Single executable that includes all POSIX command line utilities, including sh, and no libc dependency.
The logo "The Swiss Army Knife of Embedded Linux" does it great justice: it is small and has many utilities, just like a swiss knife.
Popular for minimal distributions, e.g. https://github.com/ivandavidov/minimal
Quickstart:
git clone git://git.busybox.net/busybox
make defconfig
make
make install
This generates a single busybox 2Mb executable. It is common to configure it to embed libc so it will work in systems without it. TODO how
It is also possible to configure exactly which utilities will be present on the output.
You can use it either with subcommands:
./busybox echo a
or with symlinks, which is the standard approach when deploying it:
ln -s busybox echo
./echo a
busybox uses argv[0] to decide the executable in this case.
make install generates an _install directory with all utilities symlinked to busybox.
BusyBox also implements a version init, which is capable of reading /etc/inittab to startup the system.
Even though Ubuntu 14.04 does not have it installed by default, its boot system adds it to the /boot/initrd* files. Poetically speaking, the initial boot environment has characteristics of an embedded system.
It is then the job of init to mount
BusyBox uses the kernel's Kconfig mechanism.
In particular, it is affected by variables like KBUILD_OUTPUT just like the kernel, and has identical targets like defconfig and mrproper.
Unlike the kernel, the KBUILD_OUTPUT directory must exist before running make... Why?
Most commonly populated by auto-mounted devtmpfs with kernel config:
CONFIG_DEVTMPFS=y
CONFIG_DEVTMPFS_MOUNT=y
Can also be repopulated with:
mdev -s
In host, try it out:
rm /dev/zero
ls /dev/zero
mdev -s
ls /dev/zero
TODO: when / how is this called at boot?