2.3. Busybox

2.3.1. References

2.3.2. Goals

  • Build a simple root file system with a minimal Busybox configuration

2.3.3. Steps

1. Download the Busybox sources:

user@host: cd ${DL_PATH}
user@host: wget https://busybox.net/downloads/busybox-1.30.1.tar.bz2

2. Extract the Busybox sources:

user@host: tar -xvf busybox-1.30.1.tar.bz2
user@host: cd busybox-1.30.1/

3. Set the empty config and enter the configuration:

user@host: make allnoconfig
user@host: make menuconfig

4. Enable the option in the configuration to build statically, symbol CONFIG_STATIC:

--> Busybox Settings
        --> Build Options
                --> Build Busybox as a static binary (no shared libs)
  1. Select ash, vi and all coreutils
  2. Select the following settings:
CONFIG_SHOW_USAGE=y
CONFIG_FEATURE_VERBOSE_USAGE=y
CONFIG_FEATURE_COMPRESS_USAGE=y
CONFIG_BUSYBOX=y
CONFIG_FEATURE_INSTALLER=y
CONFIG_PLATFORM_LINUX=y
CONFIG_INSTALL_APPLET_SYMLINKS=y

7. Build and inspect:

user@host: make -j 9
user@host: file busybox

8. Install Busybox tree:

user@host: make CONFIG_PREFIX=../bb_install install

2.3.4. Assignments

  • Create a cpio archive containing the Busybox install tree
  • Run the cpio archive as initramfs
  • Show a directory listing of the root file system in run time

2.3.5. Questions

  • Create a file /root/hello containing “hello world”, reboot, what happens to the file? Why?
  • What directories are missing in the root file system?

2.3.6. Busybox init System

  • Busybox provides a simple init system equivalent to System V init
  • It does not offer a lot of features, like runlevels, but it is a very simple implementation
  • /sbin/init The init binary (or soft link)
  • /etc/inittab Jobs description file for the init system
user@host: cat bb_example/etc/inittab
# /etc/inittab
::sysinit:/etc/init.d/rcS
ttyS0::respawn:/sbin/getty -L ttyS0 0 vt100
  • /etc/init.d/rcS Init script to kickstart all other startup scripts in this directory.
user@host: cat bb_example/etc/init.d/rcS
#!/bin/sh

mount -a
  • etc/fstab Description of administered system mount points; all mounted during init.
user@host: cat bb_example/etc/fstab
# /etc/fstab
#device         mount-point  type      options           dump  fsck order
/dev/mmcblk0p1  /boot        ext4      rw,noauto         0     1
proc            /proc        proc      defaults          0     0
sysfs           /sys         sysfs     defaults          0     0
devtmpfs        /dev         devtmpfs  mode=0755,nosuid  0     0
tmpfs           /tmp         tmpfs     defaults          0     0
tmpfs           /run         tmpfs     defaults          0     0

2.3.7. Assignments

  • Enable the standard configuration for Busybox (make defconfig)
  • Create the necessary files to start a shell at the console
  • Create or adapt a simple init script that runs a hello-world binary

2.3.8. Questions

  • What starts the tty on the serial console?
  • Which other init systems exist? Which ones are suitable for embedded systems and why?

2.3.9. Adding User Files

user@host: cat bb_example/etc/passwd
root:x:0:0::/root:/bin/sh

user@host: cat bb_example/etc/shadow
root::0::::::