One script which generates fully functional live Linux ISO image with minimal effort. This is based on the first published version of Minimal Linux Live with some improvements taken from the next releases. All empty lines and comments have been removed and the script has been modified to reduce the overall length.
The script below uses Linux kernel 4.19.12, BusyBox 1.29.3 and Syslinux 6.03. The source bundles are downloaded and compiled automatically. If you are using Ubuntu or Linux Mint, you should be able to resolve all build dependencies by executing the following command:
sudo apt install wget make gawk gcc bc bison flex xorriso libelf-dev libssl-dev
After that simply run the below script. It doesn't require root privileges. In the end you should have a bootable ISO image named minimal_linux_live.iso in the same directory where you executed the script.
wget http://kernel.org/pub/linux/kernel/v4.x/linux-4.19.12.tar.xz
wget http://busybox.net/downloads/busybox-1.29.3.tar.bz2
wget http://kernel.org/pub/linux/utils/boot/syslinux/syslinux-6.03.tar.xz
mkdir isoimage
tar -xvf linux-4.19.12.tar.xz
tar -xvf busybox-1.29.3.tar.bz2
tar -xvf syslinux-6.03.tar.xz
cd busybox-1.29.3
make distclean defconfig
sed -i "s|.*CONFIG_STATIC.*|CONFIG_STATIC=y|" .config
make busybox install
cd _install
rm -f linuxrc
mkdir dev proc sys
echo '#!/bin/sh' > init
echo 'dmesg -n 1' >> init
echo 'mount -t devtmpfs none /dev' >> init
echo 'mount -t proc none /proc' >> init
echo 'mount -t sysfs none /sys' >> init
echo 'setsid cttyhack /bin/sh' >> init
chmod +x init
find . | cpio -R root:root -H newc -o | gzip > ../../isoimage/rootfs.gz
cd ../../linux-4.19.12
make mrproper defconfig bzImage
cp arch/x86/boot/bzImage ../isoimage/kernel.gz
cd ../isoimage
cp ../syslinux-6.03/bios/core/isolinux.bin .
cp ../syslinux-6.03/bios/com32/elflink/ldlinux/ldlinux.c32 .
echo 'default kernel.gz initrd=rootfs.gz' > ./isolinux.cfg
xorriso \
-as mkisofs \
-o ../minimal_linux_live.iso \
-b isolinux.bin \
-c boot.cat \
-no-emul-boot \
-boot-load-size 4 \
-boot-info-table \
./
cd ..
Note that this script produces very small live Linux OS with working shell only and no network support. The network functionality has been implemented properly in the Minimal Linux Live project which is extensively documented and more feature rich, yet still produces very small live Linux ISO image.