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.
Understanding Unix Concepts: Conceptual discussion of Unix and Unix-like operating systems.
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 for Advanced Users: Description of startup/shutdown, process management and memory management.
Unix for Beginners: Brian Kernighan's 1979 classic introduction to the facilities Unix offers, from the 7th Edition manual.
Unix Guru Universe: Searchable collection of articles and guides for Unix administrators and power-users. Offers materials on all major and many minor distributions.
Unix Help Files: Searchable database from U-Texas.
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.
UNIX Signals and Process Groups: A short survey of UNIX signal handling and process groups, both conceptually and with interface descriptions. Compares System V Release 3, System V Release 4, BSD, and POSIX variants.
Unix Sysadmin Guide: Description of Unix internals.