EMAC OE Boot Process Customization

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. init is process id 1 and the parent of all processes in the system.

Runlevels

System V Init uses different runlevels to control the boot process. Each runlevel has 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.
id:5:initdefault:

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, such as to configure different levels of user functionality in each runlevel.

During boot, the scripts /etc/init.d/rcS and /etc/init.d/rc are executed to run the scripts in /etc/rcS.d/ starting with an 'S' in lexicographic order followed by the scripts in /etc/rc5.d/ (assuming that 5 is the default runlevel). 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 filename is prefixed with a number from 00-99. 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:

si::sysinit:/etc/init.d/rcS
....
l5:5:wait:/etc/init.d/rc 5
....

Initscripts

The directory /etc/init.d/ holds 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. The files in the /etc/rc*.d/ directories are 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

Use the other boot scripts on the system for 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.

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