UnixPower - Help site and portal documenting all major Unix distributions.

Examware - Unix tutorials, exercises and resources. Features on-line tests and training materials.

First Edition Unix Manual - The original Unix users manual.

Online Single Unix Specification - Ordering page for the Open Group's Single Unix Specification.

pyttymon - A simple daemon that monitors and limits usage of ttys. It is implemented in Python. A simple config file can specify monthly, daily, session, and idle limits based on tty, user, group, and ttys in use.

Unix and Linux Commands - Basic as well as more obscure commands for Unix/Linux operating systems.

Unix Filesystem - Description of how the filesystem works internally.

Unix Manuals - Brief Unix tutorial, online man pages and reference materials for the Unix operating system and common Unix utilities.

Unix Seventh Edition Manual - Source material from Bell Labs.