From Rosetta Code

BusyBox "The Swiss Army Knife of Embedded Linux" is a multiuse-utility, designed for embedded Linux-systems:

BusyBox combines tiny versions of many common UNIX utilities into a single small executable.
It provides replacements for most of the utilities you usually find in GNU fileutils, shellutils, etc.

A working system may consist of just a Linux kernel, some device nodes in /dev, a few configuration files in /etc, BusyBox, and maybe a bootmanager.

For example, BusyBox is used in Tiny Core Linux.

BusyBox can provide most of the functionality of the many programs typical found in /bin, /sbin, /usr/bin, /usr/sbin, /usr/local/bin, all in a single binary, thus saving space on small systems.

As a shell, BusyBox provides ash.

As a calculator, BusyBox provides dc, a "Tiny RPN calculator".

As an editor, BusyBox provides sed and Vi.

BusyBox can be configured (at compile-time) to include as little or as much "applets" as needed/desired.

BusyBox can be compiled to include an AWK-implementation.