The shell runs a program by asking init to load it (init alone may
create processes), then gives the program the terminal as its output
and a read-only storage grant to the current directory, writes its
arguments onto a page it moves into the program, starts it and waits
for it to die. Then it takes the grant back.

Built-ins: help, echo, clear, cd, pwd. Anything else is a program on
disk: `name args...` runs name.elf.