OpenWrt Init System

The OpenWrt Linux Distro uses an init system similar to sysvinit common on most Linux Distros.

Here is the boot process in a nutshell:

  1. bootloader configures enough low level hardware to load kernel and jump to it with a kernel cmdline
  2. kernel inits hardware for everything built 'static' in the kernel
  3. kernel mounts the root filesystem (via the root= rootfstype= etc parameters in the kernel cmdline)
  4. kernel executes the 'init' process (PID 1).
    • this is done in the kernel's init_post function in init/main.c and typically will execute whichever is found first: /sbin/init, /etc/init, /bin/init, /bin/sh
    • for OpenWrt this function is patched so that it looks for and executes only /etc/preinit
  5. /etc/preinit does some low level work such as mounting filesystems etc
  6. /etc/init.d/rcS is run which executes all scripts in /etc/rc.d/S*
    • files here are typically symlinks to scripts in /etc/init.d and have a number preceeding the name to dictate a priority affects order of execution
    • /etc/init.d/ scripts include /etc/rc.common which implements enable/disable functions which create the symlinks. The start/stop priority numbers are defined in the init scripts

For more info see OpenWrt documentation:

Failsafe bootup

The /etc/preinit script has a 'failsafe' mechanism which allows you a few seconds to answer a prompt to go into 'failsafe mode'. This simply stops execution of the init scripts in case you have a mis-configuration or other issue that is keeping you from configuring the target board. There are many configurations for the failsafe mechanism at the top of /etc/preinit (network config, timeout, messages, etc)

Adding an init script

If you want something simple to be done 'near the end of every boot' a good place to put it may be in /etc/rc.local as this is executed by the /etc/init.d/done script which is symlinked to /etc/rc.d/S95done. Note that this isn't the 'last' init script run but is for sure near the end. You can see all the startup scripts in order with 'ls /etc/rc.d/S*'

Keep in mind that init scripts run with stdout/stderr supressed from the linux serial console. If you want to disable this supression you can put the following in your init script to redirect stdout/stderr to the serial console:

exec 1>/dev/console  ;# redirect stdout to serial console
exec 2>/dev/console  ;# redirect stderr to serial console

If you want to add a more complex init process that perhaps has a start/stop and a priority you can easily create your own init script by following the examples in /etc/init.d.

Note that if using 'sysupgrade' there is a specified set of files that get backed up and restored to the newly upgraded image along with all of UCI. This is configured in /etc/sysupgrade.conf. If you want your init changes to persist across a sysupgrade be sure to configure things properly. You may want to create an OpenWrt package and use UCI configuration to help do that for you.


Below is an example of a simple script placed on OpenWrt at /etc/init.d/gst-httpd-watchdog. This script basically checks if a process is running, and if it doesn't find it, then it restarts it.

The key once creating this script is to then enable it by typing

/etc/init.d/gst-httpd-watchdog enable

This will place a symlink in /etc/rc.d/ as noted below


Here is the actual code /etc/init.d/ryantest

root@OpenWrt:/etc/init.d# cat ryantest 
#!/bin/sh /etc/rc.common


ps -aef | grep gst | grep -v grep

while [ 1 ] 

proc=$(ps -aef | grep gst | grep -v grep )

echo 'proc='$proc

if [ -z "$proc" ]
        echo 'gst process is not found, now restarting'
        /etc/init.d/gst-httpd restart
        echo 'gst process is running'
sleep 10s;



( watchdog ) &
Last modified 3 years ago Last modified on 11/15/2021 05:25:28 PM
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