One doesn't memorize a dictionary in order to increase one's vocabulary; one reads books, instead. At least,
that's a better way of doing it. Similarly, if you want to learn Unix commands you should learn them in context.
Possible places to start:
1. Introduction to Linux
. See also other documentation on the tldp
3. Official documentation for your distro, e.g. Here
, for Red Hat.
4. The FreeBSD Handbook
is excellent, even for Linux users.