Executing unix commands in ObjC
NSTask is your friend if speed isn't necessary. If it is, use the native system calls.
If you're only concerned about listing the contents of a directory, read the Guide about Low-Level File Management. Especially Listing the Contents of a Directory could be interesting.$
If that still isn't fast enough, use the C API. See this question: How do you get a directory listing in C.
As far as I'm aware, Objective C is built on C so you should have access to all the standard UNIXy capabilities, among them:
system().fork().exec()family.popen().
Don't forget about posix_spawn(), for when you need to exert dictatorial-level control over your sub-processes.
Of course, if you're just looking to do file system management and introspection from Cocoa proper, look no further than NSFileManager.