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9.2. The Emacs Editor

Text editors are among the most important applications in the Unix world. They are used so often that many people spend more time within an editor than anywhere else on their Unix system. The same holds true for Linux.

The choice of an editor can be a religious one. Many editors exist, but the Unix community has arranged itself into two major groups: the Emacs camp and the vi camp. Because of its somewhat nonintuitive user interface, many people (newcomers and seasoned users alike) prefer Emacs over vi. However, long-time users of vi (and single-finger typists) use it more efficiently than a more complex editor such as Emacs.

If vi is one end of the text-editor spectrum, Emacs is the other; they are widely different in their design and philosophy. Emacs is partly the brainchild of Richard Stallman, founder of the Free Software Foundation and author of much of the GNU software.

Emacs is a very large system with more features than any single Unix application to date (some people would even go so far as not to call it an editor but an "integrated environment"). It contains its own LISP language engine you can use to write extensions for the editor. (Many of the functions within Emacs are written in Emacs LISP). Emacs includes extensions for everything from compiling and debugging programs to reading and sending electronic mail to X Window System support and more. Emacs also includes its own online tutorial and documentation. The book Learning GNU Emacs by Debra Cameron, Bill Rosenblatt, and Eric Raymond, is a popular guide to the editor.

Most Linux distributions include two variants of Emacs. GNU Emacs is the original version, which is still being developed, but development seems to have slowed down. XEmacs is larger, slightly slower, but much more user-friendly and better integrated with the X Window System (even though you can also use it from the command line, despite its name). If you are not tight on memory and have a reasonably fast computer, we suggest using XEmacs. We will not cover the differences here, though; the discussion in this section applies to both.

9.2.1. Firing It Up

GNU Emacs is simply invoked as:

$ emacs options
likewise, XEmacs is invoked as
$ xemacs options
Most of the time, you don't need options. You can specify filenames on the command line, but it's more straightforward to read them in after starting the program.

In Emacs lingo, C-x means Ctrl-X, and M-p is equivalent to Alt-P. As you might guess, C-M-p means Ctrl-Alt-P.

Using these conventions, press C-x followed by C-f to read in a file or create a new one. The keystrokes display a prompt at the bottom of your screen showing your current working directory. You can create a buffer now to hold what will end up to be the content of a new file; let's call the file wibble.txt. We now see the following:

  


-----Emacs: wibble.txt           (Fundamental)----All-------------------

The mode line at the bottom indicates the name of the file as well as the type of buffer you're in (which here is Fundamental). Emacs supports many kinds of editing modes; Fundamental is the default for plain-text files, but other modes exist for editing C and TeX source, modifying directories, and so on. Each mode has certain key bindings and commands associated with it, as we'll see soon. Emacs typically determines the mode of the buffer based on the filename extension.

To the right of the buffer type is the word All, which means that you are currently looking at the entire file (which is empty). Typically, you will see a percentage, which represents how far into the file you are.

If you're running Emacs under the X Window System, a new window will be created for the editor with a menu bar at the top, scrollbars, and other goodies. In the section "Section 11.4.7, "Emacs"," in Chapter 11, "Customizing Your X Environment", we discuss Emacs's special features when used within X.

9.2.2. Simple Editing Commands

Emacs is more straightforward than vi when it comes to basic text editing. The arrow keys should move the cursor around the buffer; if they don't (in case Emacs is not configured for your terminal), use the keys C-p (previous line), C-n (next line), C-f (forward character), and C-b (backward character).

If you find using the Alt key uncomfortable, press Esc and then p. Pressing and releasing Esc is equivalent to holding down Alt.

Already we must take the first aside on our tour of Emacs. Literally every command and key within Emacs is customizable. That is, with a "default" Emacs configuration, C-p maps to the internal function previous-line, which moves the cursor (also called "point") to the previous line. However, you can easily rebind different keys to these functions, or write new functions and bind keys to them, and so forth. Unless otherwise stated, the keys we introduce here work for the default Emacs configuration. Later we'll show you how to customize the keys for your own use.

Back to editing: using the arrow keys or one of the equivalents moves the cursor around the current buffer. Just start typing text, and it is inserted at the current cursor location. Pressing the Backspace or Delete key should delete text at the cursor. If it doesn't, we'll show how to fix it in the section "Section 9.2.8, "Tailoring Emacs"" later in this chapter. Now begin to type:

The rain falls on the main plains in Spain.
  


--**-Emacs: wibble.txt       (Fundamental)----All-----------------------

The keys C-a and C-e move the cursor to the beginning and end of the current line, respectively. C-v moves forward a page; M-v moves back a page. There are many more basic editing commands, but we'll allow the Emacs online documentation (discussed shortly) to fill those in.

In order to get out of Emacs, use the command C-x C-c. This is the first of the extended commands we've seen; many Emacs commands require several keys. C-x alone is a "prefix" to other keys. In this case, pressing C-x followed by C-c quits Emacs, first asking for confirmation if you want to quit without saving changes to the buffer.

You can use C-x C-s to save the current file, and C-x C-f to "find" another file to edit. For example, typing C-x C-f presents you with a prompt such as:

Find file: /home/loomer/mdw/
where the current directory is displayed. After this, type the name of the file to find. Pressing the Tab key will do filename completion similar to that used in bash and tcsh. For example, entering:
Find file: /home/loomer/mdw/.bash
and pressing Tab opens another buffer, showing all possible completions, as so:
Possible completions are: 
.bash_profile                      .bashrc 


--**-Emacs: *Completions*     (Fundamental)----All----------------------

After you complete the filename, the *Completions* buffer goes away and the new file is displayed for editing. This is one example of how Emacs uses temporary buffers to present information.

Emacs allows you to use multiple buffers when editing text; each buffer may contain a different file you're editing. When you load a file with C-x C-f, a new buffer is created to edit the file, but the original buffer isn't deleted.

You can switch to another buffer using the C-x b command, which asks you for the name of the buffer (usually the name of the file within the buffer). For example, pressing C-x b presents the prompt:

Switch to buffer: (default wibble.txt)
The default buffer is the previous one visited. Press Enter to switch to the default buffer, or type another buffer name. Using C-x C-b will present a buffer list (in a buffer of its own), as so:
 MR Buffer        Size   Mode        File 
 -- ------        ----   ----        ---- 
.   wibble.txt     44    Fundamental /home/loomer/mdw/wibble.txt 
    .bashrc        1763  Fundamental /home/loomer/mdw/.bashrc 
    *scratch*      0     Lisp Interaction 
 *  *Buffer List*  265   Buffer Menu

Popping up the buffer menu splits the Emacs screen into two "windows," which you can switch between using C-x o. More than two concurrent windows are possible as well. In order to view just one window at a time, switch to the appropriate one and press C-x 1. This hides all of the other windows, but you can switch to them later using the C-x b command just described. Using C-x k actually deletes a buffer from Emacs's memory.

9.2.3. Tutorial and Online Help

Already Emacs looks a bit complex; that is simply because it's such a flexible system. Before we go any further, it is instructive to introduce Emacs's built-in online help and tutorial. This documentation has also been published in book form.

Using the C-h command gives you a list of help options on the last line of the display. Pressing C-h again describes what they are. In particular, C-h followed by t drops you into the Emacs tutorial. It should be self-explanatory, and an interactive tutorial about Emacs tells you more about the system than we can hope to cover here.

After going through the Emacs tutorial you should get accustomed to the Info system, where the rest of the Emacs documentation resides. C-h followed by i enters the Info reader. A mythical Info page might look like this:

File: intercal.info,  Node: Top,  Next: Instructions,  Up: (dir) 
 
   This file documents the Intercal interpreter for Linux. 
 
   * Menu: 
 
   * Instructions::        How to read this manual. 
   * Overview::            Preliminary information. 
   * Examples::            Example Intercal programs and bugs. 
   * Concept Index::       Index of concepts.

As you see, text is presented along with a menu to other "nodes." Pressing m and then entering a node name from the menu will allow you to read that note. You can read nodes sequentially by pressing the space bar, which jumps to the next node in the document (indicated by the information line at the top of the buffer). Here, the next node is Instructions, which is the first node in the menu.

Each node also has a link to the parent node (Up), which here is (dir), meaning the Info page directory. Pressing u takes you to the parent node. In addition, each node has a link to the previous node, if it exists (in this case, it does not). The p command moves to the previous node. The l command returns you to the node most recently visited.

Within the Info reader, pressing ? gives you a list of commands and pressing h presents you with a short tutorial on using the system. Since you're running Info within Emacs, you can use Emacs commands as well (such as C-x b to switch to another buffer).

If you think that the Info system is arcane and obsolete, please keep in my mind that it was designed to work on all kinds of systems, including those lacking graphics or powerful processing capabilities.

Other online help is available within Emacs. Pressing C-h C-h gives you a list of help options. One of these is C-h k, after which you press a key, and documentation about the function that is bound to that key appears.

9.2.4. Deleting, Copying, and Moving Text

There are various ways to move and duplicate blocks of text within Emacs. These methods involve use of the mark, which is simply a "remembered" cursor location you can set using various commands. The block of text between the current cursor location (point) and the mark is called the region.

The mark can be set using the key C-@ (or C-Space on most systems). Moving the cursor to a location and pressing C-@ sets the mark at that position. You can now move the cursor to another location within the document, and the region is defined as the text between mark and point.

Many Emacs commands operate on the region. The most important of these commands deal with deleting and yanking text. The command C-w deletes the current region and saves it in the kill ring. The kill ring is a list of text blocks that have been deleted. You can then paste (yank) the text at another location, using the C-y command. (Note that the semantics of the term yank differ between vi and Emacs. In vi, "yanking" text is equivalent to adding it to the undo register without deleting it, while in Emacs, "yank" means to paste text.) Using the kill ring, you can paste not only the most recently deleted block of text but also blocks of text that were deleted previously.

For example, type the following text into an Emacs buffer:

The rain falls on the main plains in Spain. 
Here is a line that we wish to move. 
She sells Bourne shells by the sea shore.
 


--**-Emacs: wibble.txt           (Fundamental)----All-------------------

Now, move the cursor to the beginning of the second line ("Here is a line…"), and set the mark with C-@. Move to the end of the line (with C-e), and delete the region, using C-w. The buffer should now look like the following:

The rain falls on the main plains in Spain. 
 
She sells Bourne shells by the sea shore.


--**-Emacs: wibble.txt           (Fundamental)----All-------------------

In order to yank the text just deleted, move the cursor to the end of the buffer, and press C-y. The line should be pasted at the new location:

The rain falls on the main plains in Spain. 

She sells Bourne shells by the sea shore.
Here is a line that we wish to move.  


--**-Emacs: wibble.txt           (Fundamental)----All-------------------

Pressing C-y repeatedly will insert the text multiple times.

You can copy text in a similar fashion. Using M-w instead of C-w will copy the region into the kill ring without deleting it. (Remember that M- means holding down the Alt key or pressing Esc before the w.)

Text that is deleted using other kill commands, such as C-k, is also added to the kill ring. This means that you don't need to set the mark and use C-w to move a block of text; any deletion command will do.

In order to recover previously deleted blocks of text (which are saved on the kill ring), use the command M-y after yanking with C-y. M-y replaces the yanked text with the previous block from the kill ring. Pressing M-y repeatedly cycles through the contents of the kill ring. This feature is useful if you wish to move or copy multiple blocks of text.

Emacs also provides a more general register mechanism, similar to that found in vi. Among other things, you can use this feature to save text you want to paste in later. A register has a one-character name; let's use a for this example:

  1. At the beginning of the text you want to save, set the mark by pressing the Control key and space bar together (or if that doesn't work, press C-@).

  2. Move point (the cursor) to the end of the region you want to save.

  3. Press C-x x followed by the name of the register (a in this case).

  4. When you want to paste the text somewhere, press C-x g followed by the name of the register, a.

9.2.5. Searching and Replacing

The most common way to search for a string within Emacs is to press C-s. This starts what is called an incremental search. You then start entering the characters you are looking for. Each time you press a character, Emacs searches forward for a string matching everything you've typed so far. If you make a mistake, just press the Delete key and continue typing the right characters. If the string cannot be found, Emacs beeps. If you find an occurrence, but you want to keep searching for another one, press C-s again.

You can also search backward this way using the C-r key. Several other types of searches exist, including a regular expression search that you can invoke by pressing M-C-s. This lets you search for something like jo.*n, which matches names like John, Joan, and Johann. (By default, searches are not case sensitive.)

To replace a string, enter M-%. You are prompted for the string that is currently in the buffer, and then the one you want to replace it with. Emacs displays each place in the buffer where the string is and asks you if you want to replace this occurrence. Press the space bar to replace the string, the Delete key to skip this string, or a period to stop the search.

If you know you want to replace all occurrences of a string that follow your current place in the buffer, without being queried for each one, enter M-x string. (The M-x key allows you to enter the name of an Emacs function and execute it, without use of a key binding. Many Emacs functions are available only via M-x, unless you bind them to keys yourself.) A regular expression can be replaced by entering M-x regexp.

9.2.7. Running Commands and Programming Within Emacs

Emacs provides interfaces for many programs, which you can run within an Emacs buffer. For example, Emacs modes exist for reading and sending electronic mail, reading Usenet news, compiling programs, and interacting with the shell. In this section, we'll introduce some of these features.

To send electronic mail from within Emacs, press C-x m. This opens up a buffer that allows you to compose and send an email message:

To: 
Subject: 
--text follows this line-- 
 



-----Emacs: *mail*               (Mail)----All--------------------------

Simply enter your message within this buffer and use C-c C-s to send it. You can also insert text from other buffers, extend the interface with your own Emacs LISP functions, and so on.

RMAIL is Emacs's interface for reading electronic mail. Many users prefer it to other mail readers, because Emacs can be used directly as the editor for both sending and reading mail. To start RMAIL, use the command M-x rmail.

When you run RMAIL, Emacs converts messages in your incoming mailbox to a special format it uses to keep track of messages, and mail will be moved to the file RMAIL in your home directory. Therefore, be careful! Your incoming mail file will be converted to RMAIL format, and to convert it back (in case you don't want to use RMAIL as your mail reader), you need to use the M-x unrmail function.

When you start RMAIL, a buffer is created, displaying the first message in your inbox. The n and p commands can be used to display the previous and next messages. (As with all Emacs modes, using C-h m gives you a summary of the available keys.) While you're viewing a message, the r key replies to that message, opening a mail buffer (as described earlier) with the header fields initialized. While in the mail buffer you can use C-c C-y to insert the original message.

Within RMAIL, the h command displays a summary of the messages in your mailbox:

  11  17-Feb                johnsonm@metalab  Re: Which release?
  12  25-Feb                             schar  Vision Group meeting 
  13  26-Feb                 okir@monad.swb.de  Re: Spaces in .ms? 
  14  26-Feb           wirzeniu@cc.helsinki.fi  Re: LDP
--%%-Emacs: RMAIL-summary    (RMAIL Summary)----50%---------------------

Several M-x commands are available in summary mode for sorting the list, and so forth. RMAIL commands such as n, p, and r may be used as well.

Similar to the RMAIL mail interface is GNUS, the Emacs-based newsreader, which you can start with the M-x gnus command. After startup (and a bit of chewing on your .newsrc file), a list of newsgroups will be presented, along with a count of unread articles for each:

     10: comp.os.linux.development 
      0: cucs.system 
     32: alt.fan.warlord 
    195: alt.folklore.urban 
--- GNUS: List of Newsgroups    (Newsgroup {cloyd.cs})--6%--------------

Using the arrow keys, you can select a newsgroup to read. Press the space bar to begin reading articles from that group. Two buffers will be displayed, one containing a list of articles and the other displaying the current article. Using n and p moves to the next and previous articles, f and F follows up to the current article (either with or without including the current article), and r and R replies to the article via electronic mail. There are many other GNUS commands; use C-h m for a list. If you're used to a newsreader such as rn, GNUS will be somewhat familiar.

Emacs provides a number of modes for editing various types of files. For example, there is C mode for editing C source code, and TeX mode for editing (surprise) TeX source. Each of these modes boast features that make editing the appropriate type of file easier.

For example, within C mode, you can use the command M-x compile, which, by default, runs make -k in the current directory and redirect errors to another buffer. For example, the compilation buffer may contain:

cd /home/loomer/mdw/pgmseq/ 
make -k 
gcc -O -O2 -I. -I../include -c stream_load.c -o stream_load.o 
stream_load.c:217: syntax error before `struct' 
stream_load.c:217: parse error before `struct'
You can move the cursor to a line containing an error message, press C-c C-c, and the cursor will jump to that line in the corresponding source buffer (opening a buffer for the appropriate source file if one does not already exist). Now you can edit and compile programs entirely within Emacs.

Emacs also provides a complete interface to the gdb debugger, which is described in the section "Section 14.1.6.3, "Using Emacs with gdb"" in Chapter 14, "Tools for Programmers".

Usually, Emacs selects the appropriate mode for the buffer based on the filename extension. For example, editing a file with the extension .c in the filename automatically selects C mode for that buffer.

Shell mode is one of the most popular Emacs extensions. Shell mode allows you to interact with the shell in an Emacs buffer, using the command M-x shell. Editing shell command lines and using command history can be done with standard Emacs commands. You can also run single shell commands from Emacs using M-!. If you use M-| instead, the contents of the current region is piped to the given shell command as standard input. This is a general interface for running subprograms from within Emacs.

9.2.8. Tailoring Emacs

The Emacs online documentation should be sufficient to get you on track to learning more about the system and growing accustomed to it. However, sometimes it is hard to locate some of the most helpful hints for getting started. Here we'll present a run-down on certain customization options many Emacs users choose to employ to make life easier.

The Emacs personal customization file is .emacs, which should reside in your home directory. This file should contain code, written in Emacs LISP, which runs or defines functions to customize your Emacs environment. (If you've never written LISP before, don't worry; most customizations using it are quite simple.)

One of the most common things users customize are key bindings. For instance, if you use Emacs to read your mail, you can bind the key C-c r to execute the rmail function. Put this in your .emacs file:

; Provide quick way to read mail. 
(global-set-key "\C-cr" 'rmail)

Comments in Emacs LISP start with a semicolon. The command that follows runs the command global-set-key. Now you don't have to type in the long sequence M-x rmail every time you get that little message "Mail" in your mode line. Just press the two characters C-c r. This works anywhere in Emacs--no matter what mode your buffer is in--because it is global.

A customization that you might want to use is making the text mode the default mode and turning on the "auto-fill" minor mode (which makes text automatically wrap if it is too long for one line) like this:

; Make text mode the default, with auto-fill
(setq default-major-mode 'text-mode)
(add-hook 'text-mode-hook 'turn-on-auto-fill)

You don't always want your key mappings to be global. As you use TeX mode, C mode, and other modes defined by Emacs, you'll find useful things you'd like to do only in a single mode. Here, we define a simple LISP function to insert some characters into C code, and then bind the function to a key for our convenience:

(defun start-if-block() 
  (interactive) 
  (insert "if () {\n}\n") 
  (backward-char 6) 
)

We start the function by declaring it "interactive" so that we can invoke it (otherwise, it would be used only internally by other functions). Then we use the insert function to put the following characters into our C buffer:

if () { 
}

Strings in Emacs can contain standard C escape characters. Here we've used \n for a newline.

Now we have a template for an if block. To put on the ribbon and the bow, our function also moves backward six characters so that point is within the parentheses, and we can immediately start typing an expression.

Our whole goal was to make it easy to insert these characters, so now let's bind our function to a key:

(define-key c-mode-map "\C-ci" 'start-if-block)

The define-key function binds a key to a function. By specifying c-mode-map, we indicate that the key works only in C mode. There is also a tex-mode-map for TeX mode, a lisp-mode-map that you will want to know about if you play with your .emacs file a lot, and so on.

If you're interested in writing your own Emacs LISP functions, you should read the Info pages for elisp, which should be available on your system.

Now here's an important customization you may need. On many terminals the Backspace key sends the character C-h, which is the Emacs help key. To fix this, you should change the internal table Emacs uses to interpret keys as follows:

(keyboard-translate ?\C-h ?\C-?)
Pretty cryptic code. \C-h is recognizable as the Control key pressed with h, which happens to produce the same ASCII code (8) as the Backspace key. \C-? represents the Delete key (ASCII code 127). Don't confuse this question mark with the question marks that precede each backslash. ?\C-h means "the ASCII code corresponding to \C-h." You could just as well specify 8 directly.

So now, both Backspace and C-h will delete. You've lost your help key. Therefore, another good customization would be to bind another key to C-h. Let's use C-\, which isn't used often for anything else. You have to double the backslash when you specify it as a key:

(keyboard-translate ?\C-\\ ?\C-h)

On the X Window System, there is a way to change the code sent by your Backspace key using the xmodmap command, but we'll have to leave it up to you to do your own research. It is not a completely portable solution (so we can't show you an example guaranteed to work), and it may be too sweeping for your taste (it also changes the meaning of the Backspace key in your xterm shell and everywhere else).

There are other key bindings you may want to use. For example, you may prefer to use the keys C-f and C-b to scroll forward (or backward) a page at a time, as in vi. In your .emacs file you might include the following lines:

(global-set-key "\C-f" 'scroll-up) 
(global-set-key "\C-b" 'scroll-down)
   Again, we have to issue a caveat: be careful not to redefine keys that have other important uses. (One way to find out is to use C-h k to tell you what a key does in the current mode. You should also consider that the key may have definitions in other modes.) In particular, you'll lose access to a lot of functions if you rebind the prefix keys that start commands, such as C-x and C-c.

You can create your own prefix keys, if you really want to extend your current mode with lots of new commands. Use something like:

(global-unset-key "\C-d") 
(global-set-key "\C-d\C-f" 'my-function)
First, we must unbind the C-d key (which simply deletes the character under the cursor), in order to use it as a prefix for other keys. Now, pressing C-d C-f will execute my-function.

You may also prefer to use another mode besides Fundamental or Text for editing "vanilla" files. Indented Text mode, for example, automatically indents lines of text relative to the previous line (as with the :set ai function in vi). To turn on this mode by default, use:

; Default mode for editing text 
(setq default-major-mode 'indented-text-mode)
You should also rebind the Enter key to indent the next line of text:
(define-key indented-text-mode-map "\C-m" 'newline-and-indent)

Emacs also provides "minor" modes, which are modes you use along with major modes. For example, Overwrite mode is a minor mode that causes text to overwrite that in the buffer, instead of inserting it. To bind the key C-r to toggle overwrite mode, use the command:

; Toggle overwrite mode. 
(global-set-key "\C-r" 'overwrite-mode)
Another minor mode is Autofill, which automatically wraps lines as you type them. That is, instead of pressing the Enter key at the end of each line of text, you may continue typing and Emacs automatically breaks the line for you. To enable Autofill mode, use the commands:
(setq text-mode-hook 'turn-on-auto-fill) 
(setq fill-column 72)
This turns on Autofill mode whenever you enter Text mode (through the text-mode-hook function). It also sets the point at which to break lines at 72 characters.



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