The UNIX Time-Sharing System

"Goals"

• a system that's fun to use for program development and research
• powerful yet small
• easy to write, test and run programs

Achievements

Simplicity (economy of features)

Full exploitation of small set of features

Important aspects

• Design not influenced by predefined objectives, very small team of designers
• Multitasking, multiuser operating system
• File system namespace hierarchy, uniformity, and ubiquity
• mapping of devices into filename space (Multics)
• same operations for devices as for files
• same namespace
• same protection mechanism
• Uninterpreted byte streams, applications have full control
• Command shell is an ordinary executable (no special privileges)
• Support for I/O redirection and unnamed pipes (Multics)
• Simple access control mechanism, set-uid, set-gid
• File system built around I-nodes
• Support for removable file systems
• File descriptors
• Portability (most of the system implemented in C)
• Class C security class (Multics has class B2)
• Small programs that do their job well. Complex tasks can be performed by combining these programs
• All UNIX software mantained under UNIX (self-supporting)
• No quota in the initial implementation

Questions

• mis-features in original UNIX?
• what happens if one deletes an open file?
• what do you think about their discussion of locking files?
• can you link across file systems?