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285 lines
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285 lines
14 KiB
Plaintext
@c This is part of the Emacs manual.
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@c Copyright (C) 1985, 1986, 1987, 1993, 1994, 1995, 1997, 2002, 2003,
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@c 2004, 2005 Free Software Foundation, Inc.
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@c See file emacs.texi for copying conditions.
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@iftex
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@chapter Characters, Keys and Commands
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This chapter explains the character sets used by Emacs for input
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commands and for the contents of files, and also explains the concepts
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of @dfn{keys} and @dfn{commands}, which are fundamental for understanding
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how Emacs interprets your keyboard and mouse input.
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@end iftex
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@ifnottex
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@raisesections
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@end ifnottex
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@node User Input, Keys, Screen, Top
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@section Kinds of User Input
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@cindex input with the keyboard
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@cindex keyboard input
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@cindex character set (keyboard)
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@cindex @acronym{ASCII}
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@cindex C-
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@cindex Control
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@cindex control characters
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GNU Emacs uses an extension of the @acronym{ASCII} character set for keyboard
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input; it also accepts non-character input events including function
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keys and mouse button actions.
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@acronym{ASCII} consists of 128 character codes. Some of these codes are
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assigned graphic symbols such as @samp{a} and @samp{=}; the rest are
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control characters, such as @kbd{Control-a} (usually written @kbd{C-a}
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for short). @kbd{C-a} gets its name from the fact that you type it by
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holding down the @key{CTRL} key while pressing @kbd{a}.
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Some @acronym{ASCII} control characters have special names, and most terminals
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have special keys you can type them with: for example, @key{RET},
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@key{TAB}, @key{DEL} and @key{ESC}. The space character is usually
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referred to below as @key{SPC}, even though strictly speaking it is a
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graphic character whose graphic happens to be blank. Some keyboards
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have a key labeled ``linefeed'' which is an alias for @kbd{C-j}.
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Emacs extends the @acronym{ASCII} character set with thousands more printing
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characters (@pxref{International}), additional control characters, and a
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few more modifiers that can be combined with any character.
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On @acronym{ASCII} terminals, there are only 32 possible control characters.
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These are the control variants of letters and @samp{@@[]\^_}. In
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addition, the shift key is meaningless with control characters:
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@kbd{C-a} and @kbd{C-A} are the same character, and Emacs cannot
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distinguish them.
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But the Emacs character set has room for control variants of all
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printing characters, and for distinguishing between @kbd{C-a} and
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@kbd{C-A}. The X Window System makes it possible to enter all these
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characters. For example, @kbd{C--} (that's Control-Minus) and @kbd{C-5}
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are meaningful Emacs commands under X.
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Another Emacs character-set extension is additional modifier bits.
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Only one modifier bit is commonly used; it is called Meta. Every
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character has a Meta variant; examples include @kbd{Meta-a} (normally
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written @kbd{M-a}, for short), @kbd{M-A} (not the same character as
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@kbd{M-a}, but those two characters normally have the same meaning in
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Emacs), @kbd{M-@key{RET}}, and @kbd{M-C-a}. For reasons of tradition,
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we usually write @kbd{C-M-a} rather than @kbd{M-C-a}; logically
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speaking, the order in which the modifier keys @key{CTRL} and @key{META}
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are mentioned does not matter.
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@cindex Meta
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@cindex M-
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@cindex @key{ESC} replacing @key{META} key
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Some terminals have a @key{META} key, and allow you to type Meta
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characters by holding this key down. Thus, @kbd{Meta-a} is typed by
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holding down @key{META} and pressing @kbd{a}. The @key{META} key
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works much like the @key{SHIFT} key. Such a key is not always labeled
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@key{META}, however, as this function is often a special option for a
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key with some other primary purpose. Sometimes it is labeled
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@key{ALT} or @key{EDIT}; on a Sun keyboard, it may have a diamond on
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it.
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If there is no @key{META} key, you can still type Meta characters
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using two-character sequences starting with @key{ESC}. Thus, you can
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enter @kbd{M-a} by typing @kbd{@key{ESC} a}. You can enter
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@kbd{C-M-a} by typing @kbd{@key{ESC} C-a}. Unlike @key{META}, which
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modifies other characters, @key{ESC} is a separate character. You
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don't hold down @key{ESC} while typing the next character; instead,
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you press it and release it, then you enter the next character.
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@key{ESC} is allowed on terminals with @key{META} keys, too, in case
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you have formed a habit of using it.
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The X Window System provides several other modifier keys that can be
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applied to any input character. These are called @key{SUPER},
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@key{HYPER} and @key{ALT}. We write @samp{s-}, @samp{H-} and @samp{A-}
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to say that a character uses these modifiers. Thus, @kbd{s-H-C-x} is
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short for @kbd{Super-Hyper-Control-x}. Not all X terminals actually
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provide keys for these modifier flags---in fact, many terminals have a
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key labeled @key{ALT} which is really a @key{META} key. The standard
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key bindings of Emacs do not include any characters with these
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modifiers. But you can assign them meanings of your own by customizing
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Emacs.
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If your keyboard lacks one of these modifier keys, you can enter it
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using @kbd{C-x @@}: @kbd{C-x @@ h} adds the ``hyper'' flag to the next
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character, @kbd{C-x @@ s} adds the ``super'' flag, and @kbd{C-x @@ a}
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adds the ``alt'' flag. For instance, @kbd{C-x @@ h C-a} is a way to
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enter @kbd{Hyper-Control-a}. (Unfortunately there is no way to add
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two modifiers by using @kbd{C-x @@} twice for the same character,
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because the first one goes to work on the @kbd{C-x}.)
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Keyboard input includes keyboard keys that are not characters at all:
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for example function keys and arrow keys. Mouse buttons are also
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outside the gamut of characters. You can modify these events with the
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modifier keys @key{CTRL}, @key{META}, @key{SUPER}, @key{HYPER} and
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@key{ALT}, just like keyboard characters.
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@cindex input event
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Input characters and non-character inputs are collectively called
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@dfn{input events}. @xref{Input Events,,, elisp, The Emacs Lisp
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Reference Manual}, for more information. If you are not doing Lisp
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programming, but simply want to redefine the meaning of some characters
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or non-character events, see @ref{Customization}.
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@acronym{ASCII} terminals cannot really send anything to the computer except
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@acronym{ASCII} characters. These terminals use a sequence of characters to
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represent each function key. But that is invisible to the Emacs user,
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because the keyboard input routines recognize these special sequences
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and convert them to function key events before any other part of Emacs
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gets to see them.
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@node Keys, Commands, User Input, Top
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@section Keys
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@cindex key sequence
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@cindex key
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A @dfn{key sequence} (@dfn{key}, for short) is a sequence of input
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events that are meaningful as a unit---as ``a single command.'' Some
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Emacs command sequences are just one character or one event; for
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example, just @kbd{C-f} is enough to move forward one character in the
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buffer. But Emacs also has commands that take two or more events to
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invoke.
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@cindex complete key
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@cindex prefix key
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If a sequence of events is enough to invoke a command, it is a
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@dfn{complete key}. Examples of complete keys include @kbd{C-a},
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@kbd{X}, @key{RET}, @key{NEXT} (a function key), @key{DOWN} (an arrow
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key), @kbd{C-x C-f}, and @kbd{C-x 4 C-f}. If it isn't long enough to be
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complete, we call it a @dfn{prefix key}. The above examples show that
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@kbd{C-x} and @kbd{C-x 4} are prefix keys. Every key sequence is either
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a complete key or a prefix key.
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Most single characters constitute complete keys in the standard Emacs
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command bindings. A few of them are prefix keys. A prefix key combines
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with the following input event to make a longer key sequence, which may
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itself be complete or a prefix. For example, @kbd{C-x} is a prefix key,
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so @kbd{C-x} and the next input event combine to make a two-event
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key sequence. Most of these key sequences are complete keys, including
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@kbd{C-x C-f} and @kbd{C-x b}. A few, such as @kbd{C-x 4} and @kbd{C-x
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r}, are themselves prefix keys that lead to three-event key
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sequences. There's no limit to the length of a key sequence, but in
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practice people rarely use sequences longer than four events.
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By contrast, you can't add more events onto a complete key. For
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example, the two-event sequence @kbd{C-f C-k} is not a key, because
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the @kbd{C-f} is a complete key in itself. It's impossible to give
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@kbd{C-f C-k} an independent meaning as a command. @kbd{C-f C-k} is two
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key sequences, not one.@refill
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All told, the prefix keys in Emacs are @kbd{C-c}, @kbd{C-h},
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@kbd{C-x}, @kbd{C-x @key{RET}}, @kbd{C-x @@}, @kbd{C-x a}, @kbd{C-x
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n}, @w{@kbd{C-x r}}, @kbd{C-x v}, @kbd{C-x 4}, @kbd{C-x 5}, @kbd{C-x 6},
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@key{ESC}, @kbd{M-o} and @kbd{M-g}. (@key{F1} and @key{F2} are aliases for
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@kbd{C-h} and @kbd{C-x 6}.) But this list is not cast in concrete; it
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is just a matter of Emacs's standard key bindings. If you customize
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Emacs, you can make new prefix keys, or eliminate these. @xref{Key
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Bindings}.
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If you do make or eliminate prefix keys, that changes the set of
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possible key sequences. For example, if you redefine @kbd{C-f} as a
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prefix, @kbd{C-f C-k} automatically becomes a key (complete, unless you
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define that too as a prefix). Conversely, if you remove the prefix
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definition of @kbd{C-x 4}, then @kbd{C-x 4 f} (or @kbd{C-x 4
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@var{anything}}) is no longer a key.
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Typing the help character (@kbd{C-h} or @key{F1}) after a prefix
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key displays a list of the commands starting with that prefix.
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There are a few prefix keys for which @kbd{C-h} does not
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work---for historical reasons, they have other meanings for @kbd{C-h}
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which are not easy to change. But @key{F1} should work for all prefix
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keys.
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@node Commands, Text Characters, Keys, Top
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@section Keys and Commands
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@cindex binding
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@cindex command
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@cindex function definition
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This manual is full of passages that tell you what particular keys
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do. But Emacs does not assign meanings to keys directly. Instead,
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Emacs assigns meanings to named @dfn{commands}, and then gives keys
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their meanings by @dfn{binding} them to commands.
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Every command has a name chosen by a programmer. The name is usually
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made of a few English words separated by dashes; for example,
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@code{next-line} or @code{forward-word}. A command also has a
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@dfn{function definition} which is a Lisp program; this is what makes
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the command do what it does. In Emacs Lisp, a command is actually a
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special kind of Lisp function; one which specifies how to read arguments
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for it and call it interactively. For more information on commands and
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functions, see @ref{What Is a Function,, What Is a Function, elisp, The
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Emacs Lisp Reference Manual}. (The definition we use in this manual is
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simplified slightly.)
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The bindings between keys and commands are recorded in various tables
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called @dfn{keymaps}. @xref{Keymaps}.
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When we say that ``@kbd{C-n} moves down vertically one line'' we are
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glossing over a distinction that is irrelevant in ordinary use but is vital
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in understanding how to customize Emacs. It is the command
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@code{next-line} that is programmed to move down vertically. @kbd{C-n} has
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this effect @emph{because} it is bound to that command. If you rebind
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@kbd{C-n} to the command @code{forward-word} then @kbd{C-n} will move
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forward by words instead. Rebinding keys is a common method of
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customization.@refill
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In the rest of this manual, we usually ignore this distinction to
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keep things simple. We will often speak of keys like @kbd{C-n} as
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commands, even though strictly speaking a key is bound to some
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command. To give the information needed for customization, we state
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the name of the command which really does the work in parentheses
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after mentioning the key that runs it. For example, we will say that
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``The command @kbd{C-n} (@code{next-line}) moves point vertically
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down,'' meaning that @code{next-line} is a command that moves
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vertically down, and @kbd{C-n} is a key that is normally bound to it.
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While we are on the subject of information for customization only,
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it's a good time to tell you about @dfn{variables}. Often the
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description of a command will say, ``To change this, set the variable
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@code{mumble-foo}.'' A variable is a name used to remember a value.
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Most of the variables documented in this manual exist just to facilitate
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customization: some command or other part of Emacs examines the variable
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and behaves differently according to the value that you set. Until you
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are interested in customizing, you can ignore the information about
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variables. When you are ready to be interested, read the basic
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information on variables, and then the information on individual
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variables will make sense. @xref{Variables}.
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@node Text Characters, Entering Emacs, Commands, Top
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@section Character Set for Text
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@cindex characters (in text)
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Text in Emacs buffers is a sequence of 8-bit bytes. Each byte can
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hold a single @acronym{ASCII} character. Both @acronym{ASCII} control characters (octal
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codes 000 through 037, and 0177) and @acronym{ASCII} printing characters (codes
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040 through 0176) are allowed; however, non-@acronym{ASCII} control characters
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cannot appear in a buffer. The other modifier flags used in keyboard
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input, such as Meta, are not allowed in buffers either.
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Some @acronym{ASCII} control characters serve special purposes in text, and have
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special names. For example, the newline character (octal code 012) is
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used in the buffer to end a line, and the tab character (octal code 011)
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is used for indenting to the next tab stop column (normally every 8
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columns). @xref{Text Display}.
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Non-@acronym{ASCII} printing characters can also appear in buffers. When
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multibyte characters are enabled, you can use any of the non-@acronym{ASCII}
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printing characters that Emacs supports. They have character codes
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starting at 256, octal 0400, and each one is represented as a sequence
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of two or more bytes. @xref{International}. Single-byte characters
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with codes 128 through 255 can also appear in multibyte buffers.
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If you disable multibyte characters, then you can use only one
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alphabet of non-@acronym{ASCII} characters, but they all fit in one byte. They
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use codes 0200 through 0377. @xref{Single-Byte Character Support}.
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@ifnottex
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@lowersections
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@end ifnottex
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@ignore
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arch-tag: 9be43eef-d1f4-4d03-a916-c741ea713a45
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@end ignore
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