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1 | @c This is part of the Emacs manual. |
2 | @c Copyright (C) 1985, 86, 87, 93, 94, 95, 1997 Free Software Foundation, Inc. | |
3 | @c See file emacs.texi for copying conditions. | |
4 | @iftex | |
5 | @chapter Characters, Keys and Commands | |
6 | ||
7 | This chapter explains the character sets used by Emacs for input | |
8 | commands and for the contents of files, and also explains the concepts | |
9 | of @dfn{keys} and @dfn{commands}, which are fundamental for understanding | |
10 | how Emacs interprets your keyboard and mouse input. | |
11 | @end iftex | |
12 | ||
13 | @node User Input, Keys, Screen, Top | |
14 | @section Kinds of User Input | |
15 | @cindex input with the keyboard | |
16 | @cindex keyboard input | |
17 | @cindex character set (keyboard) | |
18 | @cindex ASCII | |
19 | @cindex C- | |
20 | @cindex Control | |
21 | @cindex control characters | |
22 | ||
23 | GNU Emacs uses an extension of the ASCII character set for keyboard | |
24 | input; it also accepts non-character input events including function | |
25 | keys and mouse button actions. | |
26 | ||
27 | ASCII consists of 128 character codes. Some of these codes are | |
28 | assigned graphic symbols such as @samp{a} and @samp{=}; the rest are | |
29 | control characters, such as @kbd{Control-a} (usually written @kbd{C-a} | |
30 | for short). @kbd{C-a} gets its name from the fact that you type it by | |
31 | holding down the @key{CTRL} key while pressing @kbd{a}. | |
32 | ||
33 | Some ASCII control characters have special names, and most terminals | |
34 | have special keys you can type them with: for example, @key{RET}, | |
35 | @key{TAB}, @key{DEL} and @key{ESC}. The space character is usually | |
36 | referred to below as @key{SPC}, even though strictly speaking it is a | |
37 | graphic character whose graphic happens to be blank. Some keyboards | |
38 | have a key labeled ``linefeed'' which is an alias for @kbd{C-j}. | |
39 | ||
40 | Emacs extends the ASCII character set with thousands more printing | |
41 | characters (@pxref{International}), additional control characters, and a | |
42 | few more modifiers that can be combined with any character. | |
43 | ||
44 | On ASCII terminals, there are only 32 possible control characters. | |
45 | These are the control variants of letters and @samp{@@[]\^_}. In | |
46 | addition, the shift key is meaningless with control characters: | |
47 | @kbd{C-a} and @kbd{C-A} are the same character, and Emacs cannot | |
48 | distinguish them. | |
49 | ||
50 | But the Emacs character set has room for control variants of all | |
51 | printing characters, and for distinguishing between @kbd{C-a} and | |
52 | @kbd{C-A}. X Windows makes it possible to enter all these characters. | |
53 | For example, @kbd{C--} (that's Control-Minus) and @kbd{C-5} are | |
54 | meaningful Emacs commands under X. | |
55 | ||
56 | Another Emacs character-set extension is additional modifier bits. | |
57 | Only one modifier bit is commonly used; it is called Meta. Every | |
58 | character has a Meta variant; examples include @kbd{Meta-a} (normally | |
59 | written @kbd{M-a}, for short), @kbd{M-A} (not the same character as | |
60 | @kbd{M-a}, but those two characters normally have the same meaning in | |
61 | Emacs), @kbd{M-@key{RET}}, and @kbd{M-C-a}. For reasons of tradition, | |
62 | we usually write @kbd{C-M-a} rather than @kbd{M-C-a}; logically | |
63 | speaking, the order in which the modifier keys @key{CTRL} and @key{META} | |
64 | are mentioned does not matter. | |
65 | ||
66 | @cindex Meta | |
67 | @cindex M- | |
68 | @cindex @key{ESC} replacing @key{META} key | |
69 | Some terminals have a @key{META} key, and allow you to type Meta | |
70 | characters by holding this key down. Thus, @kbd{Meta-a} is typed by | |
71 | holding down @key{META} and pressing @kbd{a}. The @key{META} key works | |
72 | much like the @key{SHIFT} key. Such a key is not always labeled | |
73 | @key{META}, however, as this function is often a special option for a key | |
74 | with some other primary purpose.@refill | |
75 | ||
76 | If there is no @key{META} key, you can still type Meta characters | |
77 | using two-character sequences starting with @key{ESC}. Thus, to enter | |
78 | @kbd{M-a}, you could type @kbd{@key{ESC} a}. To enter @kbd{C-M-a}, you | |
79 | would type @kbd{@key{ESC} C-a}. @key{ESC} is allowed on terminals with | |
80 | @key{META} keys, too, in case you have formed a habit of using it. | |
81 | ||
82 | X Windows provides several other modifier keys that can be applied to | |
83 | any input character. These are called @key{SUPER}, @key{HYPER} and | |
84 | @key{ALT}. We write @samp{s-}, @samp{H-} and @samp{A-} to say that a | |
85 | character uses these modifiers. Thus, @kbd{s-H-C-x} is short for | |
86 | @kbd{Super-Hyper-Control-x}. Not all X terminals actually provide keys | |
87 | for these modifier flags---in fact, many terminals have a key labeled | |
88 | @key{ALT} which is really a @key{META} key. The standard key bindings | |
89 | of Emacs do not include any characters with these modifiers. But you | |
90 | can assign them meanings of your own by customizing Emacs. | |
91 | ||
92 | Keyboard input includes keyboard keys that are not characters at all: | |
93 | for example function keys and arrow keys. Mouse buttons are also | |
94 | outside the gamut of characters. You can modify these events with the | |
95 | modifier keys @key{CTRL}, @key{META}, @key{SUPER}, @key{HYPER} and | |
96 | @key{ALT}, just like keyboard characters. | |
97 | ||
98 | @cindex input event | |
99 | Input characters and non-character inputs are collectively called | |
100 | @dfn{input events}. @xref{Input Events,,, elisp, The Emacs Lisp | |
101 | Reference Manual}, for more information. If you are not doing Lisp | |
102 | programming, but simply want to redefine the meaning of some characters | |
103 | or non-character events, see @ref{Customization}. | |
104 | ||
105 | ASCII terminals cannot really send anything to the computer except | |
106 | ASCII characters. These terminals use a sequence of characters to | |
107 | represent each function key. But that is invisible to the Emacs user, | |
108 | because the keyboard input routines recognize these special sequences | |
109 | and convert them to function key events before any other part of Emacs | |
110 | gets to see them. | |
111 | ||
112 | @node Keys, Commands, User Input, Top | |
113 | @section Keys | |
114 | ||
115 | @cindex key sequence | |
116 | @cindex key | |
117 | A @dfn{key sequence} (@dfn{key}, for short) is a sequence of input | |
118 | events that are meaningful as a unit---as ``a single command.'' | |
119 | Some Emacs command sequences are just one character or one event; for | |
120 | example, just @kbd{C-f} is enough to move forward one character. But | |
121 | Emacs also has commands that take two or more events to invoke. | |
122 | ||
123 | @cindex complete key | |
124 | @cindex prefix key | |
125 | If a sequence of events is enough to invoke a command, it is a | |
126 | @dfn{complete key}. Examples of complete keys include @kbd{C-a}, | |
127 | @kbd{X}, @key{RET}, @key{NEXT} (a function key), @key{DOWN} (an arrow | |
128 | key), @kbd{C-x C-f}, and @kbd{C-x 4 C-f}. If it isn't long enough to be | |
129 | complete, we call it a @dfn{prefix key}. The above examples show that | |
130 | @kbd{C-x} and @kbd{C-x 4} are prefix keys. Every key sequence is either | |
131 | a complete key or a prefix key. | |
132 | ||
133 | Most single characters constitute complete keys in the standard Emacs | |
134 | command bindings. A few of them are prefix keys. A prefix key combines | |
135 | with the following input event to make a longer key sequence, which may | |
136 | itself be complete or a prefix. For example, @kbd{C-x} is a prefix key, | |
137 | so @kbd{C-x} and the next input event combine to make a two-character | |
138 | key sequence. Most of these key sequences are complete keys, including | |
139 | @kbd{C-x C-f} and @kbd{C-x b}. A few, such as @kbd{C-x 4} and @kbd{C-x | |
140 | r}, are themselves prefix keys that lead to three-character key | |
141 | sequences. There's no limit to the length of a key sequence, but in | |
142 | practice people rarely use sequences longer than four events. | |
143 | ||
144 | By contrast, you can't add more events onto a complete key. For | |
145 | example, the two-character sequence @kbd{C-f C-k} is not a key, because | |
146 | the @kbd{C-f} is a complete key in itself. It's impossible to give | |
147 | @kbd{C-f C-k} an independent meaning as a command. @kbd{C-f C-k} is two | |
148 | key sequences, not one.@refill | |
149 | ||
150 | All told, the prefix keys in Emacs are @kbd{C-c}, @kbd{C-h}, | |
151 | @kbd{C-x}, @kbd{C-x @key{RET}}, @kbd{C-x @@}, @kbd{C-x a}, @kbd{C-x n}, @w{@kbd{C-x | |
152 | r}}, @kbd{C-x v}, @kbd{C-x 4}, @kbd{C-x 5}, @kbd{C-x 6}, @key{ESC}, | |
153 | @kbd{M-g} and @kbd{M-j}. But this list is not cast in concrete; it is | |
154 | just a matter of Emacs's standard key bindings. If you customize Emacs, | |
155 | you can make new prefix keys, or eliminate these. @xref{Key Bindings}. | |
156 | ||
157 | If you do make or eliminate prefix keys, that changes the set of | |
158 | possible key sequences. For example, if you redefine @kbd{C-f} as a | |
159 | prefix, @kbd{C-f C-k} automatically becomes a key (complete, unless you | |
160 | define it too as a prefix). Conversely, if you remove the prefix | |
161 | definition of @kbd{C-x 4}, then @kbd{C-x 4 f} (or @kbd{C-x 4 | |
162 | @var{anything}}) is no longer a key. | |
163 | ||
164 | Typing the help character (@kbd{C-h} or @key{F1}) after a prefix | |
165 | character displays a list of the commands starting with that prefix. | |
166 | There are a few prefix characters for which @kbd{C-h} does not | |
167 | work---for historical reasons, they have other meanings for @kbd{C-h} | |
168 | which are not easy to change. But @key{F1} should work for all prefix | |
169 | characters. | |
170 | ||
171 | @node Commands, Text Characters, Keys, Top | |
172 | @section Keys and Commands | |
173 | ||
174 | @cindex binding | |
175 | @cindex function | |
176 | @cindex command | |
177 | @cindex function definition | |
178 | This manual is full of passages that tell you what particular keys | |
179 | do. But Emacs does not assign meanings to keys directly. Instead, | |
180 | Emacs assigns meanings to named @dfn{commands}, and then gives keys | |
181 | their meanings by @dfn{binding} them to commands. | |
182 | ||
183 | Every command has a name chosen by a programmer. The name is usually | |
184 | made of a few English words separated by dashes; for example, | |
185 | @code{next-line} or @code{forward-word}. A command also has a | |
186 | @dfn{function definition} which is a Lisp program; this is what makes | |
187 | the command do what it does. In Emacs Lisp, a command is actually a | |
188 | special kind of Lisp function; one which specifies how to read arguments | |
189 | for it and call it interactively. For more information on commands and | |
190 | functions, see @ref{What Is a Function,, What Is a Function, elisp, The | |
191 | Emacs Lisp Reference Manual}. (The definition we use in this manual is | |
192 | simplified slightly.) | |
193 | ||
194 | The bindings between keys and commands are recorded in various tables | |
195 | called @dfn{keymaps}. @xref{Keymaps}. | |
196 | ||
197 | When we say that ``@kbd{C-n} moves down vertically one line'' we are | |
198 | glossing over a distinction that is irrelevant in ordinary use but is vital | |
199 | in understanding how to customize Emacs. It is the command | |
200 | @code{next-line} that is programmed to move down vertically. @kbd{C-n} has | |
201 | this effect @emph{because} it is bound to that command. If you rebind | |
202 | @kbd{C-n} to the command @code{forward-word} then @kbd{C-n} will move | |
203 | forward by words instead. Rebinding keys is a common method of | |
204 | customization.@refill | |
205 | ||
206 | In the rest of this manual, we usually ignore this subtlety to keep | |
207 | things simple. To give the information needed for customization, we | |
208 | state the name of the command which really does the work in parentheses | |
209 | after mentioning the key that runs it. For example, we will say that | |
210 | ``The command @kbd{C-n} (@code{next-line}) moves point vertically | |
211 | down,'' meaning that @code{next-line} is a command that moves vertically | |
212 | down and @kbd{C-n} is a key that is standardly bound to it. | |
213 | ||
214 | While we are on the subject of information for customization only, | |
215 | it's a good time to tell you about @dfn{variables}. Often the | |
216 | description of a command will say, ``To change this, set the variable | |
217 | @code{mumble-foo}.'' A variable is a name used to remember a value. | |
218 | Most of the variables documented in this manual exist just to facilitate | |
219 | customization: some command or other part of Emacs examines the variable | |
220 | and behaves differently according to the value that you set. Until you | |
221 | are interested in customizing, you can ignore the information about | |
222 | variables. When you are ready to be interested, read the basic | |
223 | information on variables, and then the information on individual | |
224 | variables will make sense. @xref{Variables}. | |
225 | ||
226 | @node Text Characters, Entering Emacs, Commands, Top | |
227 | @section Character Set for Text | |
228 | @cindex characters (in text) | |
229 | ||
230 | Text in Emacs buffers is a sequence of 8-bit bytes. Each byte can | |
231 | hold a single ASCII character. Both ASCII control characters (octal | |
232 | codes 000 through 037, and 0177) and ASCII printing characters (codes | |
233 | 040 through 0176) are allowed; however, non-ASCII control characters | |
234 | cannot appear in a buffer. The other modifier flags used in keyboard | |
235 | input, such as Meta, are not allowed in buffers either. | |
236 | ||
237 | Some ASCII control characters serve special purposes in text, and have | |
238 | special names. For example, the newline character (octal code 012) is | |
239 | used in the buffer to end a line, and the tab character (octal code 011) | |
240 | is used for indenting to the next tab stop column (normally every 8 | |
241 | columns). @xref{Text Display}. | |
242 | ||
243 | Non-ASCII printing characters can also appear in buffers. When | |
244 | multibyte characters are enabled, you can use any of the non-ASCII | |
245 | printing characters that Emacs supports. They have character codes | |
246 | starting at 256, octal 0400, and each one is represented as a sequence | |
247 | of two or more bytes. @xref{International}. | |
248 | ||
249 | If you disable multibyte characters, then you can use only one | |
250 | alphabet of non-ASCII characters, but they all fit in one byte. They | |
a3ddb43a | 251 | use codes 0200 through 0377. @xref{Single-Byte Character Support}. |