man (online command by command documentation)
Use the "man" (for "manual") command when you already know the name of
a command to view its online
documentation. For example
% man sort
will describe the Unix sorting command. This is especially
helpful when you've used the command before but can't
remember all the options and switches. Try this with
"sort" to see all the different kinds of sorting Unix
provides.
Unix History
Genetic Relationships among different "Unix-type" Systems
(from OpenSource.org)
An excellent online discussion
of the history of Unix (as well as ongoing lawsuits
respecting its lineage) can be found
here.