Executables vs. Built-ins
Most UNIX commands invoke utility programs that are stored as executable files in the directory hierarchy
Shells also contains several built-in commands, which it executes internally
Type man shell_builtins for a partial listing
Built-in commands execute as subroutines, and do not spawn a child-shell via fork()
- Expect built-in (e.g. cd) to be faster than external (e.g. ls)