What is BusyBox and why is it needed?

Hello folks,

today I want to explain how the Swiss knife of Linux command line tools works and why it is needed, especially in embedded systems.

On a normal Linux desktop system, tools like ls, cp, mount or sh are all separate binaries. Each of those binaries depends on libraries and contains its own startup code. This is usually not a problem on desktops or servers.

But in embedded systems, storage and memory are limited. Every single binary takes space, and every copy of common code is wasted space. Memory is rare in embedded devices.

This is where BusyBox comes in.

One single binary

BusyBox combines many common Unix command line tools into one single executable. Instead of having many binaries in /bin and /sbin, there is only one: the BusyBox executable.

If every tool were a separate binary, all that code would have to be stored again and again. BusyBox avoids that by sharing everything in one file.

This saves a lot of space, which is exactly what embedded systems need.

Symlinks instead of real binaries

In a BusyBox-based system, the usual command names still exist:

  • /bin/ls

  • /bin/cp

  • /bin/sh

  • /sbin/mount

But these files are not real binaries. They are symbolic links pointing to the BusyBox executable.

Example:

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/bin/ls   -> busybox
/bin/cp   -> busybox
/bin/sh   -> busybox

So when you type ls, you are actually starting BusyBox.

How BusyBox knows what to do

When a program is started on Linux, it receives its command line arguments in argv[]. The first element, argv[0], contains the name that was used to start the program.

BusyBox looks at argv[0] to find out which command it should behave like.

In simple terms, it does the following:

  • read argv[0]

  • remove the path, so /bin/ls becomes ls

  • search for ls in its internal list of commands

  • execute the matching implementation

This is why the same binary can act as ls, sh, mount, or many other tools.

Calling BusyBox directly

BusyBox can also be called without symlinks:

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busybox ls -l
busybox sh

In this case, the first argument tells BusyBox which command should be executed. Internally, the same dispatch logic is used.

Why this is still useful

BusyBox is not meant to replace the full GNU toolchain. Many features are missing or simplified. But for embedded systems, this is often good enough.

BusyBox is still widely used because:

  • it is small

  • it has very few dependencies

  • it works well in minimal systems

  • it is easy to understand and debug

Summary

BusyBox is simple and effective:

  • one executable

  • many commands

  • very small footprint

That is why BusyBox is still used today and why it is so important in embedded Linux systems.