Case is significant when completing case-sensitive arguments, such as command names. For example, when completing command names, ‘AU’ does not complete to ‘auto-fill-mode’. Case differences are ignored when completing arguments in which case does not matter.
When completing file names, case differences are ignored if the
variable read-file-name-completion-ignore-case is
non-nil. The default value is nil on systems that have
case-sensitive file-names, such as GNU/Linux; it is non-nil on
systems that have case-insensitive file-names, such as Microsoft
Windows. When completing buffer names, case differences are ignored
if the variable read-buffer-completion-ignore-case is
non-nil; the default is nil.
When completing file names, Emacs usually omits certain alternatives
that are considered unlikely to be chosen, as determined by the list
variable completion-ignored-extensions. Each element in the
list should be a string; any file name ending in such a string is
ignored as a completion alternative. Any element ending in a slash
(/) represents a subdirectory name. The standard value of
completion-ignored-extensions has several elements including
".o", ".elc", and "~". For example, if a
directory contains ‘foo.c’ and ‘foo.elc’, ‘foo’
completes to ‘foo.c’. However, if all possible
completions end in otherwise-ignored strings, they are not ignored: in the
previous example, ‘foo.e’ completes to ‘foo.elc’. Emacs
disregards completion-ignored-extensions when showing
completion alternatives in the completion list.
Shell completion is an extended version of filename completion, see Shell Mode Options.
If completion-auto-help is set to nil, the completion
commands never display the completion list buffer; you must type
? to display the list. If the value is lazy, Emacs only
shows the completion list buffer on the second attempt to complete.
In other words, if there is nothing to complete, the first TAB
echoes ‘Next char not unique’; the second TAB shows the
completion list buffer. If the value is always, the completion
list buffer is always shown when completion is attempted.
The display of the completion list buffer after it is shown for the
first time is also controlled by completion-auto-help. If the
value is t or lazy, the window showing the completions
pops down when Emacs is able to complete (and may pop up again if
Emacs is again unable to complete after you type some more text); if
the value is always, the window pops down only when you exit
the completion. The value visible is a hybrid: it behaves like
t when it decides whether to pop up the window showing the
completion list buffer, and like always when it decides whether
to pop it down.
Emacs can optionally select the window showing the completions when
it shows that window. To enable this behavior, customize the user
option completion-auto-select to t, which changes the
behavior of TAB when Emacs pops up the completions: pressing
TAB will switch to the completion list buffer, and you can
then move to a candidate by cursor motion commands and select it with
RET. If the value of completion-auto-select is
second-tab, then the first TAB will pop up the
completions list buffer, and the second one will switch to it.
If completion-cycle-threshold is non-nil, completion
commands can cycle through completion alternatives. Normally, if
there is more than one completion alternative for the text in the
minibuffer, a completion command completes up to the longest common
substring. If you change completion-cycle-threshold to
t, the completion command instead completes to the first of
those completion alternatives; each subsequent invocation of the
completion command replaces that with the next completion alternative,
in a cyclic manner. If you give completion-cycle-threshold a
numeric value n, completion commands switch to this cycling
behavior only when there are n or fewer alternatives.
When displaying completions, Emacs will normally pop up a new buffer
to display the completions. The completions will by default be sorted
horizontally, using as many columns as will fit in the window-width,
but this can be changed by customizing the completions-format
user option. If its value is vertical, Emacs will sort the
completions vertically instead, and if it’s one-column, Emacs
will use just one column.
The completions-sort user option controls the order in which
the completions are sorted in the ‘*Completions*’ buffer. The
default is alphabetical, which sorts in alphabetical order.
The value nil disables sorting. The value can also be a
function, which will be called with the list of completions, and
should return the list in the desired order.
When completions-max-height is non-nil, it limits the
size of the completions window. It is specified in lines and include
mode, header line and a bottom divider, if any. For a more complex
control of the Completion window display properties, you can use
display-buffer-alist (see Action Alists for Buffer Display in The Emacs Lisp
Reference Manual).
The variable completions-header-format is a format spec string to
control the informative line shown before the completions list of
candidates. If it contains a ‘%s’ construct, that get replaced
by the number of completions shown in the completion list buffer. To
suppress the display of the heading line, customize this variable to
nil. The string that is the value of this variable can have
text properties to change the visual appearance of the heading line;
some useful properties face or cursor-intangible
(see Properties with Special Meanings in The Emacs Lisp Reference Manual).
When completions-highlight-face names a face, the current
completion candidate, the one that will be selected by typing
RET or clicking the mouse, will be highlighted using that
face. The default value of this variable is
completions-highlight; the value is nil disables this
highlighting. This feature uses the special text property
cursor-face.