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There are many Emacs commands which operate on an arbitrary contiguous part of the current buffer. To specify the text for such a command to operate on, you set the mark at one end of it, and move point to the other end. The text between point and the mark is called the region. You can move point or the mark to adjust the boundaries of the region. It doesn't matter which one is set first chronologically, or which one comes earlier in the text.
Once the mark has been set, it remains where you put it until it is set again at another place. The mark remains fixed with respect to the preceding character if text is inserted or deleted in the buffer. Each Emacs buffer has its own mark, so that when you return to a buffer that had been selected previously, it has the same mark it had before.
Many commands that insert text, such as C-y (yank) and
M-x insert-buffer, position point and the mark at opposite ends of
the inserted text, so that the region contains the text just inserted.
Aside from delimiting the region, the mark is also useful for remembering a spot that you may want to go back to. To make this feature more useful, each buffer remembers 16 previous locations of the mark in the mark ring.
Here are some commands for setting the mark:
set-mark-command).
exchange-point-and-mark).
For example, suppose you wish to convert part of the buffer to all
upper-case, using the C-x C-u (upcase-region) command
which operates on the text in the region. You can first go to the
beginning of the text to be capitalized, type C-SPC to put
the mark there, move to the end, and then type C-x C-u. Or, you
can set the mark at the end of the text, move to the beginning, and then
type C-x C-u.
The most common way to set the mark is with the C-SPC command
(set-mark-command). This sets the mark where point is. Then you
can move point away, leaving the mark behind.
Ordinary terminals have only one cursor, so there is no way for Emacs
to show you where the mark is located. You have to remember. The usual
solution to this problem is to set the mark and then use it soon, before
you forget where it is. Alternatively, you can see where the mark is
with the command C-x C-x (exchange-point-and-mark) which
puts the mark where point was and point where the mark was. The extent
of the region is unchanged, but the cursor and point are now at the
previous position of the mark.
C-x C-x is also useful when you are satisfied with the position
of point but want to move the mark; do C-x C-x to put point at
that end of the region, and then move it. A second use of C-x
C-x, if necessary, puts the mark at the new position with point back at
its original position.
There is no such character as C-SPC in ASCII; when you
type SPC while holding down CTRL, what you get on most
ordinary terminals is the character C-@. This key is actually
bound to set-mark-command. But unless you are unlucky enough to
have a terminal where typing C-SPC does not produce
C-@, you might as well think of this character as
C-SPC. Under X, C-SPC is actually a distinct
character, but its binding is still set-mark-command.
Many Emacs commands move the mark and invisibly set new regions. This means that there is almost always some region that you can act on. This is convenient, provided you get used to keeping track of the mark's position.
Some people prefer a more rigid mode of operation in which you must
set up a region for each command that uses one--in which the region
"lasts" only temporarily. This is called Transient Mark mode. It
is particularly well-suited to window systems such as X, since Emacs
can highlight the region when it is active.
To enable Transient Mark mode, type M-x transient-mark-mode. This command toggles the mode, so you can issue it again to return to the normal Emacs way of handling the mark and the region.
Here are the details of Transient Mark mode:
set-mark-command).
This makes the mark active; as you move point, you will see the region
highlighting change in extent.
exchange-point-and-mark).
Transient Mark mode is also sometimes known as "Zmacs mode" because the Zmacs editor on the MIT Lisp Machine handled the mark in a similar way.
When multiple windows show the same buffer, they can have different regions, because they can have different values of point. In Transient Mark mode, each window highlights its own region. The part that is highlighted in the selected window is the region that editing commands use. See section Multiple Windows.
Once you have set up a region and the mark is active, you can do many things to the text in it:
Most commands that operate on the text in the
region have the word region in their names.
Here are the commands for placing point and the mark around a textual object such as a word, list, paragraph or page.
mark-word). This command and
the following one do not move point.
mark-sexp).
mark-paragraph).
mark-defun).
mark-whole-buffer).
mark-page).
M-@ (mark-word) puts the mark at the end of the next word,
while C-M-@ (mark-sexp) puts it at the end of the next Lisp
expression. These commands handle arguments just like M-f and
C-M-f.
Other commands set both point and mark, to delimit an object in the
buffer. For example, M-h (mark-paragraph) moves point to
the beginning of the paragraph that surrounds or follows point, and puts
the mark at the end of that paragraph (see section Paragraphs). It prepares
the region so you can indent, case-convert, or kill a whole paragraph.
C-M-h (mark-defun) similarly puts point before and the
mark after the current or following defun (see section Defuns). C-x
C-p (mark-page) puts point before the current page, and mark at
the end (see section Pages). The mark goes after the terminating page
delimiter (to include it), while point goes after the preceding page
delimiter (to exclude it). A numeric argument specifies a later page
(if positive) or an earlier page (if negative) instead of the current
page.
Finally, C-x h (mark-whole-buffer) sets up the entire
buffer as the region, by putting point at the beginning and the mark at
the end.
Aside from delimiting the region, the mark is also useful for
remembering a spot that you may want to go back to. To make this
feature more useful, each buffer remembers 16 previous locations of the
mark, in the mark ring. Commands that set the mark also push the
old mark onto this ring. To return to a marked location, use C-u
C-SPC (or C-u C-@); this is the command
set-mark-command given a numeric argument. It moves point to
where the mark was, and restores the mark from the ring of former
marks. Thus, repeated use of this command moves point to all of the old
marks on the ring, one by one. The mark positions you move through in
this way are not lost; they go to the end of the ring.
Each buffer has its own mark ring. All editing commands use the current buffer's mark ring. In particular, C-u C-SPC always stays in the same buffer.
Many commands that can move long distances, such as M-<
(beginning-of-buffer), start by setting the mark and saving the
old mark on the mark ring. This is to make it easier for you to move
back later. Searches set the mark if they move point. You can tell
when a command sets the mark because it displays `Mark Set' in the
echo area.
If you want to move back to the same place over and over, the mark
ring may not be convenient enough. If so, you can record the position
in a register for later retrieval (see section Saving Positions in Registers).
The variable mark-ring-max specifies the maximum number of
entries to keep in the mark ring. If that many entries exist and
another one is pushed, the last one in the list is discarded. Repeating
C-u C-SPC circulates through the positions currently in the
ring.
The variable mark-ring holds the mark ring itself, as a list of
marker objects in the order most recent first. This variable is local
in every buffer.
Each Emacs buffer has its own mark ring. In addition, Emacs has a single global mark ring.
The global mark ring stores positions in various Emacs buffers. Setting the mark always makes an entry on the current buffer's mark ring. If you have switched buffers since the previous mark setting, the new mark position makes an entry on the global mark ring also. The result is that the global mark ring holds a sequence of buffers that you have been in, and for each buffer, it records a place where you set the mark.
The command C-x C-SPC (pop-global-mark) jumps to
the buffer and position of the latest entry in the global ring. It also
rotates the ring, so that successive uses of C-x C-SPC take
you to earlier and earlier buffers.
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