Watch mode & dev servers

Add -w to a command and fabr keeps running: it watches the files your targets actually depend on and reacts whenever they change, until you stop it with Ctrl-C. Because fabr already knows the exact input set of every target, watch mode is precise — only the work whose inputs changed re-runs; everything else stays a cache hit.

-w applies to the graph-building verbs (build, test) and to run. A burst of edits (an editor saving several files at once) is coalesced before fabr reacts, so a multi-file save triggers one rebuild, not several.

Rebuild and retest on change

fabr build -w mylib     # recompile whenever a source changes
fabr test -w mylib      # recompile and re-run the tests on every change

Each cycle reports what it rebuilt; an unchanged run reports nothing new. fabr test -w reprints the per-target report summary each time, so you get a live red/green as you edit.

Relaunch a program on change

For a runnable target, -w restages and relaunches the program when its inputs change:

fabr run -w mytool --flag arg

Fabr rebuilds the install, stops the old process, and starts the new one with the same arguments and inherited stdio. This is the basic dev loop for a CLI or a long-running program you’re editing.

Dev servers with serve

A serve target describes a long-lived server plus the content it serves — the shape that gets the most out of -w. It decorates any runnable (tool) with the files it should serve (files) and any extra support files (deps):

serve site {
  tool  = @npm:http-server:14.1.1;   # any runnable: a script, a js_script, or an external bin
  files = mysite;                    # the built content to serve
  args  = -c-1 .;                    # http-server: disable caching so edits show on reload
}

fabr run -w site

Under -w, fabr distinguishes two kinds of change and reacts to each as cheaply as it can:

• A content-only change (something in files) is synced into the running server’s directory in place — no restart. Fabr reports Updating site content (N files), and the server’s own file watcher picks up the change. This keeps a live-reload dev server fast: editing served content never bounces the process.
• A change to the program itself — tool, deps, or args — restarts the server, since the running install is no longer the right one.

Unlike a plain fabr run (which launches in your current directory), a serve target launches with its working directory at its own staged install, so a stock static file server serves files with no path wrangling.

Fabr’s own documentation site is built and previewed exactly this way — a generate target produces the static site, and a serve target runs http-server over it under fabr run -w.