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In most ways, a minibuffer is a normal Emacs buffer. Most operations within a buffer, such as editing commands, work normally in a minibuffer. However, many operations for managing buffers do not apply to minibuffers. The name of a minibuffer always has the form ‘ *Minibuf-number*’, and it cannot be changed. Minibuffers are displayed only in special windows used only for minibuffers; these windows always appear at the bottom of a frame. (Sometimes frames have no minibuffer window, and sometimes a special kind of frame contains nothing but a minibuffer window; see Minibuffers and Frames.)
The text in the minibuffer always starts with the prompt string,
the text that was specified by the program that is using the minibuffer
to tell the user what sort of input to type. This text is marked
read-only so you won't accidentally delete or change it. It is also
marked as a field (see Fields), so that certain motion functions,
including beginning-of-line, forward-word,
forward-sentence, and forward-paragraph, stop at the
boundary between the prompt and the actual text.
The minibuffer's window is normally a single line; it grows
automatically if the contents require more space. Whilst the minibuffer
is active, you can explicitly resize its window temporarily with the
window sizing commands; the window reverts to its normal size when the
minibuffer is exited. When the minibuffer is not active, you can resize
its window permanently by using the window sizing commands in the
frame's other window, or dragging the mode line with the mouse. (Due to
details of the current implementation, for this to work
resize-mini-windows must be nil.) If the frame contains
just a minibuffer window, you can change its size by changing the
frame's size.
Use of the minibuffer reads input events, and that alters the values
of variables such as this-command and last-command
(see Command Loop Info). Your program should bind them around the
code that uses the minibuffer, if you do not want that to change them.
Under some circumstances, a command can use a minibuffer even if
there is an active minibuffer; such a minibuffer is called a
recursive minibuffer. The first minibuffer is named
‘ *Minibuf-1*’. Recursive minibuffers are named by
incrementing the number at the end of the name. (The names begin with
a space so that they won't show up in normal buffer lists.) Of
several recursive minibuffers, the innermost (or most recently
entered) is the active minibuffer. We usually call this the
minibuffer. You can permit or forbid recursive minibuffers by setting
the variable enable-recursive-minibuffers, or by putting
properties of that name on command symbols (See Recursive Mini.)
Like other buffers, a minibuffer uses a local keymap (see Keymaps) to specify special key bindings. The function that invokes the minibuffer also sets up its local map according to the job to be done. See Text from Minibuffer, for the non-completion minibuffer local maps. See Completion Commands, for the minibuffer local maps for completion.
When a minibuffer is inactive, its major mode is
minibuffer-inactive-mode, with keymap
minibuffer-inactive-mode-map. This is only really useful if
the minibuffer is in a separate frame. See Minibuffers and Frames.
When Emacs is running in batch mode, any request to read from the minibuffer actually reads a line from the standard input descriptor that was supplied when Emacs was started. This supports only basic input: none of the special minibuffer features (history, completion, etc.) are available in batch mode.