Covers basic unix user commands, shell programming, and beyond.
Read as needed, but required is the "Introduction to Linux" in (2.) below.

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(1.) Unix for Windows (Cygwin)
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A Linux environment for Windows (XP, Vista, etc.).
An intro for installing and so forth:
http://www.physionet.org/physiotools/cygwin/
Binaries, source, installation instructions, documentation:
http://www.cygwin.com/

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(2.) Unix and Shells, Short intros
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A really short, really basic unix intro:
http://www.clockwatchers.com/linux_intro.html

Shortest possible intro to shell commands:
http://resist.ca/shell

Two short, and good, general intros to unix for beginners:
http://www.ee.surrey.ac.uk/Teaching/Unix/
http://www.ctssn.com/

Another very short intro to shell commands:
(from DOS perspective?)
http://leaf.sourceforge.net/doc/guide/bk07ch03.html

A faster paced unix intro, covers more ground, 
hits certain tools (tar, vi, csh, sh, C, pointers):
http://heather.cs.ucdavis.edu/~matloff/unix.html

"C-Shell programming"
This is a very short see-and-do sort of intro to the C-shell.
Good for picking up a few basics.
http://www.geog.ucl.ac.uk/~plewis/teaching/unix/csh.html.

A longer tcsh programming tutorial:
http://www.ms.unimelb.edu.au/~jrlooker/dloads/advunix.pdf

A one page, Linux quick reference (.pdf)
http://homepage.powerup.com.au/~squadron/

LINUX set of commands, very handy:
http://physics.usask.ca/~angie/ep414/linux_quick_ref.html

Bash by Example, an sh-like shell (as opposed to csh):
http://www-128.ibm.com/developerworks/library/l-bash2.html

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(3.) Unix, Shells: Long Intros
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"Introduction to Linux" by Machtelt Garrels:

This is required reading. Gives a general overview of the Linux system 
as well as some of the environment and tools. Scan through this, 
skipping sections that are not of immediate interest. You will 
find lots of tidbits relating to shells and utilities that will 
give you hints about what exists and where to look when you 
want more information. Very handy are several tables on differences 
between the various shells (sh, bash, csh, kcsh, ...) (also see the appendix).
    http://tldp.org/LDP/intro-linux/html/index.html (1-piece document)
    http://www.tldp.org/LDP/intro-linux/html/index.html (linked document)

"Using CSH and TCSH"
This site provides links to other materials:
    http://www.kitebird.com/csh-tcsh-book/
--see, in particular, the first 15 or so pages of
Bill Joy's "An Introduction to the C Shell".

"Bash Guide for Beginners", by Machtelt Garrels:
While this is bash-specific, all topics are relevant to tcsh, 
but w/ syntax differences. Includes brief intros for grep, sed, and awk. 
Also covers the differences between various shells. Again, scan 
through the document to see what's around so you can
go back and look it up. A very good intro document. There is also a followup
article on advanced Bash.

    http://www.tldp.org/LDP/Bash-Beginners-Guide/html/Bash-Beginners-Guide.html.
    http://www.tldp.org/LDP/Bash-Beginners-Guide/html/index.html (linked document.)

Some specific topics from the above:
Regular Expressions: 
    http://www.tldp.org/LDP/Bash-Beginners-Guide/html/chap_04.html
(see also man pages for regexp, etc.)
SED:
    http://www.tldp.org/LDP/Bash-Beginners-Guide/html/chap_05.html
AWK:
    http://www.tldp.org/LDP/Bash-Beginners-Guide/html/chap_06.html
Common Shell Features:
    http://www.tldp.org/LDP/Bash-Beginners-Guide/html/x6345.html
Unique Shell Features:
    http://www.tldp.org/LDP/Bash-Beginners-Guide/html/x6474.html

A DOS-to-Linux primer, for Gnu-linux, (extensive, w/ good refs)
(has a good, but short, DOS shell command summary w/ Linux equivalents
and lots of DOS equivalents throughout its somewhat long Linux intro. Shell is bash.)
    http://www.ibiblio.org/pub/Linux/docs/HOWTO/DOS-Win-to-Linux-HOWTO

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(4.) Linux, Unix Programming: References and other sources
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Linux.org documentation:
    http://www.linux.org/docs/
Linux Documentation Project:
    http://www.tldp.org/tldp-redirect.php?url=/

Guides and HowTos:
    http://www.tldp.org/guides.html

Big site, long turtorials, VI, REGEX, EMACS, AWK:
    http://www.unix-manuals.com/