Build a minimal Linux filesystem with BusyBox. To build a minimal bootable filesystem, the standard Linux filesystem directory architecture should be prepared at first.

Apart from the /bin, /sbin, and /usr directories that are created by default, the /dev, /proc, /sys, and /etc directories must be created manually:

$ cd ~/tools/busybox/ramdisk/
$ mkdir dev sys etc proc

/etc contains the configuration files for the Linux system. It includes two important files: inittab and fstab. inittab can be used by init to determine which programs should be run and fstab describes the filesystems mounting parameters. The init process is the first process invoked by the Linux kernel after the kernel finishes its initialization (to learn more about the generic boot sequence of a Linux system, read the manual page of the boot process by running the man boot command):

$ cat > ./etc/inittab
::sysinit:/etc/init.d/rcS
ttyS2::askfirst:/bin/sh
::restart:/sbin/init

$ cat > ./etc/fstab
proc /proc proc defaults 0 0
sysfs /sys sysfs defaults 0 0

$ mkdir ./etc/init.d
$ cat > ./etc/init.d/rcS
#!/bin/sh
mount -a
echo /sbin/mdev > /proc/sys/kernel/hotplug
mdev -s

$ chmod 777 ./etc/init.d/rcS
$ ln -s ./sbin/init init

With the previous setting, during the booting of our Linux system, the init process parses our inittab file and runs /etc/init.d/rcS as the first action. It mounts all filesystems specified in fstab, and starts the minimal mdev device management tool (like the more famous tool, udev) to create all device nodes exported by low-level drivers under /dev for late operations. It also creates a console with /bin/sh, and attaches it at ttyS2. Since /sbin/init is the first user process, the /etc/inittab also let it respawn for system availability.

Lastly, the console based on /bin/sh is provided as the basic shell environment for the user interaction.

Package the filesystem into an image and name it ramdisk.img.

Now, let's compress it with the following command:

$ cd ramdisk/
$ find . | cpio -H newc -o | gzip -9 > ../ramdisk.img

Cross-compile a new Android Linux kernel.

To start our own ramdisk, since it doesn't work with the prebuilt kernel image (zImage) in Android ADT (Android Development Toolkit), we should build our own Linux kernel image from the Android Linux kernel source code. Go to the just downloaded goldfish/ directory and compile it as follows:

$ cd ~/tools/busybox/goldfish/
$ git checkout android-goldfish-3.4
$ make goldfish_armv7_defconfig
$ export CROSS_COMPILE=arm-linux-gnueabi-
$ make zImage
$ cp arch/arm/boot/zImage ~/tools/busybox/

Run our new embedded system on a virtual Android device.

Now, start our own kernel image with the just-built minimal filesystem as follows:

$ emulator @busybox-avd -shell -show-kernel -no-window \
-kernel ./zImage -ramdisk ./ramdisk.img
Uncompressing Linux... done, booting the kernel.
...

Please press Enter to activate this console.

According to this message, we can press Enter to activate the console terminal and execute the commands that we want to run.