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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 | |
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52 | @kbd{C-A}. The X Window System makes it possible to enter all these |
53 | characters. For example, @kbd{C--} (that's Control-Minus) and @kbd{C-5} | |
54 | are meaningful Emacs commands under X. | |
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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 | |
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71 | holding down @key{META} and pressing @kbd{a}. The @key{META} key |
72 | works 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 | |
74 | key with some other primary purpose. Sometimes it is labeled | |
75 | @key{ALT} or @key{EDIT}; on a Sun keyboard, it may have a diamond on | |
76 | it. | |
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77 | |
78 | If there is no @key{META} key, you can still type Meta characters | |
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79 | using two-character sequences starting with @key{ESC}. Thus, you can |
80 | enter @kbd{M-a} by typing @kbd{@key{ESC} a}. You can enter | |
81 | @kbd{C-M-a} by typing @kbd{@key{ESC} C-a}. Unlike @key{META}, which | |
82 | modifies other characters, @key{ESC} is a separate character. You | |
83 | don't hold down @key{ESC} while typing the next character; instead, | |
84 | you press it and release it, then you enter the next character. | |
85 | @key{ESC} is allowed on terminals with @key{META} keys, too, in case | |
86 | you have formed a habit of using it. | |
177c0ea7 | 87 | |
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88 | The X Window System provides several other modifier keys that can be |
89 | applied to any input character. These are called @key{SUPER}, | |
90 | @key{HYPER} and @key{ALT}. We write @samp{s-}, @samp{H-} and @samp{A-} | |
91 | to say that a character uses these modifiers. Thus, @kbd{s-H-C-x} is | |
92 | short for @kbd{Super-Hyper-Control-x}. Not all X terminals actually | |
93 | provide keys for these modifier flags---in fact, many terminals have a | |
94 | key labeled @key{ALT} which is really a @key{META} key. The standard | |
95 | key bindings of Emacs do not include any characters with these | |
96 | modifiers. But you can assign them meanings of your own by customizing | |
97 | Emacs. | |
6bf7aab6 | 98 | |
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99 | If your keyboard lacks one of these modifier keys, you can enter it |
100 | using @kbd{C-x @@}: @kbd{C-x @@ h} adds the ``hyper'' flag to the next | |
101 | character, @kbd{C-x @@ s} adds the ``super'' flag, and @kbd{C-x @@ a} | |
102 | adds the ``alt'' flag. For instance, @kbd{C-x @@ h C-a} is a way to | |
103 | enter @kbd{Hyper-Control-a}. (Unfortunately there is no way to add | |
104 | two modifiers by using @kbd{C-x @@} twice for the same character, | |
105 | because the first one goes to work on the @kbd{C-x}.) | |
106 | ||
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107 | Keyboard input includes keyboard keys that are not characters at all: |
108 | for example function keys and arrow keys. Mouse buttons are also | |
109 | outside the gamut of characters. You can modify these events with the | |
110 | modifier keys @key{CTRL}, @key{META}, @key{SUPER}, @key{HYPER} and | |
111 | @key{ALT}, just like keyboard characters. | |
112 | ||
113 | @cindex input event | |
114 | Input characters and non-character inputs are collectively called | |
115 | @dfn{input events}. @xref{Input Events,,, elisp, The Emacs Lisp | |
116 | Reference Manual}, for more information. If you are not doing Lisp | |
117 | programming, but simply want to redefine the meaning of some characters | |
118 | or non-character events, see @ref{Customization}. | |
119 | ||
120 | ASCII terminals cannot really send anything to the computer except | |
121 | ASCII characters. These terminals use a sequence of characters to | |
122 | represent each function key. But that is invisible to the Emacs user, | |
123 | because the keyboard input routines recognize these special sequences | |
124 | and convert them to function key events before any other part of Emacs | |
125 | gets to see them. | |
126 | ||
127 | @node Keys, Commands, User Input, Top | |
128 | @section Keys | |
129 | ||
130 | @cindex key sequence | |
131 | @cindex key | |
132 | A @dfn{key sequence} (@dfn{key}, for short) is a sequence of input | |
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133 | events that are meaningful as a unit---as ``a single command.'' Some |
134 | Emacs command sequences are just one character or one event; for | |
135 | example, just @kbd{C-f} is enough to move forward one character in the | |
136 | buffer. But Emacs also has commands that take two or more events to | |
137 | invoke. | |
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138 | |
139 | @cindex complete key | |
140 | @cindex prefix key | |
141 | If a sequence of events is enough to invoke a command, it is a | |
142 | @dfn{complete key}. Examples of complete keys include @kbd{C-a}, | |
143 | @kbd{X}, @key{RET}, @key{NEXT} (a function key), @key{DOWN} (an arrow | |
144 | key), @kbd{C-x C-f}, and @kbd{C-x 4 C-f}. If it isn't long enough to be | |
145 | complete, we call it a @dfn{prefix key}. The above examples show that | |
146 | @kbd{C-x} and @kbd{C-x 4} are prefix keys. Every key sequence is either | |
147 | a complete key or a prefix key. | |
148 | ||
149 | Most single characters constitute complete keys in the standard Emacs | |
150 | command bindings. A few of them are prefix keys. A prefix key combines | |
151 | with the following input event to make a longer key sequence, which may | |
152 | itself be complete or a prefix. For example, @kbd{C-x} is a prefix key, | |
8e7692a9 | 153 | so @kbd{C-x} and the next input event combine to make a two-event |
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154 | key sequence. Most of these key sequences are complete keys, including |
155 | @kbd{C-x C-f} and @kbd{C-x b}. A few, such as @kbd{C-x 4} and @kbd{C-x | |
8e7692a9 | 156 | r}, are themselves prefix keys that lead to three-event key |
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157 | sequences. There's no limit to the length of a key sequence, but in |
158 | practice people rarely use sequences longer than four events. | |
159 | ||
160 | By contrast, you can't add more events onto a complete key. For | |
8e7692a9 | 161 | example, the two-event sequence @kbd{C-f C-k} is not a key, because |
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162 | the @kbd{C-f} is a complete key in itself. It's impossible to give |
163 | @kbd{C-f C-k} an independent meaning as a command. @kbd{C-f C-k} is two | |
164 | key sequences, not one.@refill | |
165 | ||
166 | All told, the prefix keys in Emacs are @kbd{C-c}, @kbd{C-h}, | |
167 | @kbd{C-x}, @kbd{C-x @key{RET}}, @kbd{C-x @@}, @kbd{C-x a}, @kbd{C-x n}, @w{@kbd{C-x | |
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168 | r}}, @kbd{C-x v}, @kbd{C-x 4}, @kbd{C-x 5}, @kbd{C-x 6}, @key{ESC}, and |
169 | @kbd{M-g}. But this list is not cast in concrete; it is | |
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170 | just a matter of Emacs's standard key bindings. If you customize Emacs, |
171 | you can make new prefix keys, or eliminate these. @xref{Key Bindings}. | |
172 | ||
173 | If you do make or eliminate prefix keys, that changes the set of | |
174 | possible key sequences. For example, if you redefine @kbd{C-f} as a | |
175 | prefix, @kbd{C-f C-k} automatically becomes a key (complete, unless you | |
cd30a00e | 176 | define that too as a prefix). Conversely, if you remove the prefix |
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177 | definition of @kbd{C-x 4}, then @kbd{C-x 4 f} (or @kbd{C-x 4 |
178 | @var{anything}}) is no longer a key. | |
179 | ||
180 | Typing the help character (@kbd{C-h} or @key{F1}) after a prefix | |
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181 | key displays a list of the commands starting with that prefix. |
182 | There are a few prefix keys for which @kbd{C-h} does not | |
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183 | work---for historical reasons, they have other meanings for @kbd{C-h} |
184 | which are not easy to change. But @key{F1} should work for all prefix | |
8e7692a9 | 185 | keys. |
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187 | @node Commands, Text Characters, Keys, Top |
188 | @section Keys and Commands | |
189 | ||
190 | @cindex binding | |
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191 | @cindex command |
192 | @cindex function definition | |
193 | This manual is full of passages that tell you what particular keys | |
194 | do. But Emacs does not assign meanings to keys directly. Instead, | |
195 | Emacs assigns meanings to named @dfn{commands}, and then gives keys | |
196 | their meanings by @dfn{binding} them to commands. | |
197 | ||
198 | Every command has a name chosen by a programmer. The name is usually | |
199 | made of a few English words separated by dashes; for example, | |
200 | @code{next-line} or @code{forward-word}. A command also has a | |
201 | @dfn{function definition} which is a Lisp program; this is what makes | |
202 | the command do what it does. In Emacs Lisp, a command is actually a | |
203 | special kind of Lisp function; one which specifies how to read arguments | |
204 | for it and call it interactively. For more information on commands and | |
205 | functions, see @ref{What Is a Function,, What Is a Function, elisp, The | |
206 | Emacs Lisp Reference Manual}. (The definition we use in this manual is | |
207 | simplified slightly.) | |
208 | ||
209 | The bindings between keys and commands are recorded in various tables | |
210 | called @dfn{keymaps}. @xref{Keymaps}. | |
211 | ||
212 | When we say that ``@kbd{C-n} moves down vertically one line'' we are | |
213 | glossing over a distinction that is irrelevant in ordinary use but is vital | |
214 | in understanding how to customize Emacs. It is the command | |
215 | @code{next-line} that is programmed to move down vertically. @kbd{C-n} has | |
216 | this effect @emph{because} it is bound to that command. If you rebind | |
217 | @kbd{C-n} to the command @code{forward-word} then @kbd{C-n} will move | |
218 | forward by words instead. Rebinding keys is a common method of | |
219 | customization.@refill | |
220 | ||
221 | In the rest of this manual, we usually ignore this subtlety to keep | |
222 | things simple. To give the information needed for customization, we | |
223 | state the name of the command which really does the work in parentheses | |
224 | after mentioning the key that runs it. For example, we will say that | |
225 | ``The command @kbd{C-n} (@code{next-line}) moves point vertically | |
226 | down,'' meaning that @code{next-line} is a command that moves vertically | |
015a26d7 | 227 | down, and @kbd{C-n} is a key that is normally bound to it. |
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228 | |
229 | While we are on the subject of information for customization only, | |
230 | it's a good time to tell you about @dfn{variables}. Often the | |
231 | description of a command will say, ``To change this, set the variable | |
232 | @code{mumble-foo}.'' A variable is a name used to remember a value. | |
233 | Most of the variables documented in this manual exist just to facilitate | |
234 | customization: some command or other part of Emacs examines the variable | |
235 | and behaves differently according to the value that you set. Until you | |
236 | are interested in customizing, you can ignore the information about | |
237 | variables. When you are ready to be interested, read the basic | |
238 | information on variables, and then the information on individual | |
239 | variables will make sense. @xref{Variables}. | |
240 | ||
241 | @node Text Characters, Entering Emacs, Commands, Top | |
242 | @section Character Set for Text | |
243 | @cindex characters (in text) | |
244 | ||
245 | Text in Emacs buffers is a sequence of 8-bit bytes. Each byte can | |
246 | hold a single ASCII character. Both ASCII control characters (octal | |
247 | codes 000 through 037, and 0177) and ASCII printing characters (codes | |
248 | 040 through 0176) are allowed; however, non-ASCII control characters | |
249 | cannot appear in a buffer. The other modifier flags used in keyboard | |
250 | input, such as Meta, are not allowed in buffers either. | |
251 | ||
252 | Some ASCII control characters serve special purposes in text, and have | |
253 | special names. For example, the newline character (octal code 012) is | |
254 | used in the buffer to end a line, and the tab character (octal code 011) | |
255 | is used for indenting to the next tab stop column (normally every 8 | |
256 | columns). @xref{Text Display}. | |
257 | ||
258 | Non-ASCII printing characters can also appear in buffers. When | |
259 | multibyte characters are enabled, you can use any of the non-ASCII | |
260 | printing characters that Emacs supports. They have character codes | |
261 | starting at 256, octal 0400, and each one is represented as a sequence | |
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262 | of two or more bytes. @xref{International}. Single-byte characters |
263 | with codes 128 through 255 can also appear in multibyte buffers. | |
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264 | |
265 | If you disable multibyte characters, then you can use only one | |
266 | alphabet of non-ASCII characters, but they all fit in one byte. They | |
a3ddb43a | 267 | use codes 0200 through 0377. @xref{Single-Byte Character Support}. |