Changes between Initial Version and Version 1 of OpenWrt/init

Timestamp:

10/22/2017 05:28:45 AM (7 years ago)

Author:

trac

Comment:

--

OpenWrt/init

@@ +1 @@
+[[PageOutline]]
+
+= 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:
+ * http://wiki.openwrt.org/doc/techref/process.boot
+ * http://wiki.openwrt.org/doc/techref/requirements.boot.process
+
+== 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.
+
+== Example ==
+
+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
+{{{
+S55gst-httpd-watchdog
+}}}
+
+Here is the actual code /etc/init.d/ryantest
+{{{
+root@OpenWrt:/etc/init.d# cat ryantest 
+#!/bin/sh /etc/rc.common
+
+NAME=gst-httpd-watchdog
+START=55
+STOP=55
+
+watchdog()
+{
+ps -aef | grep gst | grep -v grep
+
+while [ 1 ] 
+do
+
+proc=$(ps -aef | grep gst | grep -v grep )
+
+echo 'proc='$proc
+
+if [ -z "$proc" ]
+then
+{
+        echo 'gst process is not found, now restarting'
+        /etc/init.d/gst-httpd restart
+}
+else
+        echo 'gst process is running'
+fi
+sleep 10s;
+
+done
+
+}
+
+start()
+{
+( watchdog ) &
+}
+}}}