Here I post my init script. It is used in my initrd/initramfs boot images for the following purposes.
- Boot from a live CD / DVD
- Boot from a read-only filesystem image, compressed in SquashFS
- Copy the filesystem image to a RAM disk and run Linux entirely on memory
- Boot Linux from a USB flash drive
- Boot Linux from a local disk partition
- Run a rescue shell without booting Linux
I have yet to implement network booting. Right now it is good enough for my current needs. It takes the following boot parameters.
- root=
Specifies the root partition to boot Linux from. For example, /dev/sda2. - label=
Specifies the label of the device to boot from. For example, label=DEBIAN. - uuid=
Specifies the UUID of the device to boot from. For example, uuid=dcfd6a0a-2a0f-4b3d-8a1a-5e7d642ebfbd - boot=
Can be cdrom, loop, ram, usb or ata. - vmode=
Specifies the screen resolution of the framebuffer video. For example, vmode=640x480 - single
Boot into the single-user mode. - nox
Boot into the console mode in runlevel 2
#!/bin/dash
# Define a function to parse kernel command line options.
get_opt() {
echo $@ | cut -d "=" -f 2
}
# Define a function to load drivers.
loadmod() {
for i in $@ ; do
for j in $(grep $i /tmp/pcimodules.txt); do
modprobe $j
done
done
}
# Define a function to guess the partition type.
gpart() {
for i in $(blkid | grep $1); do
case $i in
*\=*)
eval $i
;;
*)
true
;;
esac
done
}
# Define a function for mounting the root partition.
mountr() {
if [ $uuid ]; then
if [ $# = 2 ]; then
mount -r -U $uuid $2
elif [ $# = 1 ]; then
mount -r -U $uuid $1
else mount -r -U $uuid /mnt
fi
elif [ $label ]; then
if [ $# = 2 ]; then
mount -r -L $label $2
elif [ $# = 1 ]; then
mount -r -L $label $1
else mount -r -L $label /mnt
fi
else
gpart $1
case $TYPE in
ext*)
e2fsck -p $1
[ $# = 2 ] && mount $1 $2 || mount $1 /mnt
;;
jfs)
jfs_fsck $1
if [ $# = 2 ]; then
mount -t jfs -o ro,iocharset=utf8 $1 $2
else mount -t jfs -o ro,iocharset=utf8 $1 /mnt
fi
;;
vfat)
if [ $# = 2 ]; then
mount -t vfat -o ro,gid=100,dmask=2,fmask=113 $1 $2
else mount -t vfat -o ro,gid=100,dmask=2,fmask=113 $1 /mnt
fi
;;
*)
[ $# = 2 ] && mount -r $1 $2 || mount -r $1 /mnt
;;
esac
fi
}
# Create a union filesystem
union() {
mount -t tmpfs none /opt/tmp
modinfo unionfs > /dev/null 2>&1 &&
mount -t unionfs -o dirs=/opt/tmp=rw:/opt=ro none /mnt ||
( mkdir /opt/tmp/.change
modprobe fuse
unionfs-fuse -o allow_other,use_ino,suid,dev,nonempty,kernel_cache \
-o cow,chroot=/opt,max_files=32768 /tmp/.change=RW:/=RO /mnt )
}
# Mount proc and sysfs.
mount -t proc none /proc
mount -t sysfs none /sys
# Find the available PCI hardware
mount -t tmpfs none /tmp
pcimodules > /tmp/pcimodules.txt
# Populate /dev (Needs kernel >= 2.6.32)
mount -t devtmpfs none /dev
mkdir -m 755 /dev/pts
mount -t devpts -o gid=5,mode=620 none /dev/pts
# Set default values
boot=ata
root=/dev/sda6
# Find the root=, label=, uuid= and boot= values on kernel command line.
for i in $(cat /proc/cmdline); do
case $i in
root\=*)
root=$(get_opt $i)
case $root in
/dev/cdr* | /dev/dvd* | /dev/sr* | /dev/scd*)
boot=cdrom
;;
0x200)
root=/dev/fd0
;;
esac
;;
label\=* | uuid\=* | boot\=* | vmode\=* )
eval $i
;;
single)
RUNLEVEL=single
;;
nox)
RUNLEVEL=2
;;
esac
done
# Activate framebuffer display devices.
if [ $vmode ]; then
if [ $boot = cdrom ]; then
modprobe uvesafb scroll=ywrap mode_option=$vmode-16
else for i in $(grep fb /tmp/pcimodules.txt); do
case $i in
atyfb)
modprobe $i mode=$vmode-16
;;
nvidiafb | rivafb)
modprobe nvidiafb mode_option=$vmode bpp=16 hwcur=1
;;
radeonfb | savagefb)
modprobe $i mode_option=$vmode-16
;;
sisfb)
modprobe $i mode=$vmodex16 mem=12288 font=SUN12x22
;;
viafb | vt8623fb)
modprobe viafb viafb_mode=$vmode viafb_bpp=16
;;
*)
modprobe $i
;;
esac
done
if grep -q i915 /tmp/pcimodules.txt; then true
else [ -c /dev/fb0 ] || modprobe uvesafb scroll=ywrap mode_option=$vmode-16
fi
fi
fi
case $boot in
cdrom)
# Boot Linux from a live CD.
loadmod ata_ ahci pdc_adma ^.hci-hcd
modprobe usb-storage &&
modprobe sr_mod &&
sleep 7
modprobe isofs
mount -t iso9660 /dev/sr0 /media
[ -d /media/isolinux -o -d /media/boot/isolinux ] ||
mount -t iso9660 /dev/sr1 /media
if [ -f /media/*.[Ss][Qq]* ]; then
SQF=$(ls -t /media/*.[Ss][Qq]* | head -n 1)
if [ $root = /dev/ram ]; then
echo "Please wait until the RAM disk is ready."
dd if=$SQF of=/dev/ram1 bs=2048 &&
mount -t squashfs /dev/ram1 /opt
else modprobe loop
mount -t squashfs -o loop $SQF /opt
fi
else
mount --move /media /opt
fi
union
;;
loop)
# Boot Linux from an image file.
loadmod ata_ ahci pdc_adma ^.hci-hcd
modprobe usb-storage &&
modprobe sd_mod &&
sleep 7
mountr $root /media
modprobe loop
if [ -f /media/*.[Ss][Qq]* ]; then
SQF=$(ls -t /media/*.[Ss][Qq]* | head -n 1)
mount -t squashfs -o loop $SQF /opt
elif [ -f /media/*.[Ii][Ss][Oo] ]; then
ISO=$(ls -t /media/*.[Ii][Ss][Oo] | head -n 1)
modprobe isofs
mount -t iso9660 -o loop $ISO /opt
fi
union
;;
ram)
# Boot Linux from ramdisk.
loadmod ata_ ahci pdc_adma ^.hci-hcd
modprobe usb-storage &&
modprobe sd_mod &&
sleep 7
mountr $root /media
echo "Please wait until the RAM disk is ready."
if [ -f /media/*.[Ss][Qq]* ]; then
SQF=$(ls -t /media/*.[Ss][Qq]* | head -n 1)
dd if=$SQF of=/dev/ram1 &&
mount -t squashfs /dev/ram1 /opt
elif [ -f /media/*.[Ii][Ss][Oo] ]; then
ISO=$(ls -t /media/*.[Ii][Ss][Oo] | head -n 1)
dd if=$ISO of=/dev/ram1 bs=2048 &&
modprobe isofs
mount -t iso9660 /dev/ram1 /opt
fi
union
;;
usb*)
# Boot Linux from a USB drive.
loadmod ^.hci-hcd
modprobe usb-storage &&
modprobe sd_mod &&
sleep 7
mountr $root
;;
ata*)
loadmod ata_ ahci pdc_adma &&
modprobe sd_mod &&
mountr $root
;;
esac
# Make sure that init exists and is executable.
if [ -x /mnt/sbin/init ]; then
mount --move /dev /mnt/dev
mount --move /proc /mnt/proc
mount --move /sys /mnt/sys
umount /tmp
# Start init from the root filesystem.
cd /mnt
[ -f /media/updates.zip ] && unzip -o /media/updates.zip
case $boot in
cdrom)
[ $root = /dev/ram ] && umount /media
[ $RUNLEVEL ] || RUNLEVEL=3
;;
loop | ram)
umount /media
[ $RUNLEVEL ] || RUNLEVEL=3
;;
*)
[ $RUNLEVEL ] || RUNLEVEL=5
;;
esac
[ -d initrd ] && pivot_root . initrd
exec chroot . /sbin/init $RUNLEVEL
fi
# Start a shell as a last resort.
echo "Error booting from the root filesystem. Starting a shell."
exec /bin/dash
The following are examples of boot parameters that can be used with my init script.
- Boot Linux from the local hard drive partition /dev/sda8
boot=ata root=/dev/sda8
- Boot Linux from the latest squashfs file (*.sq*) on /dev/sda1
boot=loop root=/dev/sda1
- Boot Linux from the CD-ROM with 1024x768 video resolution
boot=cdrom vmode=1024x768
- Copy the squashfs image from CD-ROM into memory and run Linux on memory
boot=cdrom root=/dev/ram ramdisk_size=573440 vmode=800x600
- Boot Linux from the second partition of a USB drive
boot=usb root=/dev/sda2
