esc is forwarded after a short disambiguation window (35 ms) unless more bytes arrive; sequences split across reads are reassembled, and a cancelled mode keeps ownership of an unfinished paste until it drains.

Panes and history

Every pane keeps up to scrollback-lines rows of history in the server while hidden, on another tab, or detached; switching tabs or workspaces and reattaching never discards it. Older rows are evicted first. Closing a pane, closing a tab or killing a workspace frees the history. There is no merged output log.

When the only pane of the only tab exits by itself the workspace retires with that exit status: attached viewers see the final screen, then exit with the code; the workspace finalizes once the viewers have seen it (or after five seconds). The manager retains bounded final records for up to 60 seconds after pane retirement, even after the workspace sockets disappear. Other natural exits close the pane, and an emptied tab closes. Confirmed close and kill send SIGHUP to the pane's process group, SIGKILL after one second, and reap it. Workspace kill and server shutdown do the same for every pane.

Headless and agent use

Everything works without a terminal. fux workspace new NAME starts the server and a workspace; fux [NAME] <command> drives it over the control protocol (new, split, focus, kill, resize, send-keys, capture, list, tab, subscribe, info, wait). Panes carry a monotonic output sequence reported by list, capture and pane.output events; fux [NAME] capture PANE --cells returns the visible grid cell by cell with the same coherent metadata as the text form. fux [NAME] wait PANE exit (or seq N) blocks server-side until the condition holds instead of polling. fux [NAME] new --env K=V --rows R --columns C -- CMD sets a pane's environment and headless size, and fux [NAME] send-keys PANE --keys "C-c Enter" sends named keys.

fux run -- COMMAND is the one-shot convenience: it creates an ephemeral workspace, runs the command in a pane of a given size and environment, waits for retained final evidence, prints the final screen and exits with the command's status. It creates its workspace atomically and never takes over or cleans up a pre-existing workspace.

fux run --rows 24 --columns 80 --env CI=1 -- pytest -q

Working with koh and zor

fux composes with the koh and zor programs through process protocols pinned by the fixtures in crates/fux/tests/verify/fixtures/; it never links, spawns or supervises them. zor lives in this repository as the separate crate crates/zor (its own binary, tests, lints and ownership boundary: agent, task, check and artifact policy stay there), so a protocol change is one PR and one gate. koh remains an independent repository, pinned at an exact commit in tools/xtask/companions.json and never patched locally: a needed koh change goes upstream first, then the pin moves.

Remote access is koh's job. On the machine running fux:

koh gateway serve --socket "$XDG_RUNTIME_DIR/fux/default.attach.sock" --allow CLIENT_ID

and on the viewer machine:

koh gateway connect SERVER_ID --socket /private/dir/fux.sock
fux attach --socket /private/dir/fux.sock

koh authenticates the peer before it opens the local socket, carries the opaque attachment stream, and resumes it across transient link loss without repeating input. Stopping either side leaves the panes and local attachments untouched.

Observation is zor's job. Point it at a pane's workspace control socket:

zor observe --socket "$XDG_RUNTIME_DIR/fux/default.sock" --pane 1 --pid PID

zor negotiates the control preface, samples list and capture, and runs its own rules. A missing, stalled, crashed or malformed observer cannot block or change a pane. The socket paths, identities and events zor consumes are documented in the control protocol.

Protocol fixtures

crates/fux/tests/verify/fixtures/ holds one golden JSON frame per schema and direction (control requests, replies and events, manager frames, attachment messages). cargo test -p fux --test fixtures round-trips each through its Rust type with deny_unknown_fields, so a schema change is a visible diff. These files are the shape koh and zor consume; the real-binary integration suites remain the final check.

Verification

cargo fmt --all --check
cargo clippy --workspace --all-targets --locked -- -D warnings
cargo test --workspace --locked -- --test-threads=1
cargo doc --workspace --no-deps --locked
cargo test --manifest-path crates/fux/tests/verify/fixture-child/Cargo.toml --locked
crates/fux/tests/verify/release-package.sh

Deterministic ECS tests inject events and time (crates/fux/tests/ecs.rs, including randomized command sequences); real-process scenarios use disposable HOME/XDG directories and owned processes only (crates/fux/tests/local_cli.rs, the fixture-child suite). The real zor integration (cargo test -p fux --test zor_integration) builds crates/zor from this checkout; ZOR_BIN overrides it with another build, and FUX_REQUIRE_ZOR_BIN=1 makes a missing binary a failure. The optional cross-repository job and cargo run --locked --manifest-path tools/xtask/Cargo.toml -- dependencies verify --build check out koh at its pinned commit (dependencies apply clones it; verify requires the exact commit with no local changes) and run the required real koh integration with explicit binary paths (FUX_BIN, KOH_REQUIRE_FUX_BIN=1), so it can never silently skip.

Documents

• docs/design.md: architecture, entity model, system order, lifecycle.
• docs/ecs-acceptance.md: requirement-by-requirement acceptance audit.
• docs/local-attachment-protocol.md and docs/local-control-protocol.md.
• docs/security.md, docs/release-readiness.md, CHANGELOG.md, HANDOFF.md.
• Earlier architectures (the koh-hosted wrapper, the standalone host) and the prompts that drove each rewrite live in git history; docs/design.md keeps a short history.

Licensed under MIT. Terminal handling reused from earlier fux releases and the koh/zor projects retains attribution in LICENSES.