[docs] Explain how to inject an autorun script
Change-Id: Ia671489228de3f46dd91a055ae6b848d46c95f4d
diff --git a/docs/kernel_cmdline.md b/docs/kernel_cmdline.md
index 1386c1b..696add4 100644
--- a/docs/kernel_cmdline.md
+++ b/docs/kernel_cmdline.md
@@ -248,6 +248,26 @@
Any `+` characters in *command* are treated as argument separators, allowing
you to pass arguments to an executable.
+### Sidebar: Injecting a personal autorun script <a name="autorun"></a>
+
+It's often useful to inject a personal autorun script into the boot filesystem,
+especially for running tests or setup tasks. You can do this with the zircon
+build system's `EXTRA_USER_MANIFEST_LINES` make variable.
+
+1. Create a local shell script with the commands you want to run; for this
+ example, call it `${HOME}/my_local_script.sh`. \
+ **NOTE**: The first line must be a `#!` line, typically `#!/boot/bin/sh`.
+2. Add your script to the boot filesystem by invoking `make` (or one of the
+ `//zircon/scripts/build-*` scripts) with \
+ `EXTRA_USER_MANIFEST_LINES="my_installed_script=${HOME}/my_local_script.sh"`
+ \
+ This will copy your local script into the boot filesystem at the path
+ `/boot/my_installed_script`. (You can change the `my_installed_script` part
+ to change the basename of the installed script, though it will always appear
+ under `/boot`.)
+3. Tell `devmgr` to invoke your script by passing a commandline like
+ `zircon.autorun.boot=/boot/my_installed_script`
+
## zircon.autorun.system=\<command>
This option requests that *command* be run once the system partition is mounted
