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Busybox, appropriately dubbed "The Swiss Army Knife of Embedded Linux", is a utility that combines tiny versions of many common UNIX utilities into a single small executable.
emerge --ask sys-apps/busybox
You can find a handful of UNIX utilities inside busybox. To use one, execute busybox, followed by the utility name. For example, the following command will open up Busybox's instance of vi.
busybox vi may exist on a system where vi does not. Busybox vi is completely separate from the coreutils installed on the system, and is found in a single binary.