build-initramfs.sh

#!/usr/bin/env bash

set -e

mkdir -p initramfs{/,/bin,/dev,/etc,/proc,/sbin,/sys,/usr,/usr/bin,/usr/sbin}

pushd initramfs

if [[ ! -x sbin/busybox ]]; then
    wget -O sbin/busybox http://www.busybox.net/downloads/binaries/latest/busybox-x86-64
    chmod a+x sbin/busybox
fi

if [[ ! -x init ]]; then
    cat > init << EOF
#!/sbin/busybox sh

set -e

/sbin/busybox --install -s

mount -t proc proc /proc
mount -t sysfs sysfs /sys

mknod -m 666 /dev/null c 1 3
mknod -m 666 /dev/zero c 1 5
mknod -m 666 /dev/ptmx c 5 2
mknod -m 666 /dev/tty c 5 0
mknod -m 444 /dev/random c 1 8
mknod -m 444 /dev/urandom c 1 9
mknod -m 620 /dev/tty1 c 4 1
mknod -m 620 /dev/tty2 c 4 2
mknod -m 620 /dev/tty3 c 4 3
mknod -m 620 /dev/tty4 c 4 4
mknod -m 620 /dev/tty5 c 4 5
mknod -m 620 /dev/tty6 c 4 6
mknod -m 620 /dev/tty7 c 4 7
mknod -m 620 /dev/tty8 c 4 8

ip link set eth0 up
ip link set lo up

exec /sbin/init
EOF
    chmod a+x init
fi

find . | cpio -H newc -o > ../initramfs.cpio

popd

gzip - < initramfs.cpio > initramfs.cpio.gz