busybox and toybox
toybox's implementation of posix unix tools
https://github.com/landley/toybox/tree/409a8e093a4ea8e0892ab302b4fd433d08f435eb/toys/posix
busybox's implementation of coreutils
https://github.com/mirror/busybox/tree/master/coreutils
and don't forget other tools you use everyday
https://github.com/mirror