EMAC OE Boot Process Customization

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TODO: {{#todo:Complete (10.31.13-11:02->MW+)(10.31.13-20:35->MD+)(11.06.13-12:17->JG+)|Michael Welling|oe 4,oe 5,mw,md,Complete}}

When designing an embedded system, it is often necessary to add or remove applications and tasks from the system initialization sequence. This guide describes the initialization method used for EMAC OE systems and provides information on customizing the boot process.

System V Initialization

EMAC OE uses the System V Initialization method. This is a simple method for system initialization using a set of scripts run in sequential order. When the kernel has finished loading, /sbin/init is started to initialize the userspace services in the system.


System V Init uses different runlevels to control the boot process. Each runlevel is associated with a set of scripts that are run sequentially to start various services on the system. The default runlevel on EMAC OE systems is 5. This is set in /etc/inittab with the line:

 # The default runlevel.

There are a total of seven runlevels available in System V Init, labeled 0 through 6. Runlevel 5 is the full user level in EMAC OE systems, regardless of whether a GUI is installed on the board or not. Runlevel 0 halts the system, and runlevel 6 is used for reboot. The other runlevels can be used for other purposes if desired configuring different levels of user functionality in each runlevel.

During boot, the scripts /etc/init.d/rcS and /etc/init.d/rc are executed to launch the scripts in /etc/rcS.d/ followed by the scripts in /etc/rc5.d/ (assuming that 5 is the default runlevel). The /etc/rcS.d/ and /etc/rc5.d/ directories contain a set of symbolic links starting with 'S'. These links are used to run scripts stored in /etc/init.d/ in a prescribed order. All of these startup scripts are passed the argument start. During halt or reboot, the scripts in /etc/rc0.d/ or /etc/rc6.d/ starting with a 'K' are run in lexicographic order with the argument stop.

To control the order in which the scripts are run, each symbolic link contains a number 00-99 immediately after the 'S' or 'K'. Symbolic links with the same numeric prefix will be run in alphanumeric order. For example, the listing below illustrates the files in /etc/rcS.d/ that will be run in order before entering the default runlevel:

root@emac-oe:/etc/rcS.d# ls
S02banner                S20modutils.sh           S40networking
S03sysfs                 S30ramdisk               S41ifplugd
S03udev                  S35mountall.sh           S45mountnfs.sh
S05devices               S37populate-volatile.sh  S55bootmisc.sh
S06alignment             S38devpts.sh             S98ipkg-configure
S10checkroot             S39hostname.sh           S99finish.sh

The action to perform at each level is specified in /etc/inittab. For example, the following lines are used to trigger the execution of the /etc/init.d/rcS and /etc/init.d/rc scripts:

 l5:5:wait:/etc/init.d/rc 5


The directory /etc/init.d/ holds all of the initialization scripts that are run by init during boot or shutdown. These scripts should be designed to accept at least three arguments: start, stop, or restart. As described in the previous section, /etc/rc*.d/ directories contain symbolic links to the scripts in /etc/init.d/. This structure allows for easy modification of the boot process and the ability for a script to be run at different places in different runlevels. The detailed listing of the /etc/rcS.d/ directory is shown below:

root@emac-oe:/etc/rcS.d# ls -l
lrwxrwxrwx    1 root     root           16 Dec 31  1969 S02banner -> ../init.d/banner
lrwxrwxrwx    1 root     root           18 Dec 31  1969 S03sysfs -> ../init.d/sysfs.sh
lrwxrwxrwx    1 root     root           14 Dec 31  1969 S03udev -> ../init.d/udev
lrwxrwxrwx    1 root     root           17 Dec 31  1969 S05devices -> ../init.d/devices
lrwxrwxrwx    1 root     root           22 Dec 31  1969 S06alignment -> ../init.d/alignment.sh
lrwxrwxrwx    1 root     root           19 Dec 31  1969 S10checkroot -> ../init.d/checkroot
lrwxrwxrwx    1 root     root           21 Dec 31  1969 S20modutils.sh -> ../init.d/modutils.sh
lrwxrwxrwx    1 root     root           17 Dec 31  1969 S30ramdisk -> ../init.d/ramdisk
lrwxrwxrwx    1 root     root           21 Dec 31  1969 S35mountall.sh -> ../init.d/mountall.sh
lrwxrwxrwx    1 root     root           30 Dec 31  1969 S37populate-volatile.sh -> ../init.d/populate-volatile.sh
lrwxrwxrwx    1 root     root           19 Dec 31  1969 S38devpts.sh -> ../init.d/devpts.sh
lrwxrwxrwx    1 root     root           21 Dec 31  1969 S39hostname.sh -> ../init.d/hostname.sh
lrwxrwxrwx    1 root     root           20 Dec 31  1969 S40networking -> ../init.d/networking
lrwxrwxrwx    1 root     root           17 Dec 31  1969 S41ifplugd -> ../init.d/ifplugd
lrwxrwxrwx    1 root     root           21 Dec 31  1969 S45mountnfs.sh -> ../init.d/mountnfs.sh
lrwxrwxrwx    1 root     root           21 Dec 31  1969 S55bootmisc.sh -> ../init.d/bootmisc.sh
lrwxrwxrwx    1 root     root           24 Dec 31  1969 S98ipkg-configure -> ../init.d/ipkg-configure
lrwxrwxrwx    1 root     root           19 Dec 31  1969 S99finish.sh -> ../init.d/finish.sh

The other boot scripts on the system can be used as examples when creating custom initscripts. The application that is being started should be stored in the system PATH, such as /usr/bin/, and started from the script. For example, the busybox-httpd initscript is shown below.

DESC="Busybox HTTP Daemon"
test -f $DAEMON || exit 0
set -e
case "$1" in
        echo -n "starting $DESC: $NAME... "
        if [ ! -d $HTTPROOT ]; then
                echo "$HTTPROOT is missing."
                exit 1
        start-stop-daemon -S -b -n $NAME -a $DAEMON -- $ARGS
        echo "done."
        echo -n "stopping $DESC: $NAME... "
        start-stop-daemon -K -n $NAME
        echo "done."
        echo "restarting $DESC: $NAME... "
        $0 stop
        $0 start
        echo "done."
        echo -n "reloading $DESC: $NAME... "
        killall -HUP $(basename ${DAEMON})
        echo "done."
        echo "Usage: $0 {start|stop|restart|reload}"
        exit 1
exit 0

Notice the different actions for each argument in the case statement. The start and stop blocks will be run directly during the boot and shutdown processes based on the location of the 'S' and 'K' links to this script. The restart and reload options are additional controls that are provided by many init scripts to make it easier to manually restart a process on a running system. For example, to restart the busybox-httpd server, you would run the command:

root@emac-oe:~# /etc/init.d/busybox-httpd restart

This would effectively stop the server and then start it again. This is generally useful for forcing reconfiguration after configuration file changes are made without needing to reboot the system.

The start-stop-daemon can be used to control the initialization and termination of the application as illustrated in the busybox-httpd initscript above. The available options for the busybox version of the start-stop-daemon included with EMAC OE are shown below.

BusyBox v1.13.2 (2009-06-24 18:00:48 CDT) multi-call binary

Usage: start-stop-daemon [OPTIONS] [-S|-K] ... [-- arguments...]

Search for matching processes, and then
-K: stop all matching processes.
-S: start a process unless a matching process is found.

Process matching:
       -u,--user USERNAME|UID  Match only this user's processes
       -n,--name NAME          Match processes with NAME
                               in comm field in /proc/PID/stat
       -x,--exec EXECUTABLE    Match processes with this command
                               in /proc/PID/cmdline
       -p,--pidfile FILE       Match a process with PID from the file
       All specified conditions must match
-S only:
       -x,--exec EXECUTABLE    Program to run
       -a,--startas NAME       Zeroth argument
       -b,--background         Background
       -N,--nicelevel N        Change nice level
       -c,--chuid USER[:[GRP]] Change to user/group
       -m,--make-pidfile       Write PID to the pidfile specified by -p
-K only:
       -s,--signal SIG         Signal to send
       -t,--test               Match only, exit with 0 if a process is found
       -o,--oknodo             Exit with status 0 if nothing is done
       -v,--verbose            Verbose
       -q,--quiet              Quiet

Adding and Removing Scripts

A new or existing initscript can be added or removed from the start process by simply creating or destroying the symbolic links from the /etc/rc*.d/ directories. A utility application, update-rc.d , is provided to automate this process. The usage of update-rc.d is shown below:

usage: update-rc.d [-n] [-f] [-r <root>] <basename> remove
       update-rc.d [-n] [-r <root>] [-s] <basename> defaults [NN | sNN kNN]
       update-rc.d [-n] [-r <root>] [-s] <basename> start|stop NN runlvl [runlvl] [...] .
                -n: not really
                -f: force
                -r: alternate root path (default is /)
                -s: invoke start methods if appropriate to current runlevel

The update-rc.d utility can be used to add or remove the startup links for any initscript in the system. For example, the /etc/init.d/busybox-httpd script has the following links to it in the boot process:


Removing the links from the system startup without actually removing the /etc/init.d/busybox-httpd file will effectively disable the daemon from running at system startup or shutdown. To accomplish this, use the remove argument to update-rc.d as illustrated below:

root@emac-oe:~# update-rc.d -f busybox-httpd remove

update-rc.d: /etc/init.d/busybox-httpd exists during rc.d purge (continuing)
 Removing any system startup links for busybox-httpd ...

To add the links for the /etc/init.d/busybox-httpd script back into the system, the defaults argument may be used. Alternatively, the start and stop number for each runlevel may be specified explicitly as shown in the example below. Both methods will yield exactly the same results in this case; the runlevels are specified in this example simply to demonstrate the syntax. Using this method is required when it is important that a script starts at a specific point in the init process such as directly after networking is started.

root@emac-oe:~# update-rc.d busybox-httpd start 20 2 3 4 5 . stop 20 0 1 6 .

 Adding system startup for /etc/init.d/busybox-httpd ...
  /etc/rc2.d/S20busybox-httpd -> ../init.d/busybox-httpd
  /etc/rc3.d/S20busybox-httpd -> ../init.d/busybox-httpd
  /etc/rc4.d/S20busybox-httpd -> ../init.d/busybox-httpd
  /etc/rc5.d/S20busybox-httpd -> ../init.d/busybox-httpd
  /etc/rc0.d/K20busybox-httpd -> ../init.d/busybox-httpd
  /etc/rc1.d/K20busybox-httpd -> ../init.d/busybox-httpd
  /etc/rc6.d/K20busybox-httpd -> ../init.d/busybox-httpd

Temporarily Disabling Scripts

Sometimes it is necessary to disable an initscript temporarily during development. The easiest way to disable a script from running is moving it out of the /etc/init.d directory. This will disable the script from being run at any runlevel. All of the symbolic links will be retained but will have no effect until the script is moved back into /etc/init.d

For example to disable the launching of the Xserver during boot:

root@emac-oe:~# cd /etc/init.d
root@emac-oe:/etc/init.d# mkdir disabled
root@emac-oe:/etc/init.d# mv xserver-nodm disabled/

This will prevent any of the runlevels from launching the Xserver. To re-enable the script:

root@emac-oe:~# cd /etc/init.d
root@emac-oe:/etc/init.d# mv disabled/xserver-nodm .


System V Initialization is used on EMAC OE filesystems and can be easily modified to alter the boot process. Configuring the /etc/inittab file allows users to define the initscripts run at various runlevels. The system init and runlevels are associated with directories containing symbolic links to scripts in /etc/init.d. The names of the links determine the action and order in which the scripts are run. Users can write simple initscripts and store them in /etc/init.d/ to launch custom applications. These scripts can be linked to in various runlevels in order to start and stop the applications according to the requirements of the application.

For more information:
Wikipedia Init Page
The Linux Documentation Project

Custom Initialization

In some designs, the target application is so specific that only one main task will be performed. After a custom application has been developed and tested, it may be possible to simplify the boot process significantly by modifying the System V Init configuration. The end result is generally a much faster boot time. It is also possible to override init completely by changing the kernel boot argument value for init, but this method offers little benefit over the customized configuration described in this document.

A complete customization of the inittab file involves overriding the /etc/init.d/rc and /etc/init.d/rcS scripts altogether. A single script is specified for init to run during boot that performs the entire system initialization. The example /etc/inittab below will start the script /etc/init.d/custom and also start a serial console for login.


# Serial console on ttyS0
S:1:respawn:/sbin/getty 115200 ttyS0

Example Custom Scripts

The contents of the custom initialization script will vary depending on the system configuration and services required for the application.

Minimal Initialization

The following example is a simple custom initialization script that mounts the sysfs and procfs filesystems, configures a static networking interface, and starts a custom application.

# mount procfs and sysfs
if [ -e /proc ] && ! [ -e /proc/mounts ]; then
  mount -t proc proc /proc
if [ -e /sys ] && grep -q sysfs /proc/filesystems; then
  mount sysfs /sys -t sysfs

# configure networking
ifconfig lo up
ifconfig eth0 netmask up

# start custom application
/usr/bin/custom-application &

Advanced Initialization

The example provided in this section is designed to do a significant amount of initialization while providing a fast boot time. Where possible, tasks are performed in the background to allow the script to continue execution. Any command or function called with an & at the end of the line will be run in the background. After the primary system initialization is completed, a custom application is executed followed by some final initialization such as starting the SSH server.

#!/bin/busybox sh

    if [ -e /proc ] && ! [ -e /proc/mounts ]; then
      /bin/mount -t proc proc /proc
    if [ -e /sys ] && grep -q sysfs /proc/filesystems; then
      /bin/mount sysfs /sys -t sysfs

    if [ -e /proc/cpu/alignment ]; then
       echo "3" > /proc/cpu/alignment

    if [ -f /etc/hostname ]
        /bin/hostname -F /etc/hostname

    /etc/init.d/modutils.sh start
    /etc/init.d/sshd start
    /etc/init.d/syslog start

    /etc/init.d/networking start >/dev/null

    /bin/mount -a

    if [ -f /etc/volatile.cache ]
        busybox sh /etc/volatile.cache

    if [ -x "/sbin/sysctl" ] && [ -f "/etc/sysctl.conf" ]
        /sbin/sysctl -p /etc/sysctl.conf

network_init &
misc_init &

# Start custom app
/usr/bin/custom-application &

delayed_init >/dev/null