[cesarb]

> mkdir(1) [...] it's so short that it fits entirely on one screen.

296 lines in the main .c file, without counting the headers and helper functions: https://git.savannah.gnu.org/cgit/coreutils.git/tree/src/mkd...

[carbocation]

And, entertainingly, it contains a FIXME! https://git.savannah.gnu.org/cgit/coreutils.git/tree/src/mkd...