Linuxrc (/linuxrc, also common name /init) on desktop OSes is on initramfs(ramdisk). Usualy this in script that probed modules, creates temporary device nodes in /dev, waits and mount rootfs, switches to real root. If initramfs is not used it may by symlinked to init.
Systemd uses udev to create /dev/ tree so it non needed.
Desktop linux uses it in initramfs to mount root. If rootfs mounted directly it can run from footfs.
Example /init extracted from router.
#!/bin/sh
#
# crucial mountpoints
mount -t proc none /proc
mount -t sysfs none /sys
mount -n tmpfs /var -t tmpfs -o size=17825792
mount -t tmpfs dev /dev
mknod /dev/console c 5 1
mknod /dev/ttyS0 c 4 64
mknod /dev/ttyS1 c 4 65
# setup console, consider using ptmx?
CIN=/dev/console
COUT=/dev/console
exec <$CIN &>$COUT
mknod /dev/null c 1 3
mknod /dev/gpio c 127 0
mknod /dev/zero c 1 5
mknod /dev/tty c 5 0
mknod /dev/tty0 c 4 0
mknod /dev/tty1 c 4 1
mknod /dev/random c 1 8
mknod /dev/urandom c 1 9
mknod /dev/ptmx c 5 2
mknod /dev/mem c 1 1
mknod /dev/watchdog c 10 130
mknod /dev/mtdblock0 b 31 0
mknod /dev/mtdblock1 b 31 1
mknod /dev/mtdblock2 b 31 2
mknod /dev/mtdblock3 b 31 3
mknod /dev/mtdblock4 b 31 4
mknod /dev/mtdblock5 b 31 5
mknod /dev/mtdblock6 b 31 6
mknod /dev/mtdblock7 b 31 7
mknod /dev/mtd0 c 90 0
mknod /dev/mtd1 c 90 2
mknod /dev/mtd2 c 90 4
mknod /dev/mtd3 c 90 6
mknod /dev/mtd4 c 90 8
mknod /dev/mtd5 c 90 10
mknod /dev/mtd6 c 90 12
mknod /dev/mtd7 c 90 14
mknod /dev/ttyUSB0 c 188 0
mknod /dev/ttyUSB1 c 188 1
mknod /dev/ttyUSB2 c 188 2
mknod /dev/ttyUSB3 c 188 3
mknod /dev/ttyUSB4 c 188 4
mknod /dev/ttyUSB5 c 188 5
mknod /dev/ttyUSB6 c 188 6
mknod /dev/ppp c 108 0
mknod /dev/i2c-0 c 89 0
mknod /dev/i2c-1 c 89 1
mknod /dev/i2c-2 c 89 2
mknod /dev/i2c-3 c 89 3
#
# Create the ubnt-poll-host char dev entries
# The major number is 189
#
# NOTE: wifiN's minor number = N
#
mknod /dev/uph_wifi0 c 189 0
mkdir /dev/pts /dev/shm
# rest of the mounts
mount none /dev/pts -t devpts
if [ -e /proc/bus/usb ]; then
mount none /proc/bus/usb -t usbfs
fi
echo "...mounts done"
mkdir -p /var/run /var/tmp /var/log /var/etc /var/etc/persistent /var/lock
echo "...filesystem init done"
# insert hal module
[ ! -f /lib/modules/*/ubnthal.ko ] || insmod /lib/modules/*/ubnthal.ko
# making sure that critical files are in place
mkdir -p /etc/rc.d /etc/init.d
# forced update
for f in inittab rc.d/rc.sysinit rc.d/rc rc.d/rc.stop ppp; do
cp -f -r /usr/etc/$f /etc/$f
done
echo "...base ok"
mkdir -p /etc/udhcpc
# do not update if exist
for f in passwd group login.defs profile hosts host.conf \
fstab udhcpc/udhcpc startup.list; do
if [ -e /etc/$f ]; then
echo -n '.'
else
cp -f /usr/etc/$f /etc/$f
fi
done
echo "...update ok"
mkdir -p /etc/sysinit
# make symlinks if do not exist
for f in services protocols shells mime.types ethertypes modules.d; do
if [ -e /etc/$f ]; then
echo -n '.'
else
ln -s /usr/etc/$f /etc/$f
fi
done
echo "...symlinks ok"
mkdir -p /etc/httpd
# check if we have uploaded certificates
for f in server.crt server.key; do
if [ -e /etc/persistent/https/$f ]; then
ln -s /etc/persistent/https/$f /etc/httpd/$f
else
ln -s /usr/etc/$f /etc/httpd/$f
fi
done
echo "...httpd ok"
CFG_SYSTEM="/tmp/system.cfg"
CFG_RUNNING="/tmp/running.cfg"
CFG_DEFAULT="/etc/default.cfg"
#Starting watchdog manager
/bin/watchdog -t 1 /dev/watchdog
# board data symlinks + default config
if [ -e /sbin/ubntconf ]; then
/sbin/ubntconf -i $CFG_DEFAULT
echo "ubntconf returned $?" >> /tmp/ubntconf.log
echo "...detect ok"
fi
# System configuration
mkdir -p /etc/sysinit/
/sbin/cfgmtd -r -p /etc/ -f $CFG_RUNNING
if [ $? -ne 0 ]; then
/sbin/cfgmtd -r -p /etc/ -t 2 -f $CFG_RUNNING
if [ $? -ne 0 ]; then
cp $CFG_DEFAULT $CFG_RUNNING
fi
fi
sort $CFG_RUNNING | tr -d "\r" > $CFG_SYSTEM
cp $CFG_SYSTEM $CFG_RUNNING
# But for DFS testing, it's useful to be able to overide this
if [ -f /etc/persistent/enable_printk ]; then
echo 9 > /proc/sys/kernel/printk
dmesg -c
else
# Do not clutter serial port, normally
echo 1 > /proc/sys/kernel/printk
fi;
# Set device date to firmware build date
BDATE=201509091722
if [ ! -z $BDATE ]; then
date -s $BDATE >/dev/null 2>&1
fi
# Run configuration parser
if [ -e /sbin/ubntconf ]; then
/sbin/ubntconf
echo "ubntconf returned $?" >> /tmp/ubntconf.log
fi
echo "...running /sbin/init"
exec /sbin/init
echo "INTERNAL ERROR!!! Cannot run /sbin/init."
