`nxinit` NXInit

Overview

In NuttX, there are various options for the init entry point, with NSH being one of the commonly used ones. However, taking NSH as an example, it is not suitable for all usage scenarios. For instance, the code size required by NSH and NSH_LIBRARY is relatively large, and NSH lacks management capabilities for daemons/services (such as restarting services).

NXInit was created by Xiaomi to solve these issues. It is compatible with most of the syntax of Android Init, which you can read about here: https://android.googlesource.com/platform/system/core/+/master/init/README.md

It is lightweight, supports command execution triggered by events, supports service/daemon management and many more.

The init.rc consists of five categories of statements, all line-oriented with parameters separated by spaces. The content is processed by a preprocessor, following C language specifications for escape rules and comment formats.

File path configured by CONFIG_SYSTEM_NXINIT_RC_FILE_PATH.

1. Actions and Commands

on <trigger>
   <command>
   <command>
   <command>

An Action (command sequence) contains trigger conditions. When satisfied, it is added to the execution queue.
Commands are the specific execution content of actions, such as setprop (set system properties), start (start services), and mount (mount file systems).

2. Services and Options

service <name> <pathname> [ <argument> ]*
   <option>
   <option>
   ...

A Service is a program started by Init, with a unique name (can be overridden via the override option).
Options are modifiers for services, affecting their running mode and timing. Examples include class (specify service category for batch start/stop), override (override previously defined services), restart_period (interval for restarting exited services), and reboot_on_failure (critical services trigger device reboot on startup failure or abnormal exit).

Triggers

The triggers of an action serve as its triggering conditions, supporting one or more triggers (which must all be satisfied simultaneously).

There are two types of triggers: event triggers (e.g., boot) and action triggers (also known as property triggers, e.g., property:a=b). Actions can include multiple action triggers but only one event trigger. Triggers can be combined with && to represent “AND” conditions.

Note

Currently, only event triggers are supported.

Todo

Implement action triggers.

1. Event Trigger Execution Order

boot: The first event after NXInit starts.
init: After boot event completation.
netinit: Optional, after netinit_bringup() returns.
finalinit: After all previous events complete.

2. Action Triggers Checking

All action triggers are automatically checked once at startup.
Property triggers are checked when the property is created or its value is updated (e.g., property:a=b is checked when a’s value changes).

Commands

The commands supported by an action fall into three types: the built-in commands of NXInit, the built-in commands of NSH (if NSH is enabled), and Builtin Apps.

The following is an explanation of some of NXInit’s built-in commands.

System Operations: mount/umount (mount/unmount file systems), setprop (set system properties), chmod/chown (modify file permissions/owner).
Service Management: start/stop/restart (start/stop/restart services), class_start/class_stop (batch start/stop service categories).
File Operations: mkdir (create directories with configurable permissions, owner, and encryption settings), copy/write (copy files/write file content).
Others: trigger (trigger events).

Examples

This is an example of enabling the basic functions of the NXInit component, with all log levels additionally enabled to facilitate debugging of the init.rc script.

defconfig:

+CONFIG_INIT_ENTRYPOINT="init_main"
+CONFIG_SYSTEM_INIT=y
+CONFIG_SYSTEM_INIT_DEBUG=y
+CONFIG_SYSTEM_INIT_INFO=y
+CONFIG_SYSTEM_INIT_WARN=y

init.rc:

on boot
    start console        /* Start the service named "console" */
    sleep 1              /* Block for 1 second */
    exec_start mkdir_tmp /* Start the "mkdir_tmp" service and wait for exit */
    start fastboot

service console sh
    class core
    restart_period 1000  /* Service restart interval (in milliseconds), calculated from service start time */

service fastboot fastbootd
    class core
    restart_period 7000

service mkdir_tmp sh -c "mkdir /tmp"
    reboot_on_failure 0 /* Reboot device with reason 0 on startup failure or abnormal exit */
    oneshot             /* This service runs only once; releases resources after exit */

nxinit NXInit