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README.md

@@ -1,3 +1,96 @@
 # bajia
 
-an init system
+an init system (PID 1) for embedded / VM targets, written in C++20 and
+configured with a small declarative language inspired by Android's init `.rc`
+format.
+
+## Status
+
+- `.rc` config parser (services + on-trigger action blocks)
+- Supervisor event loop built on `signalfd` + `epoll`
+- service spawn / reap / respawn (per-service restart policy)
+- action commands: `start`, `stop`, `restart`, `exec`, `mkdir`, `chmod`,
+  `chown`, `setenv`, `write`, `symlink`, `mount`, `log`
+- logger with a ring buffer that flushes to the console once available
+
+roadmap:
+
+- user/group privilege drop (`user`, `group`, supplementary groups)
+- dependency ordering between services
+- `SIGCHLD` crash-window limiting (rate-limited restarts)
+- property triggers (`property:<k>=<v>`) and `setprop`/`getprop`
+- per-service logging to files
+- `reboot`/`poweroff` path with ordered unmount
+- `SIGHUP` config reload
+- SELinux
+
+## building
+
+requires a C++20 compiler and [Ninja](https://ninja-build.org/)
+
+```sh
+python3 configure.py     # generates build/build.ninja
+ninja -C build           # produces build/bajia
+```
+
+`configure.py` also writes a thin `Makefile` convenience wrapper
+(`make`, `make clean`, `make format`, `--asan`, `--debug`).
+
+## running
+
+As a real init, the kernel must launch it as PID 1:
+
+```
+init=/path/to/bajia
+```
+
+Or by hand against a config (useful for development, may not behave like a
+real boot). bajia normally refuses to start unless it is PID 1; pass
+`--run-as-user` to override:
+
+```sh
+./build/bajia --run-as-user etc/init.rc
+```
+
+If no files are given it looks for `/etc/bajia/init.rc`.
+
+## configuration language
+
+See [`etc/init.rc`](etc/init.rc) for a complete example.
+
+### services
+
+```rc
+service NAME /path/to/exe [args...]
+    user root|other          # privilege level (drop, planned)
+    group GROUP [GROUP...]
+    oneshot                  # run once and exit, never respawn
+    disabled                 # not started by the boot sequence
+    console                  # bind stdio to /dev/console
+    class NAME               # grouping (default "default")
+    respawn never|on-failure|always   # restart policy (default always)
+    crash-threshold N        # restarts allowed per window
+    crash-window SECS
+    setenv K=V               # extra environment (repeatable)
+    cwd /path
+```
+
+### actions
+
+```rc
+on TRIGGER
+    start NAME | stop NAME | restart NAME
+    exec /cmd args...
+    mkdir PATH [mode]
+    chmod PATH mode
+    chown PATH uid gid
+    setenv K V
+    write PATH CONTENT
+    symlink TARGET LINK
+    mount SOURCE TARGET FSTYPE
+    log message
+```
+
+boot triggers fire in order: `early-init`, `init`, `boot`. `shutdown` is
+reserved (planned wiring to the signal path). property/`service-*` triggers
+are on the roadmap.