* Why directory like that?
	- for organization
	- for protection
	- directory cannot be written by unprivileged program --> system
	control contents of directory

* Why fork() and exec()?
	- easier to implement shell, setting environment, ...

* protection domain: user id (coarse grained)	

* set uid

	

Take away:
Why Unix is so popular?
- simple
- easy to use
	+ for programmer, easy to write, run, test program 
		> interactive system rather than batch
		> sys call is simple
- open, hence easy to review, fix and run again

List all simple/easy to use stuff in UNIX:
- . and .. (constraint on dir hierarchy)
- file and IO almost the same:
	+ access IO like a file
	+ protection
	+ from programmer stand point, no distinction among between 
	random/sequential access
- no user visible locks in file system
- separate fork and exec, why? for flexible
after fork, can setup any appropriate environment variable to execute
(hence can easy redirect the output in shell: a.out > file)