emacs/man/commands.texi

@c This is part of the Emacs manual.
@c Copyright (C) 1985, 1986, 1987, 1993, 1994, 1995, 1997, 2002, 2003,
@c   2004, 2005, 2006 Free Software Foundation, Inc.
@c See file emacs.texi for copying conditions.
@iftex
@chapter Characters, Keys and Commands

  This chapter explains the character sets used by Emacs for input
commands and for the contents of files, and the fundamental concepts of
@dfn{keys} and @dfn{commands}, whereby Emacs interprets your keyboard
and mouse input.
@end iftex

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@node User Input, Keys, Screen, Top
@section Kinds of User Input
@cindex input with the keyboard
@cindex keyboard input
@cindex character set (keyboard)
@cindex @acronym{ASCII}
@cindex C-
@cindex Control
@cindex control characters

  GNU Emacs uses an extension of the @acronym{ASCII} character set for keyboard
input; it also accepts non-character input events including function
keys and mouse button actions.

  @acronym{ASCII} consists of 128 character codes.  Some of these codes are
assigned graphic symbols such as @samp{a} and @samp{=}; the rest are
control characters, such as @kbd{Control-a} (usually written @kbd{C-a}
for short).  @kbd{C-a} gets its name from the fact that you type it by
holding down the @key{CTRL} key while pressing @kbd{a}.

  Some @acronym{ASCII} control characters have special names, and most
terminals have special keys you can type them with: for example,
@key{RET}, @key{TAB}, @key{DEL} and @key{ESC}.  The space character is
usually known as @key{SPC}, even though strictly speaking it is a
graphic character that is blank.

  Emacs extends the @acronym{ASCII} character set with thousands more printing
characters (@pxref{International}), additional control characters, and a
few more modifiers that can be combined with any character.

  On @acronym{ASCII} terminals, there are only 32 possible control characters.
These are the control variants of letters and @samp{@@[]\^_}.  In
addition, the shift key is meaningless with control characters:
@kbd{C-a} and @kbd{C-A} are the same character, and Emacs cannot
distinguish them.

  The Emacs character set has room for control variants of all
printing characters, and distinguishes @kbd{C-A} from @kbd{C-a}.
Graphical terminals make it possible to enter all these characters.
For example, @kbd{C--} (that's Control-Minus) and @kbd{C-5} are
meaningful Emacs commands on a graphical terminal.

  Another Emacs character-set extension is additional modifier bits.
Only one modifier bit is commonly used; it is called Meta.  Every
character has a Meta variant; examples include @kbd{Meta-a} (normally
written @kbd{M-a}, for short), @kbd{M-A} (different from @kbd{M-a},
but they are normally equivalent in Emacs), @kbd{M-@key{RET}}, and
@kbd{M-C-a}.  That last means @kbd{a} with both the @key{CTRL} and
@key{META} modifiers.  We usually write it as @kbd{C-M-a} rather than
@kbd{M-C-a}, for reasons of tradition.

@cindex Meta
@cindex M-
@cindex @key{ESC} replacing @key{META} key
  Some terminals have a @key{META} key, and allow you to type Meta
characters by holding this key down.  Thus, you can type @kbd{Meta-a}
by holding down @key{META} and pressing @kbd{a}.  The @key{META} key
works much like the @key{SHIFT} key.  In fact, this key is more often
labeled @key{ALT} or @key{EDIT}, instead of @key{META}; on a Sun
keyboard, it may have a diamond on it.

  If there is no @key{META} key, you can still type Meta characters
using two-character sequences starting with @key{ESC}.  Thus, you can
enter @kbd{M-a} by typing @kbd{@key{ESC} a}.  You can enter
@kbd{C-M-a} by typing @kbd{@key{ESC} C-a}.  Unlike @key{META}, which
modifies other characters, @key{ESC} is a separate character.  You
don't hold down @key{ESC} while typing the next character; instead,
you press it and release it, then you enter the next character.
@key{ESC} is allowed on terminals with @key{META} keys, too, in case
you have formed a habit of using it.

  Emacs defines several other modifier keys that can be applied to any
input character.  These are called @key{SUPER}, @key{HYPER} and
@key{ALT}.  We write @samp{s-}, @samp{H-} and @samp{A-} to say that a
character uses these modifiers.  Thus, @kbd{s-H-C-x} is short for
@kbd{Super-Hyper-Control-x}.  Not all graphical terminals actually
provide keys for these modifier flags---in fact, many terminals have a
key labeled @key{ALT} which is really a @key{META} key.  The standard
key bindings of Emacs do not include any characters with these
modifiers.  But you can assign them meanings of your own by
customizing Emacs.

  If your keyboard lacks one of these modifier keys, you can enter it
using @kbd{C-x @@}: @kbd{C-x @@ h} adds the ``hyper'' flag to the next
character, @kbd{C-x @@ s} adds the ``super'' flag, and @kbd{C-x @@ a}
adds the ``alt'' flag.  For instance, @kbd{C-x @@ h C-a} is a way to
enter @kbd{Hyper-Control-a}.  (Unfortunately there is no way to add
two modifiers by using @kbd{C-x @@} twice for the same character,
because the first one goes to work on the @kbd{C-x}.)

  Keyboard input includes keyboard keys that are not characters at
all, such as function keys and arrow keys.  Mouse buttons are also not
characters.  However, you can modify these events with the modifier
keys @key{CTRL}, @key{META}, @key{SUPER}, @key{HYPER} and @key{ALT},
just like keyboard characters.

@cindex input event
  Input characters and non-character inputs are collectively called
@dfn{input events}.  @xref{Input Events,,, elisp, The Emacs Lisp
Reference Manual}, for the full Lisp-level details.  If you are not
doing Lisp programming, but simply want to redefine the meaning of
some characters or non-character events, see @ref{Customization}.

  @acronym{ASCII} terminals cannot really send anything to the computer except
@acronym{ASCII} characters.  These terminals use a sequence of characters to
represent each function key.  But that is invisible to the Emacs user,
because the keyboard input routines catch these special sequences
and convert them to function key events before any other part of Emacs
gets to see them.

@node Keys, Commands, User Input, Top
@section Keys

@cindex key sequence
@cindex key
  A @dfn{key sequence} (@dfn{key}, for short) is a sequence of input
events that is meaningful as a unit---a ``single command.''  Some
Emacs command sequences are invoked by just one character or one
event; for example, just @kbd{C-f} moves forward one character in the
buffer.  But Emacs also has commands that take two or more events to
invoke.

@cindex complete key
@cindex prefix key
  If a sequence of events is enough to invoke a command, it is a
@dfn{complete key}.  Examples of complete keys include @kbd{C-a},
@kbd{X}, @key{RET}, @key{NEXT} (a function key), @key{DOWN} (an arrow
key), @kbd{C-x C-f}, and @kbd{C-x 4 C-f}.  If it isn't long enough to be
complete, we call it a @dfn{prefix key}.  The above examples show that
@kbd{C-x} and @kbd{C-x 4} are prefix keys.  Every key sequence is either
a complete key or a prefix key.

  Most single characters constitute complete keys in the standard Emacs
command bindings.  A few of them are prefix keys.  A prefix key combines
with the following input event to make a longer key sequence, which may
itself be complete or a prefix.  For example, @kbd{C-x} is a prefix key,
so @kbd{C-x} and the next input event combine to make a two-event
key sequence.  Most of these key sequences are complete keys, including
@kbd{C-x C-f} and @kbd{C-x b}.  A few, such as @kbd{C-x 4} and @kbd{C-x
r}, are themselves prefix keys that lead to three-event key
sequences.  There's no limit to the length of a key sequence, but in
practice people rarely use sequences longer than four events.

  You can't add input events onto a complete key.  For example, the
two-event sequence @kbd{C-f C-k} is not a key, because the @kbd{C-f}
is a complete key in itself.  It's impossible to give @kbd{C-f C-k} an
independent meaning as a command.  @kbd{C-f C-k} is two key sequences,
not one.@refill

  All told, the prefix keys in Emacs are @kbd{C-c}, @kbd{C-h},
@kbd{C-x}, @kbd{C-x @key{RET}}, @kbd{C-x @@}, @kbd{C-x a}, @kbd{C-x
n}, @w{@kbd{C-x r}}, @kbd{C-x v}, @kbd{C-x 4}, @kbd{C-x 5}, @kbd{C-x
6}, @key{ESC}, @kbd{M-g}, and @kbd{M-o}.  (@key{F1} and @key{F2} are
aliases for @kbd{C-h} and @kbd{C-x 6}.)  This list is not cast in stone;
it describes the standard key bindings.  If you customize Emacs, you can make
new prefix keys, or eliminate some of the standard ones (not
recommended for most users).  @xref{Key Bindings}.

  If you make or eliminate prefix keys, that changes the set of
possible key sequences.  For example, if you redefine @kbd{C-f} as a
prefix, @kbd{C-f C-k} automatically becomes a key (complete, unless
you define that too as a prefix).  Conversely, if you remove the
prefix definition of @kbd{C-x 4}, then @kbd{C-x 4 f} and @kbd{C-x 4
@var{anything}} are no longer keys.

  Typing the help character (@kbd{C-h} or @key{F1}) after a prefix key
displays a list of the commands starting with that prefix.  There are
a few prefix keys after which @kbd{C-h} does not work---for historical
reasons, they define other meanings for @kbd{C-h} which are painful to
change.  @key{F1} works after all prefix keys.

@node Commands, Text Characters, Keys, Top
@section Keys and Commands

@cindex binding
@cindex command
@cindex function definition
  This manual is full of passages that tell you what particular keys
do.  But Emacs does not assign meanings to keys directly.  Instead,
Emacs assigns meanings to named @dfn{commands}, and then gives keys
their meanings by @dfn{binding} them to commands.

  Every command has a name chosen by a programmer.  The name is
usually made of a few English words separated by dashes; for example,
@code{next-line} or @code{forward-word}.  A command also has a
@dfn{function definition} which is a Lisp program; this is how the
command does its work.  In Emacs Lisp, a command is a Lisp function with
special options to read arguments and for interactive use.  For more
information on commands and functions, see @ref{What Is a Function,,
What Is a Function, elisp, The Emacs Lisp Reference Manual}.  (The
definition here is simplified slightly.)

  The bindings between keys and commands are recorded in tables called
@dfn{keymaps}.  @xref{Keymaps}.

  When we say that ``@kbd{C-n} moves down vertically one line'' we are
glossing over a subtle distinction that is irrelevant in ordinary use,
but vital for Emacs customization.  The command @code{next-line} does
a vertical move downward.  @kbd{C-n} has this effect @emph{because} it
is bound to @code{next-line}.  If you rebind @kbd{C-n} to the command
@code{forward-word}, @kbd{C-n} will move forward one word instead.
Rebinding keys is an important method of customization.

  In the rest of this manual, we usually ignore this distinction to
keep things simple.  We will often speak of keys like @kbd{C-n} as
commands, even though strictly speaking the key is bound to a command.
Usually we state the name of the command which really does the work in
parentheses after mentioning the key that runs it.  For example, we
will say that ``The command @kbd{C-n} (@code{next-line}) moves point
vertically down,'' meaning that the command @code{next-line} moves
vertically down, and the key @kbd{C-n} is normally bound to it.

  Since we are discussing customization, we should tell you about
@dfn{variables}.  Often the description of a command will say, ``To
change this, set the variable @code{mumble-foo}.''  A variable is a
name used to store a value.  Most of the variables documented in this
manual are meant for customization: some command or other part of
Emacs examines the variable and behaves differently according to the
value that you set.  You can ignore the information about variables
until you are interested in customizing them.  Then read the basic
information on variables (@pxref{Variables}) and the information about
specific variables will make sense.

@node Text Characters, Entering Emacs, Commands, Top
@section Character Set for Text
@cindex characters (in text)

  Text in Emacs buffers is a sequence of characters.  In the simplest
case, these are @acronym{ASCII} characters, each stored in one 8-bit
byte.  Both @acronym{ASCII} control characters (octal codes 000
through 037, and 0177) and @acronym{ASCII} printing characters (codes
040 through 0176) are allowed.  The other modifier flags used in
keyboard input, such as Meta, are not allowed in buffers.

  Non-@acronym{ASCII} printing characters can also appear in buffers,
when multibyte characters are enabled.  They have character codes
starting at 256, octal 0400, and each one is represented as a sequence
of two or more bytes.  @xref{International}.  Single-byte characters
with codes 128 through 255 can also appear in multibyte buffers.
However, non-@acronym{ASCII} control characters cannot appear in a
buffer.

  Some @acronym{ASCII} control characters serve special purposes in text, and have
special names.  For example, the newline character (octal code 012) is
used in the buffer to end a line, and the tab character (octal code 011)
is used for indenting to the next tab stop column (normally every 8
columns).  @xref{Text Display}.

  If you disable multibyte characters, then you can use only one
alphabet of non-@acronym{ASCII} characters, which all fit in one byte.
They use octal codes 0200 through 0377.  @xref{Unibyte Mode}.

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