µlfs: the simplest Linux From Scratch. Part of a series of posts: https://jlxip.net/blog

µlfs.sh

#!/bin/bash -eu
# µlfs

IMGSZ=128M
KERNEL_VARIANT=linux-virt
KERNEL_FILE=vmlinuz-virt

# ----- IMG FILE -----
# Generate img file
IMG=hdd.img
dd if=/dev/zero of=$IMG bs=$IMGSZ count=1

# Partition it
echo "n




a
w
" | fdisk $IMG

# Set up loop device
LOOP="$(sudo losetup --partscan --show --find $IMG)"

# Format partition and get UUID
PART="$LOOP"p1
sudo mkfs.ext4 $PART
UUID="$(sudo blkid -s UUID $PART | cut -d'"' -f2)"

# Mount partition
mkdir -p mnt/
sudo mount $PART mnt/

# Basic filesystem
sudo mkdir -p mnt/{proc,sys,run,dev,etc,root}



# ----- KERNEL -----
# Steal a kernel from Alpine Linux
ALPINE="https://dl-cdn.alpinelinux.org/alpine/latest-stable/main/x86_64/"
if [ ! -d linux ]; then	
	PKG="$(curl $ALPINE | grep -e "\"$KERNEL_VARIANT-[0-9]" | cut -d'"' -f2)"
	mkdir linux
	cd linux
	wget "$ALPINE/$PKG"
	tar xvfz $PKG
	cd ..
fi
sudo cp -ar linux/{boot,lib,usr} mnt/
KERNEL="$(ls linux/lib/modules)"



# ----- INIT -----
# Get hummingbird
if [ ! -d hummingbird ]; then
	git clone https://github.com/Sweets/hummingbird --depth 1
	cd hummingbird
	make CC="gcc -static" hummingbird
	cd ..
fi

# Install hummingbird
cd hummingbird
DESTDIR="`pwd`/../mnt" make install
cd ..



# ----- SHELL AND COREUTILS -----
# Get and install busybox
if [ ! -d busybox ]; then
	# This mirror is way faster than the official one
	git clone https://github.com/mirror/busybox --depth 1
	cd busybox
	make defconfig
	CONFIG_STATIC=y make -j`nproc`
	CONFIG_STATIC=y make install # Installs to _install/
	cd ..
fi
sudo cp -ar busybox/_install/* mnt/



# ----- INITRAMFS -----
mkdir -p initramfs/{bin,dev,etc,lib,lib64,mnt/root,proc,root,sbin,sys}
sudo cp -ar busybox/_install/* initramfs/
cp -ar linux/lib initramfs/

echo "#!/bin/busybox sh
echo Oh, hello

mount -t proc none /proc
mount -t sysfs none /sys

find /sys -name modalias | xargs sort -u | xargs -n 1 /bin/busybox modprobe
mount -t devtmpfs none /dev

mount -t ext4 UUID=$UUID /mnt/root

umount /dev
umount /sys
umount /proc

exec switch_root /mnt/root /usr/bin/hummingbird
" | sudo tee initramfs/init
sudo chmod +x initramfs/init

cd initramfs
find . -print0 | cpio --null --create --verbose --format=newc | gzip --best > ../mnt/boot/initramfs.cpio.gz
cd ..



# ----- PASSWD -----
echo "root:x:0:0:root:/root:/bin/sh" | sudo tee mnt/etc/passwd
echo "root:::0:::::" | sudo tee mnt/etc/shadow
sudo chmod 0644 mnt/etc/passwd
sudo chmod 0640 mnt/etc/shadow



# ----- BOOTLOADER -----
# Copy syslinux files
sudo mkdir -p mnt/boot/syslinux/
sudo cp -a /usr/lib/syslinux/bios/*.c32 mnt/boot/syslinux/

# Install
sudo extlinux --install mnt/boot/syslinux

# Config
echo "
DEFAULT µlfs
TIMEOUT 0
PROMPT 0

LABEL µlfs
	LINUX ../$KERNEL_FILE
	APPEND root=UUID=$UUID loglevel=3 rw
	INITRD ../initramfs.cpio.gz
" | sudo tee mnt/boot/syslinux/syslinux.cfg

# Unmount
sudo umount -R mnt/
sudo losetup -d $LOOP

# Write MBR
dd bs=440 conv=notrunc count=1 if=/usr/lib/syslinux/bios/mbr.bin of=hdd.img

# That's it! Good luck. Try it out with:
# qemu-system-x86_64 -hda hdd.img