build.sh

#!/bin/sh

# Kernel, BusyBox, and musl Toolchain Versions and URLs
KERNEL_VERSION="5.4.295"
KERNEL_URL="https://github.com/danielsboringfloppy/linux-$KERNEL_VERSION.git"
TOYBOX_VERSION="0.8.13"
TOYBOX_URL="https://github.com/landley/toybox.git"
MUSL_TOOLCHAIN_URL="https://musl.cc/i486-linux-musl-cross.tgz"

# Directories
BUILD_ROOT_DIR=$(pwd)
CONFIG_DIR="./configs"
CONFIG_FILE="$CONFIG_DIR/linux.config"
BUILD_DIR="$BUILD_ROOT_DIR/build"
TOOLCHAIN_DIR="$BUILD_DIR/toolchain"
ROOTFS_DIR="$BUILD_DIR/rootfs"  # Root filesystem directory
FLOPPY_IMG="$BUILD_DIR/floppy.img"  # Virtual floppy disk image file
MNT_DIR="$BUILD_DIR/mnt"  # Temporary mount directory for the floppy
SYSLINUX_CFG="$MNT_DIR/syslinux.cfg"  # Syslinux configuration file path

set -xe

# Function to download and extract Linux kernel
download_kernel() {
    mkdir -p "$BUILD_DIR"
    cd "$BUILD_DIR" || exit 1
    cd linux-$KERNEL_VERSION || git clone $KERNEL_URL
}

# Function to download and extract BusyBox
download_toybox() {
    cd "$BUILD_DIR" || exit 1
    cd toybox || git clone $TOYBOX_URL -b $TOYBOX_VERSION
}

# Function to download and extract musl toolchain
download_toolchain() {
    mkdir -p "$TOOLCHAIN_DIR"
    cd "$TOOLCHAIN_DIR" || exit 1
    wget -c "$MUSL_TOOLCHAIN_URL" -O "i486-linux-musl-cross.tgz"
    tar xf "i486-linux-musl-cross.tgz"
}

# Function to copy kernel config
copy_config() {
    cp "$1" "$2"
}

# Function to build Linux kernel
build_kernel() {
    cd "$BUILD_DIR/linux-$KERNEL_VERSION" || exit 1
    make dbf_pentium_defconfig
    make -j"$(nproc)"
}

# Function to build BusyBox using musl toolchain
build_toybox() {
    cd "$BUILD_DIR/toybox" || exit 1

    # Set up the cross-compilation environment
    export CROSS_COMPILE="$TOOLCHAIN_DIR/i486-linux-musl-cross/bin/i486-linux-musl-"

    CROSS_COMPILE=$CROSS_COMPILE LDFLAGS="--static" make defconfig
    make -j"$(nproc)"
    PREFIX=$ROOTFS_DIR make install
}

# Function to create the root filesystem structure
setup_rootfs() {
    mkdir -p "$ROOTFS_DIR"

    cd "$ROOTFS_DIR" || exit 1

    mkdir -p bin dev etc sbin usr lib proc sys

    cd -
}

setup_init() {
    cp -a "$BUILD_ROOT_DIR/init/." "$ROOTFS_DIR/"
}

# Function to create CPIO archive of the root filesystem
create_cpio_archive() {
    cd "$ROOTFS_DIR" || exit 1
    find . | cpio -o -H newc | gzip > "$BUILD_DIR/rootfs.cpio.xz"
}

# Function to create and format a virtual floppy image in FAT32
create_floppy_image() {
    # Size of floppy image in bytes (1.44MB)
    FLOPPY_SIZE=$((1440 * 1024))

    # Create a blank floppy image
    dd if=/dev/zero of="$FLOPPY_IMG" bs=512 count=$((FLOPPY_SIZE / 512))

    mkfs.fat -F 12 "$FLOPPY_IMG"
}

# Function to install Syslinux bootloader on the floppy image
install_syslinux() {
    mkdir -p "$MNT_DIR"
    # Mount the floppy image
    sudo mount -o loop "$FLOPPY_IMG" "$MNT_DIR"

    # Create Syslinux configuration file
    sudo bash -c "cat > $SYSLINUX_CFG" <<EOF
	DEFAULT linux
	LABEL linux
  	KERNEL /bzImage
  	APPEND initrd=/rootfs.cpio.xz
EOF

    # Unmount the floppy image
    sudo umount "$MNT_DIR"
    sudo syslinux --install "$FLOPPY_IMG"
}

# Function to mount the floppy image and copy the kernel and rootfs to it
copy_files_to_floppy() {
    mkdir -p "$MNT_DIR"
    
    # Mount the floppy image
    sudo mount -o loop "$FLOPPY_IMG" "$MNT_DIR"

    sudo cp "$BUILD_DIR/linux-$KERNEL_VERSION/arch/x86/boot/bzImage" "$MNT_DIR/bzImage"
    sudo cp "$BUILD_DIR/rootfs.cpio.xz" "$MNT_DIR/rootfs.cpio.xz"

    # Unmount the floppy image
    sudo umount "$MNT_DIR"
}

# Main execution
download_toolchain
download_kernel
build_kernel

download_toybox
build_toybox

setup_rootfs
setup_init
create_cpio_archive

create_floppy_image
copy_files_to_floppy
install_syslinux