Emacs ("Eight Megs And Counting Still") is a unix text editor you can invoke by running 'emacs' (sometimes a more friendly version is available under x-windows called 'xemacs'.) Here are some useful (or essential!) emacs keys to know:

A key like M-x is typed by pressing ESC then x.
A key C-s is just control-s.
A key like C-x C-f is typed by holding down control while you press x and f in sequence.
A key like C-x b is typed like control-x (release control) then b.

C-x C-f Open File Works on empty files to create a new file. Use tab to complete filenames (ie, type hel and hit tab to complete to hello.cc if there's a file called hello.cc)
C-x C-s Save File Saves the current buffer. C-x s (release control for the s) will save all open buffers, prompting you for each one.
C-/ Undo Multiple undo is availible, by repeatedly pressing C-/.
C-x C-c Exit Prompts to save then closes emacs.
C-k Cut line From cursor to the end of the line. Puts the cut text in the clipboard, which emacs perversely calls the "kill ring".
C-y Paste Pastes the most recent cut (or copy, etc.). If multiple lines were cut in one operation, they are all pasted. You are not limited to pasting the last cut/copy: After pasting use M-y to cycle through older copied text in the "kill ring", pasting it instead.
C-space Start marking text Starts marking text. After pressing C-space, move the cursor and use commands like C-w or M-w to operate on that region. In xemacs this will probably hilight the text; in regular emacs you might not be able to see anything, depending on your terminal type.
C-w Cut text Cuts the hilighted text into the "kill ring".
M-w Copy text Copy the hilighted text into the "kill ring" (does not remove it).
C-g Quit command Abort a command or operation. For instance, if emacs is prompting you for a file to open, and you don't want to open a file, press C-g.
C-s Incremental search (forwards) Allows you to search for text forwards or backwards from the cursor. To see the next match, press C-s or C-r again.
C-rIncremental search (backwards)
C-h Help Press C-h a few times for an overwhelming but accurate help system. Note that this is the same as the backspace key on some unenlightened terminals.
C-x 1 Maximize this window Use these to remove and move the cursor between split-windows that emacs will pop up (while in the help system, for instance).
C-x 0 Maximize other window
C-x o Move cursor to other window
C-x b Switch to buffer Switch between the various open buffers (by typing in the name of the file). C-x C-b gets you a list of available buffers.
C-v Pagedown 
M-v Pageup 

Emacs is most useful in cc-mode, which it will be in whenever you are editing a file ending with .c or .cc. Under this mode, there are a number of useful additional keys; you can see these by typing C-h m. The most useful are probably C-c C-c (which comments out the marked region) and tab, which properly indents the current line. C-c C-q will automatically indent the whole function under your cursor.

To facilitate debugging your programs, I recommend you add a goto-line keybinding. Add the following to your .emacs in your home directory (run emacs ~/.emacs to edit it):

(global-set-key "\M-g" 'goto-line)

This sets the key M-g (typed ESC-g) to the command goto-line. (By default goto-line is not bound to anything. You can run it by typing ESC-x goto-line (enter) line number (enter), but that gets tedious quickly.) Now when g++ gives you an error like:

foo.c: In function `main':
foo.c:42: invalid lvalue in assignment

You can go ESC g42 (enter) to go to line 42 of your source file and witness the error.

Carnage Melon
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