What is BusyBox?

# What is BusyBox? Source: [https://specular.fi/post/what-is-busybox](https://specular.fi/post/what-is-busybox) before you even know, this moment will be a memoryI've often stumbled upon[BusyBox](https://busybox.net/), but only after noticing Alpine, by default, has wget installed did I start to investigate where it comes from\. Except the wget version was "BusyBox Wget"\. But what's that? ``` docker run --rm -it alpine sh / which wget /usr/bin/wget / ls -lah /usr/bin/wget lrwxrwxrwx 1 root root 12 Apr 15 04:51 /usr/bin/wget -> /bin/busybox ``` After running`ls \-la /usr/bin`, my mind was blown: nearly 130\+ executables are from a single binary\! That explains the name "multicall binary"\. But\.\.\. how? How does BusyBox work? How does a single binary know which executable to run? Well, I guessed it right: ``` applet_name = argv[0]; if (applet_name[0] == '-') applet_name++; applet_name = bb_basename(applet_name); ``` So, an explicit invocation would also work: ``` / busybox ls -1 bin dev etc home / busybox meheh meheh: applet not found ``` So, to run wget, we find it by name and run it: ``` int applet = find_applet_by_name(name); // ... run_applet_no_and_exit(applet, name, argv); // ... xfunc_error_retval = applet_main[applet_no](argc, argv); ``` Each applet has its own C file, wget being`wget\.c`\. Each applet also appears to have some sort of "config" defined in code comments: ``` //config:config WGET //config: bool "wget (41 kb)" //config: default y //config: help //config: wget is a utility for non-interactive download of files from HTTP //config: and FTP servers. //applet:IF_WGET(APPLET(wget, BB_DIR_USR_BIN, BB_SUID_DROP)) //kbuild:lib-$(CONFIG_WGET) += wget.o ``` And eventually, we invoke the wget main: ``` int wget_main(int argc UNUSED_PARAM, char **argv) ``` Other interesting bits: BusyBox also supports hard links\. ``` busybox --install -s ``` `\-s`creates symlinks\. You can also list what commands it has compiled in: ``` / busybox --list | wc -l 304 ``` So, Alpine is more like an interface to BusyBox\-based binaries\. Each binary seems to be a bit stripped\-down version of the actual full\-blown one\. I'd still be interested to know if these are reimplementations or just the original source code of the utility shrunk down\.