Shellscript to build the most minimalistic linux live CD which just starts /sbin/init from the CD root directory and outputs "Hello World!"

my_hello_world_distro.sh

#!/bin/bash

# Vorhandensein der Programme prüfen
MKISOFS=( $(which genisoimage mkisofs) )

if ! [ -x "$MKISOFS" ]; then echo "genisoimage aka mkisofs is missing"; exit 1; fi
if ! [ -x "$(which gcc)" ]; then echo "gcc is missing"; exit 1; fi
if ! [ -x "$(which nasm)" ]; then echo "nasm is missing"; exit 1; fi
if ! [ -x "$(which cpio)" ]; then echo "cpio is missing"; exit 1; fi
if ! [ -x "$(which tar)" ]; then echo "tar is missing"; exit 1; fi

set -xe # Befehle ausgeben und bei Fehler abbrechen

# Label der ISO zum finden des rootfs
LABEL="Hello World Distribution"
KERNEL="https://cdn.kernel.org/pub/linux/kernel/v4.x/linux-4.14.15.tar.xz"

# Wichtigste Verzeichnisse anlegen
mkdir my_hello_world_distro
cd my_hello_world_distro
mkdir src initramfs bin/{,musl} rootfs/{,boot,sbin}

pushd src

# Kernel erstellen
wget "$KERNEL"
tar -xf "$(basename $KERNEL)"
pushd "$(basename $KERNEL .tar.xz)"
make olddefconfig # Kernel Config vom Hostsystem übernehmen
# make localyesconfig # Minimale Kernel Config für diesen PC
make -j 9
# make INSTALL_PATH=../../rootfs/boot/ install # This line is commented because it calls /sbin/installkernel, which may do unwanted things
cp "$(make image_name)" ../../rootfs/boot/vmlinuz
make INSTALL_MOD_PATH=../../rootfs/ modules_install
popd

# Musl libc builden
git clone git://git.musl-libc.org/musl
pushd musl
./configure --prefix="$PWD/../../bin/musl"
make -j 9
make install
popd

# Das init/Hello World Program erstellen
mkdir init
pushd init
cat > init.c <<EOF
#include <stdio.h>
#include <unistd.h>

int main(){
  sleep(1);
  printf("\033[3J\033[H\033[2J"); // Clear screen

  printf("Hello World!\n"); // Print Hello World

  while(1) sleep(10); // Wait forever
}
EOF
../../bin/musl/bin/musl-gcc -static init.c -o ../../rootfs/sbin/init
popd

# initrd/initramfs erstellen (wird gebraucht um die CD zu suchen & module zu laden)
mkdir initramfs

# Kopieren aller Module, die zum Mounten von / benötigt werden könnten
MODDIR=( $(basename ../rootfs/lib/modules/*) )
cat > ../initramfs/module_list <<EOF
kernel/lib/crc16.ko
kernel/crypto/xor.ko
kernel/crypto/crc32_generic.ko
kernel/crypto/crc32.ko
kernel/fs/mbcache.ko
kernel/fs/jbd2/jbd2.ko
kernel/drivers/cdrom/cdrom.ko
kernel/drivers/scsi/scsi_mod.ko
kernel/drivers/ata/libata.ko
kernel/drivers/usb/common/usb-common.ko
kernel/drivers/usb/core/usbcore.ko
kernel/drivers/usb/misc/ldusb.ko
kernel/drivers/usb/misc/ezusb.ko
kernel/drivers/ata/libahci.ko
kernel/drivers/scsi/sr_mod.ko
kernel/drivers/scsi/sd_mod.ko
kernel/drivers/scsi/sg.ko
kernel/drivers/scsi/scsi_transport_iscsi.ko
kernel/drivers/scsi/libiscsi.ko
kernel/drivers/scsi/scsi_transport_sas.ko
kernel/drivers/scsi/libsas/libsas.ko
kernel/drivers/scsi/isci/isci.ko
kernel/drivers/mmc/core/mmc_core.ko
kernel/drivers/mmc/card/mmc_block.ko
kernel/drivers/mmc/host/sdhci.ko
kernel/drivers/mmc/host/sdhci-pci.ko
kernel/drivers/mmc/host/sdhci-acpi.ko
kernel/drivers/memstick/core/memstick.ko
kernel/drivers/mfd/mfd-core.ko
kernel/drivers/mfd/rtsx_usb.ko
kernel/drivers/memstick/host/rtsx_usb_ms.ko
kernel/drivers/mmc/host/rtsx_usb_sdmmc.ko
kernel/drivers/usb/storage/usb-storage.ko
kernel/drivers/usb/storage/ums-alauda.ko
kernel/drivers/usb/storage/ums-freecom.ko
kernel/drivers/usb/storage/ums-onetouch.ko
kernel/drivers/usb/storage/ums-usbat.ko
kernel/drivers/usb/storage/ums-cypress.ko
kernel/drivers/usb/storage/ums-isd200.ko
kernel/drivers/usb/storage/ums-realtek.ko
kernel/drivers/usb/storage/ums-datafab.ko
kernel/drivers/usb/storage/ums-jumpshot.ko
kernel/drivers/usb/storage/ums-sddr09.ko
kernel/drivers/usb/storage/ums-eneub6250.ko
kernel/drivers/usb/storage/ums-karma.ko
kernel/drivers/usb/storage/ums-sddr55.ko
kernel/drivers/usb/host/ohci-hcd.ko
kernel/drivers/usb/host/xhci-hcd.ko
kernel/drivers/usb/host/uhci-hcd.ko
kernel/drivers/usb/host/ehci-hcd.ko
kernel/drivers/usb/host/ohci-pci.ko
kernel/drivers/usb/host/ehci-pci.ko
kernel/drivers/ata/pata_atiixp.ko
kernel/drivers/ata/pata_rdc.ko
kernel/drivers/ata/sata_via.ko
kernel/drivers/ata/pata_ninja32.ko
kernel/drivers/ata/sata_svw.ko
kernel/drivers/ata/pata_ns87415.ko
kernel/drivers/ata/pata_oldpiix.ko
kernel/drivers/ata/pata_hpt366.ko
kernel/drivers/ata/ata_piix.ko
kernel/drivers/ata/sata_promise.ko
kernel/drivers/ata/pata_triflex.ko
kernel/drivers/ata/pata_cmd64x.ko
kernel/drivers/ata/sata_qstor.ko
kernel/drivers/ata/pata_sil680.ko
kernel/drivers/ata/sata_uli.ko
kernel/drivers/ata/pata_artop.ko
kernel/drivers/ata/sata_sil24.ko
kernel/drivers/ata/pata_pdc2027x.ko
kernel/drivers/pcmcia/pcmcia_core.ko
kernel/drivers/pcmcia/pcmcia.ko
kernel/drivers/ata/pata_pcmcia.ko
kernel/drivers/ata/pata_jmicron.ko
kernel/drivers/ata/sata_sil.ko
kernel/drivers/ata/pata_it821x.ko
kernel/drivers/ata/pata_sis.ko
kernel/drivers/ata/pata_ali.ko
kernel/drivers/ata/pata_efar.ko
kernel/drivers/ata/pata_ns87410.ko
kernel/drivers/ata/pata_netcell.ko
kernel/drivers/ata/pata_amd.ko
kernel/drivers/ata/sata_sx4.ko
kernel/drivers/ata/sata_nv.ko
kernel/drivers/ata/pdc_adma.ko
kernel/drivers/ata/acard-ahci.ko
kernel/drivers/ata/pata_via.ko
kernel/drivers/ata/pata_piccolo.ko
kernel/drivers/ata/pata_rz1000.ko
kernel/drivers/ata/pata_it8213.ko
kernel/drivers/ata/ata_generic.ko
kernel/drivers/ata/ahci.ko
kernel/drivers/ata/pata_serverworks.ko
kernel/drivers/ata/pata_marvell.ko
kernel/drivers/ata/pata_mpiix.ko
kernel/drivers/ata/sata_mv.ko
kernel/drivers/ata/pata_sch.ko
kernel/drivers/ata/pata_hpt37x.ko
kernel/drivers/ata/sata_vsc.ko
kernel/drivers/ata/pata_pdc202xx_old.ko
kernel/drivers/ata/pata_atp867x.ko
kernel/drivers/ata/sata_sis.ko
kernel/fs/ext4/ext4.ko
kernel/fs/isofs/isofs.ko
EOF
while read mod
do
  m="$MODDIR/$mod"
  if [ ! -f "../rootfs/lib/modules/$m" ]; then continue; fi
  mkdir -p $(dirname "../initramfs/lib/modules/$m")
  cp "../rootfs/lib/modules/$m" "../initramfs/lib/modules/$m"
done < ../initramfs/module_list

# Init Program für initramfs zum laden der Module, mounten des echten root FS & strten des eigentlichen init
pushd initramfs
cat > init.c <<EOF
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mount.h>
#include <sys/syscall.h>
#include <fcntl.h>
#include <stdbool.h>
#include <dirent.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

void exitfunc(void){
  sleep(10);
}

// search for a device containing an iso with the right label
char* find_root(void){
  char* root = 0;
  char label[33] = LABEL;
  label[32] = 0;

  puts("Searching root FS");

  int dev = open("/dev/",O_DIRECTORY);
  DIR* dir = fdopendir(dev);
  struct dirent* entry;
  while(entry=readdir(dir)){
    if( strncmp(entry->d_name,"sr",2)
     && strncmp(entry->d_name,"mmcblk",2)
     && strncmp(entry->d_name,"sd",2)
    ) continue;
    printf("Checking /dev/%s\n",entry->d_name);
    int fd = openat(dev,entry->d_name,O_RDONLY);
    if( fd == -1 )
      continue;
    if( lseek(fd, 33342, SEEK_SET) == -1 )
      goto next;
    char result[33] = {0};
    if( read(fd,result,strlen(label)) != strlen(label) )
      goto next;
    close(fd);
    if( !strcmp(label, result) ){
      root = malloc(5+strlen(entry->d_name)+1);
      memcpy(root,"/dev/",5);
      strcpy(root+5, entry->d_name);
      break;
    }
    continue;
   next:
    close(fd);
  }
  closedir(dir);

  return root;
}

// Modul laden
bool load_module(int fd, const char* name){
  int init_module(void *module_image, unsigned long len, const char *param_values);
  FILE* f = fdopen(fd, "r");
  if(!f){
    perror("load_module: fdopen failed");
    return false;
  }
  if( fseek(f, 0, SEEK_END) == -1 ){
    perror("load_module: fseek failed");
    goto failed_fdopen;
  }
  long l = ftell(f);
  if( l < 0 ){
    perror("load_module: ftell failed");
    goto failed_fdopen;
  }
  if( fseek(f, 0, SEEK_SET) == -1 ){
    perror("load_module: fseek failed");
    goto failed_fdopen;
  }
  char* buf = malloc(l);
  if(!buf){
    perror("load_module: malloc failed");
    goto failed_fdopen;
  }
  if( fread(buf,1,l,f) == -1 ){
    perror("load_module: fread failed");
    goto failed_malloc;
  }
  char* params = alloca(5+strlen(name)+1);
  strcpy(params,"name=");
  strcpy(params+5,name);
  if( init_module(buf,l,params) == -1 ){
    perror("load_module: init_module failed");
    goto failed_malloc;
  }
  free(buf);
  fclose(f);
  return true;
failed_malloc:
  free(buf);
failed_fdopen:
  fclose(f);
  return false;
}

int main(int argc, const char* argv[]){
  atexit(exitfunc);

  // Mount /dev
  if( mount("none","/dev","devtmpfs",0,"rw") == -1 ){
    perror("mounting devtmpfs failed\n");
    return 1;
  }

  int dir = open("/lib/modules/" MODDIR, O_DIRECTORY|O_RDONLY);
  FILE* list = fopen("/module_list","r");
  if( dir != -1 && list ){
    char buf[257];
    while(fgets(buf,256,list)){
      unsigned long l = strlen(buf);
      if(l && buf[l-1]=='\n') buf[l-1] = 0;
      const char* s = strrchr(buf,'/');
      if(!s) continue;
      int f = openat(dir,buf,O_RDONLY);
      if(f == -1) continue;
      printf("Loading module %s ",buf);
      fflush(stdout);
      bool res = load_module(f,s+1);
      puts(res ? "OK" : "");
      close(f);
    }
  }
  close(dir);
  fclose(list);

  char* root;
  for( int i=0; !(root=find_root()) && i<3; i++)
    sleep(i+1);
  if(!root){
    fprintf(stderr,"Root FS not found\n");
    return 2;
  }else
    printf("Root found at %s\n",root);

  // mount root & switch to real root & start real init
  if( mount(root,"/root/","iso9660",MS_RDONLY,"") == -1 ){
    perror("mount failed");
    return 3;
  }
  free(root);
  umount("/dev/");
  if( chroot("/root/") ){
    perror("chroot failed");
    return 4;
  }
  argv[0] = "/sbin/init";
  execv("/sbin/init",(void*)argv);
  perror("execv /sbin/init failed");

  return 5;
}
EOF
../../bin/musl/bin/musl-gcc -D "LABEL=\"$LABEL\"" -D "MODDIR=\"$MODDIR\"" -static init.c -o ../../initramfs/init
pushd ../../initramfs/
find . -print0 | cpio --null -ov --format=newc | gzip -9 > ../rootfs/boot/initramfs.cpio.gz
popd
popd

# Syslinux bootloader builden
wget https://www.kernel.org/pub/linux/utils/boot/syslinux/Testing/6.04/syslinux-6.04-pre1.tar.gz
tar -xf syslinux-6.04-pre1.tar.gz
pushd syslinux-6.04-pre1
make
popd
popd

# Syslinux dateien kopieren
mkdir rootfs/efi/{,boot}
mkdir rootfs/syslinux
cp src/syslinux-6.04-pre1/efi64/efi/syslinux.efi rootfs/efi/boot/bootx64.efi
cp src/syslinux-6.04-pre1/bios/com32/elflink/ldlinux/ldlinux.c32 rootfs/syslinux/
cp src/syslinux-6.04-pre1/bios/core/isolinux.bin rootfs/syslinux/
cat > rootfs/syslinux/syslinux.cfg <<EOF
default linux
prompt 0
timeout 0

label linux
  initrd /boot/initramfs.cpio.gz
  kernel /boot/vmlinuz quiet
EOF

# ISO erstellen
rm -r rootfs/lib/ # Nicht benötigte Module löschen
"$MKISOFS" -r -l -o hello_world_distro.iso -b syslinux/isolinux.bin -c syslinux/boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table -A "$LABEL" -input-charset utf-8 rootfs
./src/syslinux-6.04-pre1/bios/utils/isohybrid hello_world_distro.iso