Busybox aims to be the smallest and simplest correct implementation of the standard Linux command line tools. First and foremost, this means the smallest executable size we can manage. We also want to have the simplest and cleanest implementation we can manage, be standards compliant, minimize run-time memory usage (heap and stack), run fast, and take over the world.
Busybox is like a swiss army knife: one thing with many functions. The busybox executable can act like many different programs depending on the name used to invoke it. Normal practice is to create a bunch of symlinks pointing to the busybox binary, each of which triggers a different busybox function. (See getting started in the FAQ for more information on usage, and the busybox documentation for a list of symlink names and what they do.)
The "one binary to rule them all" approach is primarily for size reasons: a single multi-purpose executable is smaller then many small files could be. This way busybox only has one set of ELF headers, it can easily share code between different apps even when statically linked, it has better packing efficiency by avoding gaps between files or compression dictionary resets, and so on.
Work is underway on new options such as "make standalone" to build separate binaries for each applet, and a "libbb.so" to make the busybox common code available as a shared library. Neither is ready yet at the time of this writing.
The directory "applets" contains the busybox startup code (applets.c and busybox.c), and several subdirectories containing the code for the individual applets.
Busybox execution starts with the main() function in applets/busybox.c, which sets the global variable bb_applet_name to argv[0] and calls run_applet_by_name() in applets/applets.c. That uses the applets[] array (defined in include/busybox.h and filled out in include/applets.h) to transfer control to the appropriate APPLET_main() function (such as cat_main() or sed_main()). The individual applet takes it from there.
This is why calling busybox under a different name triggers different functionality: main() looks up argv[0] in applets[] to get a function pointer to APPLET_main().
Busybox applets may also be invoked through the multiplexor applet "busybox" (see busybox_main() in applets/busybox.c), and through the standalone shell (grep for STANDALONE_SHELL in applets/shell/*.c). See getting started in the FAQ for more information on these alternate usage mechanisms, which are just different ways to reach the relevant APPLET_main() function.
The applet subdirectories (archival, console-tools, coreutils, debianutils, e2fsprogs, editors, findutils, init, loginutils, miscutils, modutils, networking, procps, shell, sysklogd, and util-linux) correspond to the configuration sub-menus in menuconfig. Each subdirectory contains the code to implement the applets in that sub-menu, as well as a Config.in file defining that configuration sub-menu (with dependencies and help text for each applet), and the makefile segment (Makefile.in) for that subdirectory.
The run-time --help is stored in usage_messages[], which is initialized at the start of applets/applets.c and gets its help text from usage.h. During the build this help text is also used to generate the BusyBox documentation (in html, txt, and man page formats) in the docs directory. See adding an applet to busybox for more information.
Most non-setup code shared between busybox applets lives in the libbb directory. It's a mess that evolved over the years without much auditing or cleanup. For anybody looking for a great project to break into busybox development with, documenting libbb would be both incredibly useful and good experience.
Common themes in libbb include allocation functions that test for failure and abort the program with an error message so the caller doesn't have to test the return value (xmalloc(), xstrdup(), etc), wrapped versions of open(), close(), read(), and write() that test for their own failures and/or retry automatically, linked list management functions (llist.c), command line argument parsing (getopt_ulflags.c), and a whole lot more.
To add a new applet to busybox, first pick a name for the applet and a corresponding CONFIG_NAME. Then do this:
The standard we're paying attention to is the "Shell and Utilities" portion of the Open Group Base Standards (also known as the Single Unix Specification version 3 or SUSv3). Note that paying attention isn't necessarily the same thing as following it.
SUSv3 doesn't even mention things like init, mount, tar, or losetup, nor commonly used options like echo's '-e' and '-n', or sed's '-i'. Busybox is driven by what real users actually need, not the fact the standard believes we should implement ed or sccs. For size reasons, we're unlikely to include much internationalization support beyond UTF-8, and on top of all that, our configuration menu lets developers chop out features to produce smaller but very non-standard utilities.
Also, Busybox is aimed primarily at Linux. Unix standards are interesting because Linux tries to adhere to them, but portability to dozens of platforms is only interesting in terms of offering a restricted feature set that works everywhere, not growing dozens of platform-specific extensions. Busybox should be portable to all hardware platforms Linux supports, and any other similar operating systems that are easy to do and won't require much maintenance.
In practice, standards compliance tends to be a clean-up step once an applet is otherwise finished. When polishing and testing a busybox applet, we ensure we have at least the option of full standards compliance, or else document where we (intentionally) fall short.