The operating system stores data permanently in named files. So most of the text you edit with Emacs comes from a file and is ultimately stored in a file.
To edit a file, you must tell Emacs to read the file and prepare a buffer containing a copy of the file's text. This is called visiting the file. Editing commands apply directly to text in the buffer; that is, to the copy inside Emacs. Your changes appear in the file itself only when you save the buffer back into the file.
In addition to visiting and saving files, Emacs can delete, copy, rename, and append to files, keep multiple versions of them, and operate on file directories.
Most Emacs commands that operate on a file require you to specify the file name. (Saving and reverting are exceptions; the buffer knows which file name to use for them.) You enter the file name using the minibuffer (see section The Minibuffer). Completion is available, to make it easier to specify long file names. See section Completion.
For most operations, there is a default file name which is used if you type just RET to enter an empty argument. Normally the default file name is the name of the file visited in the current buffer; this makes it easy to operate on that file with any of the Emacs file commands.
Each buffer has a default directory, normally the same as the
directory of the file visited in that buffer. When you enter a file
name without a directory, the default directory is used. If you specify
a directory in a relative fashion, with a name that does not start with
a slash, it is interpreted with respect to the default directory. The
default directory is kept in the variable default-directory,
which has a separate value in every buffer.
For example, if the default file name is `/u/rms/gnu/gnu.tasks' then the default directory is `/u/rms/gnu/'. If you type just `foo', which does not specify a directory, it is short for `/u/rms/gnu/foo'. `../.login' would stand for `/u/rms/.login'. `new/foo' would stand for the file name `/u/rms/gnu/new/foo'.
The command M-x pwd prints the current buffer's default
directory, and the command M-x cd sets it (to a value read using
the minibuffer). A buffer's default directory changes only when the
cd command is used. A file-visiting buffer's default directory
is initialized to the directory of the file that is visited there. If
you create a buffer with C-x b, its default directory is copied
from that of the buffer that was current at the time.
The default directory actually appears in the minibuffer when the
minibuffer becomes active to read a file name. This serves two
purposes: it shows you what the default is, so that you can type
a relative file name and know with certainty what it will mean, and it
allows you to edit the default to specify a different directory.
This insertion of the default directory is inhibited if the variable
insert-default-directory is set to nil.
Note that it is legitimate to type an absolute file name after you enter the minibuffer, ignoring the presence of the default directory name as part of the text. The final minibuffer contents may look invalid, but that is not so. For example, if the minibuffer starts out with `/usr/tmp/' and you add `/x1/rms/foo', you get `/usr/tmp//x1/rms/foo'; but Emacs ignores everything through the first slash in the double slash; the result is `/x1/rms/foo'. See section Minibuffers for File Names.
`$' in a file name is used to substitute environment variables.
For example, if you have used the shell command `export
FOO=rms/hacks' to set up an environment variable named FOO, then
you can use `/u/$FOO/test.c' or `/u/${FOO}/test.c' as an
abbreviation for `/u/rms/hacks/test.c'. The environment variable
name consists of all the alphanumeric characters after the `$';
alternatively, it may be enclosed in braces after the `$'. Note
that shell commands to set environment variables affect Emacs only if
done before Emacs is started.
To access a file with `$' in its name, type `$$'. This pair is converted to a single `$' at the same time as variable substitution is performed for single `$'. Alternatively, quote the whole file name with `/:' (see section Quoted File Names).
The Lisp function that performs the substitution is called
substitute-in-file-name. The substitution is performed only on
file names read as such using the minibuffer.
You can include non-ASCII characters in file names if you set the
variable file-name-coding-system to a non-nil value.
See section Specifying a Coding System.
find-file).
find-file-read-only).
find-alternate-file).
find-file-other-window). Don't
alter what is displayed in the selected window.
find-file-other-frame). Don't
alter what is displayed in the selected frame.
Visiting a file means copying its contents into an Emacs buffer so you can edit them. Emacs makes a new buffer for each file that you visit. We say that this buffer is visiting the file that it was created to hold. Emacs constructs the buffer name from the file name by throwing away the directory, keeping just the name proper. For example, a file named `/usr/rms/emacs.tex' would get a buffer named `emacs.tex'. If there is already a buffer with that name, a unique name is constructed by appending `<2>', `<3>', or so on, using the lowest number that makes a name that is not already in use.
Each window's mode line shows the name of the buffer that is being displayed in that window, so you can always tell what buffer you are editing.
The changes you make with editing commands are made in the Emacs buffer. They do not take effect in the file that you visited, or any place permanent, until you save the buffer. Saving the buffer means that Emacs writes the current contents of the buffer into its visited file. See section Saving Files.
If a buffer contains changes that have not been saved, we say the buffer is modified. This is important because it implies that some changes will be lost if the buffer is not saved. The mode line displays two stars near the left margin to indicate that the buffer is modified.
To visit a file, use the command C-x C-f (find-file). Follow
the command with the name of the file you wish to visit, terminated by a
RET.
The file name is read using the minibuffer (see section The Minibuffer), with defaulting and completion in the standard manner (see section File Names). While in the minibuffer, you can abort C-x C-f by typing C-g.
Your confirmation that C-x C-f has completed successfully is the appearance of new text on the screen and a new buffer name in the mode line. If the specified file does not exist and could not be created, or cannot be read, then you get an error, with an error message displayed in the echo area.
If you visit a file that is already in Emacs, C-x C-f does not make another copy. It selects the existing buffer containing that file. However, before doing so, it checks that the file itself has not changed since you visited or saved it last. If the file has changed, a warning message is printed. See section Protection against Simultaneous Editing.
What if you want to create a new file? Just visit it. Emacs prints `(New File)' in the echo area, but in other respects behaves as if you had visited an existing empty file. If you make any changes and save them, the file is created.
If the file you specify is actually a directory, C-x C-f invokes
Dired, the Emacs directory browser, so that you can "edit" the contents
of the directory (see section Dired, the Directory Editor). Dired is a convenient way to delete,
look at, or operate on the files in the directory. However, if the
variable find-file-run-dired is nil, then it is an error
to try to visit a directory.
If you visit a file that the operating system won't let you modify,
Emacs makes the buffer read-only, so that you won't go ahead and make
changes that you'll have trouble saving afterward. You can make the
buffer writable with C-x C-q (vc-toggle-read-only).
See section Miscellaneous Buffer Operations.
Occasionally you might want to visit a file as read-only in order to
protect yourself from entering changes accidentally; do so by visiting
the file with the command C-x C-r (find-file-read-only).
If you visit a nonexistent file unintentionally (because you typed the
wrong file name), use the C-x C-v command
(find-alternate-file) to visit the file you really wanted.
C-x C-v is similar to C-x C-f, but it kills the current
buffer (after first offering to save it if it is modified). When it
reads the file name to visit, it inserts the entire default file name in
the buffer, with point just after the directory part; this is convenient
if you made a slight error in typing the name.
If you find a file which exists but cannot be read, C-x C-f signals an error.
C-x 4 f (find-file-other-window) is like C-x C-f
except that the buffer containing the specified file is selected in another
window. The window that was selected before C-x 4 f continues to
show the same buffer it was already showing. If this command is used when
only one window is being displayed, that window is split in two, with one
window showing the same buffer as before, and the other one showing the
newly requested file. See section Multiple Windows.
C-x 5 f (find-file-other-frame) is similar, but opens a
new frame, or makes visible any existing frame showing the file you
seek. This feature is available only when you are using a window
system. See section Frames and X Windows.
If you wish to edit a file as a sequence of characters with no special encoding or conversion, use the M-x find-file-literally command. It visits a file, like C-x C-f, but does not do format conversion (see section Editing Formatted Text), character code conversion (see section Coding Systems), or automatic uncompression (see section Accessing Compressed Files). If you already have visited the same file in the usual (non-literal) manner, this command asks you whether to visit it literally instead.
Two special hook variables allow extensions to modify the operation of
visiting files. Visiting a file that does not exist runs the functions
in the list find-file-not-found-hooks; this variable holds a list
of functions, and the functions are called one by one until one of them
returns non-nil. Any visiting of a file, whether extant or not,
expects find-file-hooks to contain a list of functions and calls
them all, one by one. In both cases the functions receive no
arguments. Of these two variables, find-file-not-found-hooks
takes effect first. These variables are not normal hooks, and
their names end in `-hooks' rather than `-hook' to indicate
that fact. See section Hooks.
There are several ways to specify automatically the major mode for editing the file (see section How Major Modes are Chosen), and to specify local variables defined for that file (see section Local Variables in Files).
Saving a buffer in Emacs means writing its contents back into the file that was visited in the buffer.
save-buffer).
save-some-buffers).
not-modified).
write-file).
When you wish to save the file and make your changes permanent, type
C-x C-s (save-buffer). After saving is finished, C-x C-s
displays a message like this:
Wrote /u/rms/gnu/gnu.tasks
If the selected buffer is not modified (no changes have been made in it since the buffer was created or last saved), saving is not really done, because it would have no effect. Instead, C-x C-s displays a message like this in the echo area:
(No changes need to be saved)
The command C-x s (save-some-buffers) offers to save any
or all modified buffers. It asks you what to do with each buffer. The
possible responses are analogous to those of query-replace:
save-some-buffers without any more saving.
save-some-buffers without even asking
about other buffers.
save-some-buffers, which asks the
question again.
C-x C-c, the key sequence to exit Emacs, invokes
save-some-buffers and therefore asks the same questions.
If you have changed a buffer but you do not want to save the changes,
you should take some action to prevent it. Otherwise, each time you use
C-x s or C-x C-c, you are liable to save this buffer by
mistake. One thing you can do is type M-~ (not-modified),
which clears out the indication that the buffer is modified. If you do
this, none of the save commands will believe that the buffer needs to be
saved. (`~' is often used as a mathematical symbol for `not'; thus
M-~ is `not', metafied.) You could also use
set-visited-file-name (see below) to mark the buffer as visiting
a different file name, one which is not in use for anything important.
Alternatively, you can cancel all the changes made since the file was
visited or saved, by reading the text from the file again. This is
called reverting. See section Reverting a Buffer. You could also undo all the
changes by repeating the undo command C-x u until you have undone
all the changes; but reverting is easier.
M-x set-visited-file-name alters the name of the file that the
current buffer is visiting. It reads the new file name using the
minibuffer. Then it specifies the visited file name and changes the
buffer name correspondingly (as long as the new name is not in use).
set-visited-file-name does not save the buffer in the newly
visited file; it just alters the records inside Emacs in case you do
save later. It also marks the buffer as "modified" so that C-x
C-s in that buffer will save.
If you wish to mark the buffer as visiting a different file and save it
right away, use C-x C-w (write-file). It is precisely
equivalent to set-visited-file-name followed by C-x C-s.
C-x C-s used on a buffer that is not visiting a file has the
same effect as C-x C-w; that is, it reads a file name, marks the
buffer as visiting that file, and saves it there. The default file name in
a buffer that is not visiting a file is made by combining the buffer name
with the buffer's default directory.
If the new file name implies a major mode, then C-x C-w switches
to that major mode, in most cases. The command
set-visited-file-name also does this. See section How Major Modes are Chosen.
If Emacs is about to save a file and sees that the date of the latest version on disk does not match what Emacs last read or wrote, Emacs notifies you of this fact, because it probably indicates a problem caused by simultaneous editing and requires your immediate attention. See section Protection against Simultaneous Editing.
If the variable require-final-newline is non-nil, Emacs
puts a newline at the end of any file that doesn't already end in one,
every time a file is saved or written. The default is nil.
On most operating systems, rewriting a file automatically destroys all
record of what the file used to contain. Thus, saving a file from Emacs
throws away the old contents of the file--or it would, except that
Emacs carefully copies the old contents to another file, called the
backup file, before actually saving. (This assumes that the
variable make-backup-files is non-nil. Backup files are
not written if this variable is nil.) Emacs does not normally
make backup files for files in `/tmp'.
At your option, Emacs can keep either a single backup file or a series of numbered backup files for each file that you edit.
Emacs makes a backup for a file only the first time the file is saved from one buffer. No matter how many times you save a file, its backup file continues to contain the contents from before the file was visited. Normally this means that the backup file contains the contents from before the current editing session; however, if you kill the buffer and then visit the file again, a new backup file will be made by the next save.
You can also explicitly request making another backup file from a buffer even though it has already been saved at least once. If you save the buffer with C-u C-x C-s, the version thus saved will be made into a backup file if you save the buffer again. C-u C-u C-x C-s saves the buffer, but first makes the previous file contents into a new backup file. C-u C-u C-u C-x C-s does both things: it makes a backup from the previous contents, and arranges to make another from the newly saved contents, if you save again.
If you choose to have a single backup file (this is the default), the backup file's name is constructed by appending `~' to the file name being edited; thus, the backup file for `eval.c' would be `eval.c~'.
If you choose to have a series of numbered backup files, backup file names are made by appending `.~', the number, and another `~' to the original file name. Thus, the backup files of `eval.c' would be called `eval.c.~1~', `eval.c.~2~', and so on, through names like `eval.c.~259~' and beyond.
If protection stops you from writing backup files under the usual names, the backup file is written as `%backup%~' in your home directory. Only one such file can exist, so only the most recently made such backup is available.
The choice of single backup or numbered backups is controlled by the
variable version-control. Its possible values are
t
nil
never
You can set version-control locally in an individual buffer to
control the making of backups for that buffer's file. For example,
Rmail mode locally sets version-control to never to make sure
that there is only one backup for an Rmail file. See section Local Variables.
If you set the environment variable VERSION_CONTROL, to tell
various GNU utilities what to do with backup files, Emacs also obeys the
environment variable by setting the Lisp variable version-control
accordingly at startup. If the environment variable's value is `t'
or `numbered', then version-control becomes t; if the
value is `nil' or `existing', then version-control
becomes nil; if it is `never' or `simple', then
version-control becomes never.
For files managed by a version control system (see section Version Control), the variable vc-make-backup-files determines whether
to make backup files. By default, it is nil, since backup files
are redundant when you store all the previous versions in a version
control system. See section VC Workfile Handling.
To prevent unlimited consumption of disk space, Emacs can delete numbered backup versions automatically. Generally Emacs keeps the first few backups and the latest few backups, deleting any in between. This happens every time a new backup is made.
The two variables kept-old-versions and
kept-new-versions control this deletion. Their values are,
respectively the number of oldest (lowest-numbered) backups to keep and
the number of newest (highest-numbered) ones to keep, each time a new
backup is made. Recall that these values are used just after a new
backup version is made; that newly made backup is included in the count
in kept-new-versions. By default, both variables are 2.
If delete-old-versions is non-nil, the excess
middle versions are deleted without a murmur. If it is nil, the
default, then you are asked whether the excess middle versions should
really be deleted.
Dired's . (Period) command can also be used to delete old versions. See section Deleting Files with Dired.
Backup files can be made by copying the old file or by renaming it. This makes a difference when the old file has multiple names. If the old file is renamed into the backup file, then the alternate names become names for the backup file. If the old file is copied instead, then the alternate names remain names for the file that you are editing, and the contents accessed by those names will be the new contents.
The method of making a backup file may also affect the file's owner and group. If copying is used, these do not change. If renaming is used, you become the file's owner, and the file's group becomes the default (different operating systems have different defaults for the group).
Having the owner change is usually a good idea, because then the owner
always shows who last edited the file. Also, the owners of the backups
show who produced those versions. Occasionally there is a file whose
owner should not change; it is a good idea for such files to contain
local variable lists to set backup-by-copying-when-mismatch
locally (see section Local Variables in Files).
The choice of renaming or copying is controlled by three variables.
Renaming is the default choice. If the variable
backup-by-copying is non-nil, copying is used. Otherwise,
if the variable backup-by-copying-when-linked is non-nil,
then copying is used for files that have multiple names, but renaming
may still be used when the file being edited has only one name. If the
variable backup-by-copying-when-mismatch is non-nil, then
copying is used if renaming would cause the file's owner or group to
change. backup-by-copying-when-mismatch is t by default
if you start Emacs as the superuser.
When a file is managed with a version control system (see section Version Control), Emacs does not normally make backups in the usual way for that file. But check-in and check-out are similar in some ways to making backups. One unfortunate similarity is that these operations typically break hard links, disconnecting the file name you visited from any alternate names for the same file. This has nothing to do with Emacs--the version control system does it.
Simultaneous editing occurs when two users visit the same file, both make changes, and then both save them. If nobody were informed that this was happening, whichever user saved first would later find that his changes were lost.
On some systems, Emacs notices immediately when the second user starts to change the file, and issues an immediate warning. On all systems, Emacs checks when you save the file, and warns if you are about to overwrite another user's changes. You can prevent loss of the other user's work by taking the proper corrective action instead of saving the file.
When you make the first modification in an Emacs buffer that is visiting a file, Emacs records that the file is locked by you. (It does this by creating a symbolic link in the same directory with a different name.) Emacs removes the lock when you save the changes. The idea is that the file is locked whenever an Emacs buffer visiting it has unsaved changes.
If you begin to modify the buffer while the visited file is locked by
someone else, this constitutes a collision. When Emacs detects a
collision, it asks you what to do, by calling the Lisp function
ask-user-about-lock. You can redefine this function for the sake
of customization. The standard definition of this function asks you a
question and accepts three possible answers:
file-locked) and the modification you
were trying to make in the buffer does not actually take place.
Note that locking works on the basis of a file name; if a file has multiple names, Emacs does not realize that the two names are the same file and cannot prevent two users from editing it simultaneously under different names. However, basing locking on names means that Emacs can interlock the editing of new files that will not really exist until they are saved.
Some systems are not configured to allow Emacs to make locks, and there are cases where lock files cannot be written. In these cases, Emacs cannot detect trouble in advance, but it still can detect the collision when you try to save a file and overwrite someone else's changes.
If Emacs or the operating system crashes, this may leave behind lock files which are stale. So you may occasionally get warnings about spurious collisions. When you determine that the collision is spurious, just use p to tell Emacs to go ahead anyway.
Every time Emacs saves a buffer, it first checks the last-modification date of the existing file on disk to verify that it has not changed since the file was last visited or saved. If the date does not match, it implies that changes were made in the file in some other way, and these changes are about to be lost if Emacs actually does save. To prevent this, Emacs prints a warning message and asks for confirmation before saving. Occasionally you will know why the file was changed and know that it does not matter; then you can answer yes and proceed. Otherwise, you should cancel the save with C-g and investigate the situation.
The first thing you should do when notified that simultaneous editing
has already taken place is to list the directory with C-u C-x C-d
(see section File Directories). This shows the file's current author. You
should attempt to contact him to warn him not to continue editing.
Often the next step is to save the contents of your Emacs buffer under a
different name, and use diff to compare the two files.
If you have made extensive changes to a file and then change your mind about them, you can get rid of them by reading in the previous version of the file. To do this, use M-x revert-buffer, which operates on the current buffer. Since reverting a buffer unintentionally could lose a lot of work, you must confirm this command with yes.
revert-buffer keeps point at the same distance (measured in
characters) from the beginning of the file. If the file was edited only
slightly, you will be at approximately the same piece of text after
reverting as before. If you have made drastic changes, the same value of
point in the old file may address a totally different piece of text.
Reverting marks the buffer as "not modified" until another change is made.
Some kinds of buffers whose contents reflect data bases other than files,
such as Dired buffers, can also be reverted. For them, reverting means
recalculating their contents from the appropriate data base. Buffers
created explicitly with C-x b cannot be reverted; revert-buffer
reports an error when asked to do so.
When you edit a file that changes automatically and frequently--for example, a log of output from a process that continues to run--it may be useful for Emacs to revert the file without querying you, whenever you visit the file again with C-x C-f.
To request this behavior, set the variable revert-without-query
to a list of regular expressions. When a file name matches one of these
regular expressions, find-file and revert-buffer will
revert it automatically if it has changed--provided the buffer itself
is not modified. (If you have edited the text, it would be wrong to
discard your changes.)
Emacs saves all the visited files from time to time (based on counting your keystrokes) without being asked. This is called auto-saving. It prevents you from losing more than a limited amount of work if the system crashes.
When Emacs determines that it is time for auto-saving, each buffer is considered, and is auto-saved if auto-saving is turned on for it and it has been changed since the last time it was auto-saved. The message `Auto-saving...' is displayed in the echo area during auto-saving, if any files are actually auto-saved. Errors occurring during auto-saving are caught so that they do not interfere with the execution of commands you have been typing.
Auto-saving does not normally save in the files that you visited, because it can be very undesirable to save a program that is in an inconsistent state when you have made half of a planned change. Instead, auto-saving is done in a different file called the auto-save file, and the visited file is changed only when you request saving explicitly (such as with C-x C-s).
Normally, the auto-save file name is made by appending `#' to the
front and rear of the visited file name. Thus, a buffer visiting file
`foo.c' is auto-saved in a file `#foo.c#'. Most buffers that
are not visiting files are auto-saved only if you request it explicitly;
when they are auto-saved, the auto-save file name is made by appending
`#%' to the front and `#' to the rear of buffer name. For
example, the `*mail*' buffer in which you compose messages to be
sent is auto-saved in a file named `#%*mail*#'. Auto-save file
names are made this way unless you reprogram parts of Emacs to do
something different (the functions make-auto-save-file-name and
auto-save-file-name-p). The file name to be used for auto-saving
in a buffer is calculated when auto-saving is turned on in that buffer.
When you delete a substantial part of the text in a large buffer, auto save turns off temporarily in that buffer. This is because if you deleted the text unintentionally, you might find the auto-save file more useful if it contains the deleted text. To reenable auto-saving after this happens, save the buffer with C-x C-s, or use C-u 1 M-x auto-save.
If you want auto-saving to be done in the visited file, set the variable
auto-save-visited-file-name to be non-nil. In this mode,
there is really no difference between auto-saving and explicit saving.
A buffer's auto-save file is deleted when you save the buffer in its
visited file. To inhibit this, set the variable delete-auto-save-files
to nil. Changing the visited file name with C-x C-w or
set-visited-file-name renames any auto-save file to go with
the new visited name.
Each time you visit a file, auto-saving is turned on for that file's
buffer if the variable auto-save-default is non-nil (but not
in batch mode; see section Entering and Exiting Emacs). The default for this variable is
t, so auto-saving is the usual practice for file-visiting buffers.
Auto-saving can be turned on or off for any existing buffer with the
command M-x auto-save-mode. Like other minor mode commands, M-x
auto-save-mode turns auto-saving on with a positive argument, off with a
zero or negative argument; with no argument, it toggles.
Emacs does auto-saving periodically based on counting how many characters
you have typed since the last time auto-saving was done. The variable
auto-save-interval specifies how many characters there are between
auto-saves. By default, it is 300.
Auto-saving also takes place when you stop typing for a while. The
variable auto-save-timeout says how many seconds Emacs should
wait before it does an auto save (and perhaps also a garbage
collection). (The actual time period is longer if the current buffer is
long; this is a heuristic which aims to keep out of your way when you
are editing long buffers, in which auto-save takes an appreciable amount
of time.) Auto-saving during idle periods accomplishes two things:
first, it makes sure all your work is saved if you go away from the
terminal for a while; second, it may avoid some auto-saving while you
are actually typing.
Emacs also does auto-saving whenever it gets a fatal error. This includes killing the Emacs job with a shell command such as `kill %emacs', or disconnecting a phone line or network connection.
You can request an auto-save explicitly with the command M-x do-auto-save.
You can use the contents of an auto-save file to recover from a loss of data with the command M-x recover-file RET file RET. This visits file and then (after your confirmation) restores the contents from its auto-save file `#file#'. You can then save with C-x C-s to put the recovered text into file itself. For example, to recover file `foo.c' from its auto-save file `#foo.c#', do:
M-x recover-file RET foo.c RET yes RET C-x C-s
Before asking for confirmation, M-x recover-file displays a directory listing describing the specified file and the auto-save file, so you can compare their sizes and dates. If the auto-save file is older, M-x recover-file does not offer to read it.
If Emacs or the computer crashes, you can recover all the files you were editing from their auto save files with the command M-x recover-session. This first shows you a list of recorded interrupted sessions. Move point to the one you choose, and type C-c C-c.
Then recover-session asks about each of the files that were
being edited during that session, asking whether to recover that file.
If you answer y, it calls recover-file, which works in its
normal fashion. It shows the dates of the original file and its
auto-save file, and asks once again whether to recover that file.
When recover-session is done, the files you've chosen to
recover are present in Emacs buffers. You should then save them. Only
this--saving them--updates the files themselves.
Interrupted sessions are recorded for later recovery in files named
`~/.saves-pid-hostname'. The `~/.saves' portion of
these names comes from the value of auto-save-list-file-prefix.
You can arrange to record sessions in a different place by setting that
variable in your `.emacs' file, but you'll have to redefine
recover-session as well to make it look in the new place. If you
set auto-save-list-file-prefix to nil in your
`.emacs' file, sessions are not recorded for recovery.
Symbolic links and hard links both make it possible for several file names to refer to the same file. Hard links are alternate names that refer directly to the file; all the names are equally valid, and no one of them is preferred. By contrast, a symbolic link is a kind of defined alias: when `foo' is a symbolic link to `bar', you can use either name to refer to the file, but `bar' is the real name, while `foo' is just an alias. More complex cases occur when symbolic links point to directories.
If you visit two names for the same file, normally Emacs makes two different buffers, but it warns you about the situation.
If you wish to avoid visiting the same file in two buffers under
different names, set the variable find-file-existing-other-name
to a non-nil value. Then find-file uses the existing
buffer visiting the file, no matter which of the file's names you
specify.
If the variable find-file-visit-truename is non-nil,
then the file name recorded for a buffer is the file's truename
(made by replacing all symbolic links with their target names), rather
than the name you specify. Setting find-file-visit-truename also
implies the effect of find-file-existing-other-name.
Version control systems are packages that can record multiple versions of a source file, usually storing the unchanged parts of the file just once. Version control systems also record history information such as the creation time of each version, who created it, and a description of what was changed in that version.
The Emacs version control interface is called VC. Its commands work with three version control systems--RCS, CVS and SCCS. The GNU project recommends RCS and CVS, which are free software and available from the Free Software Foundation.
VC allows you to use a version control system from within Emacs, integrating the version control operations smoothly with editing. VC provides a uniform interface to version control, so that regardless of which version control system is in use, you can use it the same way.
This section provides a general overview of version control, and describes the version control systems that VC supports. You can skip this section if you are already familiar with the version control system you want to use.
VC currently works with three different version control systems or "back ends": RCS, CVS, and SCCS.
RCS is a free version control system that is available from the Free Software Foundation. It is perhaps the most mature of the supported back ends, and the VC commands are conceptually closest to RCS. Almost everything you can do with RCS can be done through VC.
CVS is built on top of RCS, and extends the features of RCS, allowing for more sophisticated release management, and concurrent multi-user development. VC supports basic editing operations under CVS, but for some less common tasks you still need to call CVS from the command line. Note also that before using CVS you must set up a repository, which is a subject too complex to treat here.
SCCS is a proprietary but widely used version control system. In terms of capabilities, it is the weakest of the three that VC supports. VC compensates for certain features missing in SCCS (snapshots, for example) by implementing them itself, but some other VC features, such as multiple branches, are not available with SCCS. You should use SCCS only if for some reason you cannot use RCS.
When a file is under version control, we also say that it is registered in the version control system. Each registered file has a corresponding master file which represents the file's present state plus its change history--enough to reconstruct the current version or any earlier version. Usually the master file also records a log entry for each version, describing in words what was changed in that version.
The file that is maintained under version control is sometimes called the work file corresponding to its master file. You edit the work file and make changes in it, as you would with an ordinary file. (With SCCS and RCS, you must lock the file before you start to edit it.) After you are done with a set of changes, you check the file in, which records the changes in the master file, along with a log entry for them.
With CVS, there are usually multiple work files corresponding to a single master file--often each user has his own copy. It is also possible to use RCS in this way, but this is not the usual way to use RCS.
A version control system typically has some mechanism to coordinate between users who want to change the same file. One method is locking (analogous to the locking that Emacs uses to detect simultaneous editing of a file, but distinct from it). The other method is to merge your changes with other people's changes when you check them in.
With version control locking, work files are normally read-only so that you cannot change them. You ask the version control system to make a work file writable for you by locking it; only one user can do this at any given time. When you check in your changes, that unlocks the file, making the work file read-only again. This allows other users to lock the file to make further changes. SCCS always uses locking, and RCS normally does.
The other alternative for RCS is to let each user modify the work file at any time. In this mode, locking is not required, but it is permitted; check-in is still the way to record a new version.
CVS normally allows each user to modify his own copy of the work file at any time, but requires merging with changes from other users at check-in time. However, CVS can also be set up to require locking. (see section Options for VC Backends).
When you visit a file that is under version control, Emacs indicates this on the mode line. For example, `RCS-1.3' says that RCS is used for that file, and the current version is 1.3.
The character between the back-end name and the version number indicates the version control status of the file. `-' means that the work file is not locked (if locking is in use), or not modified (if locking is not in use). `:' indicates that the file is locked, or that it is modified. If the file is locked by some other user (for instance, `jim'), that is displayed as `RCS:jim:1.3'.
The principal VC command is an all-purpose command that performs either locking or check-in, depending on the situation.
Strictly speaking, the command for this job is vc-next-action,
bound to C-x v v. However, the normal meaning of C-x C-q is
to make a read-only buffer writable, or vice versa; we have extended it
to do the same job properly for files managed by version control, by
performing the appropriate version control operations. When you type
C-x C-q on a registered file, it acts like C-x v v.
The precise action of this command depends on the state of the file, and whether the version control system uses locking or not. SCCS and RCS normally use locking; CVS normally does not use locking.
If locking is used for the file (as with SCCS, and RCS in its default mode), C-x C-q can either lock a file or check it in:
These rules also apply when you use CVS in locking mode, except that there is no such thing as stealing a lock.
When there is no locking--the default for CVS--work files are always writable; you do not need to do anything before you begin to edit a file. The status indicator on the mode line is `-' if the file is unmodified; it flips to `:' as soon as you save any changes in the work file.
Here is what C-x C-q does when using CVS:
These rules also apply when you use RCS in the mode that does not require locking, except that automatic merging of changes from the master file is not implemented. Unfortunately, this means that nothing informs you if another user has checked in changes in the same file since you began editing it, and when this happens, his changes will be effectively removed when you check in your version (though they will remain in the master file, so they will not be entirely lost). You must therefore verify the current version is unchanged, before you check in your changes. We hope to eliminate this risk and provide automatic merging with RCS in a future Emacs version.
In addition, locking is possible with RCS even in this mode, although it is not required; C-x C-q with an unmodified file locks the file, just as it does with RCS in its normal (locking) mode.
When you check in changes, C-x C-q first reads a log entry. It pops up a buffer called `*VC-Log*' for you to enter the log entry. When you are finished, type C-c C-c in the `*VC-Log*' buffer. That is when check-in really happens.
To abort check-in, just don't type C-c C-c in that buffer. You can switch buffers and do other editing. As long as you don't try to check in another file, the entry you were editing remains in the `*VC-Log*' buffer, and you can go back to that buffer at any time to complete the check-in.
If you change several source files for the same reason, it is often convenient to specify the same log entry for many of the files. To do this, use the history of previous log entries. The commands M-n, M-p, M-s and M-r for doing this work just like the minibuffer history commands (except that these versions are used outside the minibuffer).
Each time you check in a file, the log entry buffer is put into VC Log
mode, which involves running two hooks: text-mode-hook and
vc-log-mode-hook. See section Hooks.
One of the convenient features of version control is the ability to examine any version of a file, or compare two versions.
To examine an old version in toto, visit the file and then type
C-x v ~ version RET (vc-version-other-window).
This puts the text of version version in a file named
`filename.~version~', and visits it in its own buffer
in a separate window. (In RCS, you can also select an old version
and create a branch from it. See section Multiple Branches of a File.)
But usually it is more convenient to compare two versions of the file,
with the command C-x v = (vc-diff). Plain C-x v =
compares the current buffer contents (saving them in the file if
necessary) with the last checked-in version of the file. C-u C-x v
=, with a numeric argument, reads a file name and two version numbers,
then compares those versions of the specified file.
If you supply a directory name instead of the name of a registered file, this command compares the two specified versions of all registered files in that directory and its subdirectories.
You can specify a checked-in version by its number; an empty input specifies the current contents of the work file (which may be different from all the checked-in versions). You can also specify a snapshot name (see section Snapshots) instead of one or both version numbers.
This command works by running the diff utility, getting the
options from the variable diff-switches. It displays the output
in a special buffer in another window. Unlike the M-x diff
command, C-x v = does not try to locate the changes in the old and
new versions. This is because normally one or both versions do not
exist as files when you compare them; they exist only in the records of
the master file. See section Comparing Files, for more information about
M-x diff.
For CVS-controlled files, you can display the result of the CVS annotate command, using colors to enhance the visual appearance. Use the command M-x vc-annotate to do this. Red means new, blue means old, and intermediate colors indicate intermediate ages. A prefix argument n specifies a stretch factor for the time scale; it makes each color cover a period n times as long.
This section explains the secondary commands of VC; those that you might use once a day.
You can put any file under version control by simply visiting it, and
then typing C-x v i (vc-register).
To register the file, Emacs must choose which version control system
to use for it. You can specify your choice explicitly by setting
vc-default-back-end to RCS, CVS or SCCS.
Otherwise, if there is a subdirectory named `RCS', `SCCS', or
`CVS', Emacs uses the corresponding version control system. In the
absence of any specification, the default choice is RCS if RCS is
installed, otherwise SCCS.
If locking is in use,C-x v i leaves the file unlocked and read-only. Type C-x C-q if you wish to start editing it. After registering a file with CVS, you must subsequently commit the initial version by typing C-x C-q.
The initial version number for a newly registered file is 1.1, by
default. You can specify a different default by setting the variable
vc-default-init-version, or you can give C-x v i a numeric
argument; then it reads the initial version number for this particular
file using the minibuffer.
If vc-initial-comment is non-nil, C-x v i reads an
initial comment to describe the purpose of this source file. Reading
the initial comment works like reading a log entry (see section Features of the Log Entry Buffer).
To view the detailed version control status and history of a file,
type C-x v l (vc-print-log). It displays the history of
changes to the current file, including the text of the log entries. The
output appears in a separate window.
If you want to discard your current set of changes and revert to the
last version checked in, use C-x v u (vc-revert-buffer).
This leaves the file unlocked; if locking is in use, you must first lock
the file again before you change it again. C-x v u requires
confirmation, unless it sees that you haven't made any changes since the
last checked-in version.
C-x v u is also the command to unlock a file if you lock it and then decide not to change it.
To cancel a change that you already checked in, use C-x v c
(vc-cancel-version). This command discards all record of the
most recent checked-in version. C-x v c also offers to revert
your work file and buffer to the previous version (the one that precedes
the version that is deleted).
If you answer no, VC keeps your changes in the buffer, and locks the file. The no-revert option is useful when you have checked in a change and then discover a trivial error in it; you can cancel the erroneous check-in, fix the error, and check the file in again.
When C-x v c does not revert the buffer, it unexpands all version control headers in the buffer instead (see section Inserting Version Control Headers). This is because the buffer no longer corresponds to any existing version. If you check it in again, the check-in process will expand the headers properly for the new version number.
However, it is impossible to unexpand the RCS `$Log$' header automatically. If you use that header feature, you have to unexpand it by hand--by deleting the entry for the version that you just canceled.
Be careful when invoking C-x v c, as it is easy to lose a lot of work with it. To help you be careful, this command always requires confirmation with yes. Note also that this command is disabled under CVS, because canceling versions is very dangerous and discouraged with CVS.
When you are working on a large program, it is often useful to find
out which files have changed within an entire directory tree, or to view
the status of all files under version control at once, and to perform
version control operations on collections of files. You can use the
command C-x v d (vc-directory) to make a directory listing
that includes only files relevant for version control.
C-x v d creates a buffer which uses VC Dired Mode. This looks
much like an ordinary Dired buffer (see section Dired, the Directory Editor); however, normally it
shows only the noteworthy files (those locked or not up-to-date). This
is called terse display. If you set the variable
vc-dired-terse-display to nil, then VC Dired shows all
relevant files--those managed under version control, plus all
subdirectories (full display). The command v t in a VC
Dired buffer toggles between terse display and full display (see section VC Dired Commands).
By default, VC Dired produces a recursive listing of noteworthy or
relevant files at or below the given directory. You can change this by
setting the variable vc-dired-recurse to nil; then VC
Dired shows only the files in the given directory.
The line for an individual file shows the version control state in the place of the hard link count, owner, group, and size of the file. If the file is unmodified, in sync with the master file, the version control state shown is blank. Otherwise it consists of text in parentheses. Under RCS and SCCS, the name of the user locking the file is shown; under CVS, an abbreviated version of the `cvs status' output is used. Here is an example using RCS:
/home/jim/project: -rw-r--r-- (jim) Apr 2 23:39 file1 -r--r--r-- Apr 5 20:21 file2
The files `file1' and `file2' are under version control, `file1' is locked by user jim, and `file2' is unlocked.
Here is an example using CVS:
/home/joe/develop: -rw-r--r-- (modified) Aug 2 1997 file1.c -rw-r--r-- Apr 4 20:09 file2.c -rw-r--r-- (merge) Sep 13 1996 file3.c
Here `file1.c' is modified with respect to the repository, and `file2.c' is not. `file3.c' is modified, but other changes have also been checked in to the repository--you need to merge them with the work file before you can check it in.
When VC Dired displays subdirectories (in the "full" display mode),
it omits some that should never contain any files under version control.
By default, this includes Version Control subdirectories such as
`RCS' and `CVS'; you can customize this by setting the
variable vc-directory-exclusion-list.
You can fine-tune VC Dired's format by typing C-u x v d---as in ordinary Dired, that allows you to specify additional switches for the `ls' command.
All the usual Dired commands work normally in VC Dired mode, except
for v, which is redefined as the version control prefix. You can
invoke VC commands such as vc-diff and vc-print-log by
typing v =, or v l, and so on. Most of these commands apply
to the file name on the current line.
The command v v (vc-next-action) operates on all the
marked files, so that you can lock or check in several files at once.
If it operates on more than one file, it handles each file according to
its current state; thus, it might lock one file, but check in another
file. This could be confusing; it is up to you to avoid confusing
behavior by marking a set of files that are in a similar state.
If any files call for check-in, v v reads a single log entry, then uses it for all the files being checked in. This is convenient for registering or checking in several files at once, as part of the same change.
You can toggle between terse display (only locked files, or files not
up-to-date) and full display at any time by typing v t
vc-dired-toggle-terse-mode. There is also a special command
* l (vc-dired-mark-locked), which marks all files currently
locked (or, with CVS, all files not up-to-date). Thus, typing * l
t k is another way to delete from the buffer all files except those
currently locked.
One use of version control is to maintain multiple "current" versions of a file. For example, you might have different versions of a program in which you are gradually adding various unfinished new features. Each such independent line of development is called a branch. VC allows you to create branches, switch between different branches, and merge changes from one branch to another. Please note, however, that branches are only supported for RCS at the moment.
A file's main line of development is usually called the trunk. The versions on the trunk are normally numbered 1.1, 1.2, 1.3, etc. At any such version, you can start an independent branch. A branch starting at version 1.2 would have version number 1.2.1.1, and consecutive versions on this branch would have numbers 1.2.1.2, 1.2.1.3, 1.2.1.4, and so on. If there is a second branch also starting at version 1.2, it would consist of versions 1.2.2.1, 1.2.2.2, 1.2.2.3, etc.
If you omit the final component of a version number, that is called a branch number. It refers to the highest existing version on that branch--the head version of that branch. The branches in the example above have branch numbers 1.2.1 and 1.2.2.
To switch between branches, type C-u C-x C-q and specify the version number you want to select. This version is then visited unlocked (write-protected), so you can examine it before locking it. Switching branches in this way is allowed only when the file is not locked.
You can omit the minor version number, thus giving only the branch number; this takes you to the head version on the chosen branch. If you only type RET, Emacs goes to the highest version on the trunk.
After you have switched to any branch (including the main branch), you stay on it for subsequent VC commands, until you explicitly select some other branch.
To create a new branch from a head version (one that is the latest in the branch that contains it), first select that version if necessary, lock it with C-x C-q, and make whatever changes you want. Then, when you check in the changes, use C-u C-x C-q. This lets you specify the version number for the new version. You should specify a suitable branch number for a branch starting at the current version. For example, if the current version is 2.5, the branch number should be 2.5.1, 2.5.2, and so on, depending on the number of existing branches at that point.
To create a new branch at an older version (one that is no longer the head of a branch), first select that version, then lock it with C-x C-q. You'll be asked to confirm, when you lock the old version, that you really mean to create a new branch--if you say no, you'll be offered a chance to lock the latest version instead.
Then make your changes and type C-x C-q again to check in a new version. This automatically creates a new branch starting from the selected version. You need not specially request a new branch, because that's the only way to add a new version at a point that is not the head of a branch.
After the branch is created, you "stay" on it. That means that subsequent check-ins create new versions on that branch. To leave the branch, you must explicitly select a different version with C-u C-x C-q. To transfer changes from one branch to another, use the merge command, described in the next section.
When you have finished the changes on a certain branch, you will
often want to incorporate them into the file's main line of development
(the trunk). This is not a trivial operation, because development might
also have proceeded on the trunk, so that you must merge the
changes into a file that has already been changed otherwise. VC allows
you to do this (and other things) with the vc-merge command.
C-x v m (vc-merge) takes a set of changes and merges it
into the current version of the work file. It first asks you for a
branch number or a pair of version numbers in the minibuffer. Then it
finds the changes from that branch, or between the two versions you
specified, and merges them into the current version of the current file.
As an example, suppose that you have finished a certain feature on branch 1.3.1. In the meantime, development on the trunk has proceeded to version 1.5. To merge the changes from the branch to the trunk, first go to the head version of the trunk, by typing C-u C-x C-q RET. Version 1.5 is now current. If locking is used for the file, type C-x C-q to lock version 1.5 so that you can change it. Next, type C-x v m 1.3.1 RET. This takes the entire set of changes on branch 1.3.1 (relative to version 1.3, where the branch started, up to the last version on the branch) and merges it into the current version of the work file. You can now check in the changed file, thus creating version 1.6 containing the changes from the branch.
It is possible to do further editing after merging the branch, before the next check-in. But it is usually wiser to check in the merged version, then lock it and make the further changes. This will keep a better record of the history of changes.
When you merge changes into a file that has itself been modified, the changes might overlap. We call this situation a conflict, and reconciling the conflicting changes is called resolving a conflict.
Whenever conflicts occur during merging, VC detects them, tells you about them in the echo area, and asks whether you want help in merging. If you say yes, it starts an Ediff session (see section `Ediff' in The Ediff Manual).
If you say no, the conflicting changes are both inserted into the file, surrounded by conflict markers. The example below shows how a conflict region looks; the file is called `name' and the current master file version with user B's changes in it is 1.11.
<<<<<<< name User A's version ======= User B's version >>>>>>> 1.11
Then you can resolve the conflicts by editing the file manually. Or
you can type M-x vc-resolve-conflicts after visiting the file.
This starts an Ediff session, as described above.
It is often useful for multiple developers to work simultaneously on different branches of a file. CVS allows this by default; for RCS, it is possible if you create multiple source directories. Each source directory should have a link named `RCS' which points to a common directory of RCS master files. Then each source directory can have its own choice of selected versions, but all share the same common RCS records.
This technique works reliably and automatically, provided that the source files contain RCS version headers (see section Inserting Version Control Headers). The headers enable Emacs to be sure, at all times, which version number is present in the work file.
If the files do not have version headers, you must instead tell Emacs explicitly in each session which branch you are working on. To do this, first find the file, then type C-u C-x C-q and specify the correct branch number. This ensures that Emacs knows which branch it is using during this particular editing session.
A snapshot is a named set of file versions (one for each registered file) that you can treat as a unit. One important kind of snapshot is a release, a (theoretically) stable version of the system that is ready for distribution to users.
There are two basic commands for snapshots; one makes a snapshot with a given name, the other retrieves a named snapshot.
C-x v s name RET
vc-create-snapshot).
C-x v r name RET
vc-retrieve-snapshot).
This command reports an error if any files are locked at or below the
current directory, without changing anything; this is to avoid
overwriting work in progress.
A snapshot uses a very small amount of resources--just enough to record the list of file names and which version belongs to the snapshot. Thus, you need not hesitate to create snapshots whenever they are useful.
You can give a snapshot name as an argument to C-x v = or C-x v ~ (see section Examining And Comparing Old Versions). Thus, you can use it to compare a snapshot against the current files, or two snapshots against each other, or a snapshot against a named version.
VC's snapshot facilities are modeled on RCS's named-configuration support. They use RCS's native facilities for this, so under VC snapshots made using RCS are visible even when you bypass VC.
For SCCS, VC implements snapshots itself. The files it uses contain name/file/version-number triples. These snapshots are visible only through VC.
A snapshot is a set of checked-in versions. So make sure that all the files are checked in and not locked when you make a snapshot.
File renaming and deletion can create some difficulties with snapshots. This is not a VC-specific problem, but a general design issue in version control systems that no one has solved very well yet.
If you rename a registered file, you need to rename its master along
with it (the command vc-rename-file does this automatically). If
you are using SCCS, you must also update the records of the snapshot, to
mention the file by its new name (vc-rename-file does this,
too). An old snapshot that refers to a master file that no longer
exists under the recorded name is invalid; VC can no longer retrieve
it. It would be beyond the scope of this manual to explain enough about
RCS and SCCS to explain how to update the snapshots by hand.
Using vc-rename-file makes the snapshot remain valid for
retrieval, but it does not solve all problems. For example, some of the
files in the program probably refer to others by name. At the very
least, the makefile probably mentions the file that you renamed. If you
retrieve an old snapshot, the renamed file is retrieved under its new
name, which is not the name that the makefile expects. So the program
won't really work as retrieved.
This section explains the less-frequently-used features of VC.
If you use RCS or CVS for a program and also maintain a change log file for it (see section Change Logs), you can generate change log entries automatically from the version control log entries:
vc-update-change-log).
This command works with RCS or CVS only, not with SCCS.
For example, suppose the first line of `ChangeLog' is dated 10 April 1992, and that the only check-in since then was by Nathaniel Bowditch to `rcs2log' on 8 May 1992 with log text `Ignore log messages that start with `#'.'. Then C-x v a visits `ChangeLog' and inserts text like this:
@medbreak
Fri May 8 21:45:00 1992 Nathaniel Bowditch <nat@apn.org>
* rcs2log: Ignore log messages that start with `#'.
@medbreak
You can then edit the new change log entry further as you wish.
Normally, the log entry for file `foo' is displayed as `* foo: text of log entry'. The `:' after `foo' is omitted if the text of the log entry starts with `(functionname): '. For example, if the log entry for `vc.el' is `(vc-do-command): Check call-process status.', then the text in `ChangeLog' looks like this:
@medbreak
Wed May 6 10:53:00 1992 Nathaniel Bowditch <nat@apn.org>
* vc.el (vc-do-command): Check call-process status.
@medbreak
When C-x v a adds several change log entries at once, it groups related log entries together if they all are checked in by the same author at nearly the same time. If the log entries for several such files all have the same text, it coalesces them into a single entry. For example, suppose the most recent check-ins have the following log entries:
* For `vc.texinfo': `Fix expansion typos.' * For `vc.el': `Don't call expand-file-name.' * For `vc-hooks.el': `Don't call expand-file-name.'
They appear like this in `ChangeLog':
@medbreak
Wed Apr 1 08:57:59 1992 Nathaniel Bowditch <nat@apn.org>
* vc.texinfo: Fix expansion typos.
* vc.el, vc-hooks.el: Don't call expand-file-name.
@medbreak
Normally, C-x v a separates log entries by a blank line, but you can mark several related log entries to be clumped together (without an intervening blank line) by starting the text of each related log entry with a label of the form `{clumpname} '. The label itself is not copied to `ChangeLog'. For example, suppose the log entries are:
* For `vc.texinfo': `{expand} Fix expansion typos.'
* For `vc.el': `{expand} Don't call expand-file-name.'
* For `vc-hooks.el': `{expand} Don't call expand-file-name.'
Then the text in `ChangeLog' looks like this:
@medbreak
Wed Apr 1 08:57:59 1992 Nathaniel Bowditch <nat@apn.org>
* vc.texinfo: Fix expansion typos.
* vc.el, vc-hooks.el: Don't call expand-file-name.
@medbreak
A log entry whose text begins with `#' is not copied to `ChangeLog'. For example, if you merely fix some misspellings in comments, you can log the change with an entry beginning with `#' to avoid putting such trivia into `ChangeLog'.
When you rename a registered file, you must also rename its master
file correspondingly to get proper results. Use vc-rename-file
to rename the source file as you specify, and rename its master file
accordingly. It also updates any snapshots (see section Snapshots) that
mention the file, so that they use the new name; despite this, the
snapshot thus modified may not completely work (see section Snapshot Caveats).
You cannot use vc-rename-file on a file that is locked by
someone else.
Sometimes it is convenient to put version identification strings directly into working files. Certain special strings called version headers are replaced in each successive version by the number of that version.
If you are using RCS, and version headers are present in your working files, Emacs can use them to determine the current version and the locking state of the files. This is more reliable than referring to the master files, which is done when there are no version headers. Note that in a multi-branch environment, version headers are necessary to make VC behave correctly (see section Multi-User Branching).
Searching for version headers is controlled by the variable
vc-consult-headers. If it is non-nil, Emacs searches for
headers to determine the version number you are editing. Setting it to
nil disables this feature.
You can use the C-x v h command (vc-insert-headers) to
insert a suitable header string.
The default header string is `$Id$' for RCS and
`%W%' for SCCS. You can specify other headers to insert by
setting the variable vc-header-alist. Its value is a list of
elements of the form (program . string) where
program is RCS or SCCS and string is the
string to use.
Instead of a single string, you can specify a list of strings; then each string in the list is inserted as a separate header on a line of its own.
It is often necessary to use "superfluous" backslashes when writing the strings that you put in this variable. This is to prevent the string in the constant from being interpreted as a header itself if the Emacs Lisp file containing it is maintained with version control.
Each header is inserted surrounded by tabs, inside comment delimiters,
on a new line at the start of the buffer. Normally the ordinary comment
start and comment end strings of the current mode are used, but for
certain modes, there are special comment delimiters for this purpose;
the variable vc-comment-alist specifies them. Each element of
this list has the form (mode starter ender).
The variable vc-static-header-alist specifies further strings
to add based on the name of the buffer. Its value should be a list of
elements of the form (regexp . format). Whenever
regexp matches the buffer name, format is inserted as part
of the header. A header line is inserted for each element that matches
the buffer name, and for each string specified by
vc-header-alist. The header line is made by processing the
string from vc-header-alist with the format taken from the
element. The default value for vc-static-header-alist is as follows:
(("\\.c$" .
"\n#ifndef lint\nstatic char vcid[] = \"\%s\";\n\
#endif /* lint */\n"))
It specifies insertion of text of this form:
#ifndef lint static char vcid[] = "string"; #endif /* lint */
Note that the text above starts with a blank line.
If you use more than one version header in a file, put them close
together in the file. The mechanism in revert-buffer that
preserves markers may not handle markers positioned between two version
headers.
There are many ways of customizing VC. The options you can set fall into four categories, described in the following sections.
You can tell RCS and CVS whether to use locking for a file or not (see section Concepts of Version Control, for a description of locking). VC automatically recognizes what you have chosen, and behaves accordingly.
For RCS, the default is to use locking, but there is a mode called non-strict locking in which you can check-in changes without locking the file first. Use `rcs -U' to switch to non-strict locking for a particular file, see the `rcs' manpage for details.
Under CVS, the default is not to use locking; anyone can change a work file at any time. However, there are ways to restrict this, resulting in behavior that resembles locking.
For one thing, you can set the CVSREAD environment variable to
an arbitrary value. If this variable is defined, CVS makes your work
files read-only by default. In Emacs, you must type C-x C-q to
make the file writeable, so that editing works in fact similar as if
locking was used. Note however, that no actual locking is performed, so
several users can make their files writeable at the same time. When
setting CVSREAD for the first time, make sure to check out all
your modules anew, so that the file protections are set correctly.
Another way to achieve something similar to locking is to use the
watch feature of CVS. If a file is being watched, CVS makes it
read-only by default, and you must also use C-x C-q in Emacs to
make it writable. VC calls cvs edit to make the file writeable,
and CVS takes care to notify other developers of the fact that you
intend to change the file. See the CVS documentation for details on
using the watch feature.
You can turn off use of VC for CVS-managed files by setting the
variable vc-handle-cvs to nil. If you do this, Emacs
treats these files as if they were not registered, and the VC commands
are not available for them. You must do all CVS operations manually.
Emacs normally does not save backup files for source files that are
maintained with version control. If you want to make backup files even
for files that use version control, set the variable
vc-make-backup-files to a non-nil value.
Normally the work file exists all the time, whether it is locked or
not. If you set vc-keep-workfiles to nil, then checking
in a new version with C-x C-q deletes the work file; but any
attempt to visit the file with Emacs creates it again. (With CVS, work
files are always kept.)
Editing a version-controlled file through a symbolic link can be dangerous. It bypasses the version control system--you can edit the file without locking it, and fail to check your changes in. Also, your changes might overwrite those of another user. To protect against this, VC checks each symbolic link that you visit, to see if it points to a file under version control.
The variable vc-follow-symlinks controls what to do when a
symbolic link points to a version-controlled file. If it is nil,
VC only displays a warning message. If it is t, VC automatically
follows the link, and visits the real file instead, telling you about
this in the echo area. If the value is ask (the default), VC
asks you each time whether to follow the link.
When deducing the locked/unlocked state of a file, VC first looks for an RCS version header string in the file (see section Inserting Version Control Headers). If there is no header string, or if you are using SCCS, VC normally looks at the file permissions of the work file; this is fast. But there might be situations when the file permissions cannot be trusted. In this case the master file has to be consulted, which is rather expensive. Also the master file can only tell you if there's any lock on the file, but not whether your work file really contains that locked version.
You can tell VC not to use version headers to determine lock status by
setting vc-consult-headers to nil. VC then always uses
the file permissions (if it can trust them), or else checks the master
file.
You can specify the criterion for whether to trust the file
permissions by setting the variable vc-mistrust-permissions. Its
value can be t (always mistrust the file permissions and check
the master file), nil (always trust the file permissions), or a
function of one argument which makes the decision. The argument is the
directory name of the `RCS', `CVS' or `SCCS'
subdirectory. A non-nil value from the function says to mistrust
the file permissions. If you find that the file permissions of work
files are changed erroneously, set vc-mistrust-permissions to
t. Then VC always checks the master file to determine the file's
status.
If vc-suppress-confirm is non-nil, then C-x C-q
and C-x v i can save the current buffer without asking, and
C-x v u also operates without asking for confirmation. (This
variable does not affect C-x v c; that operation is so drastic
that it should always ask for confirmation.)
VC mode does much of its work by running the shell commands for RCS,
CVS and SCCS. If vc-command-messages is non-nil, VC
displays messages to indicate which shell commands it runs, and
additional messages when the commands finish.
You can specify additional directories to search for version control
programs by setting the variable vc-path. These directories are
searched before the usual search path. But the proper files are usually
found automatically.
The file system groups files into directories. A directory listing is a list of all the files in a directory. Emacs provides commands to create and delete directories, and to make directory listings in brief format (file names only) and verbose format (sizes, dates, and authors included). There is also a directory browser called Dired; see section Dired, the Directory Editor.
list-directory).
The command to display a directory listing is C-x C-d
(list-directory). It reads using the minibuffer a file name
which is either a directory to be listed or a wildcard-containing
pattern for the files to be listed. For example,
C-x C-d /u2/emacs/etc RET
lists all the files in directory `/u2/emacs/etc'. Here is an example of specifying a file name pattern:
C-x C-d /u2/emacs/src/*.c RET
Normally, C-x C-d prints a brief directory listing containing just file names. A numeric argument (regardless of value) tells it to make a verbose listing including sizes, dates, and authors (like `ls -l').
The text of a directory listing is obtained by running ls in an
inferior process. Two Emacs variables control the switches passed to
ls: list-directory-brief-switches is a string giving the
switches to use in brief listings ("-CF" by default), and
list-directory-verbose-switches is a string giving the switches to
use in a verbose listing ("-l" by default).
The command M-x diff compares two files, displaying the
differences in an Emacs buffer named `*Diff*'. It works by running
the diff program, using options taken from the variable
diff-switches, whose value should be a string.
The buffer `*Diff*' has Compilation mode as its major mode, so you can use C-x ` to visit successive changed locations in the two source files. You can also move to a particular hunk of changes and type RET or C-c C-c, or click Mouse-2 on it, to move to the corresponding source location. You can also use the other special commands of Compilation mode: SPC and DEL for scrolling, and M-p and M-n for cursor motion. See section Running Compilations under Emacs.
The command M-x diff-backup compares a specified file with its most
recent backup. If you specify the name of a backup file,
diff-backup compares it with the source file that it is a backup
of.
The command M-x compare-windows compares the text in the current window with that in the next window. Comparison starts at point in each window, and each starting position is pushed on the mark ring in its respective buffer. Then point moves forward in each window, a character at a time, until a mismatch between the two windows is reached. Then the command is finished. For more information about windows in Emacs, section Multiple Windows.
With a numeric argument, compare-windows ignores changes in
whitespace. If the variable compare-ignore-case is
non-nil, it ignores differences in case as well.
See also section Merging Files with Emerge, for convenient facilities for merging two similar files.
Emacs has commands for performing many other operations on files. All operate on one file; they do not accept wildcard file names.
M-x view-file allows you to scan or read a file by sequential
screenfuls. It reads a file name argument using the minibuffer. After
reading the file into an Emacs buffer, view-file displays the
beginning. You can then type SPC to scroll forward one windowful,
or DEL to scroll backward. Various other commands are provided
for moving around in the file, but none for changing it; type ?
while viewing for a list of them. They are mostly the same as normal
Emacs cursor motion commands. To exit from viewing, type q.
The commands for viewing are defined by a special major mode called View
mode.
A related command, M-x view-buffer, views a buffer already present in Emacs. See section Miscellaneous Buffer Operations.
M-x insert-file inserts a copy of the contents of the specified file into the current buffer at point, leaving point unchanged before the contents and the mark after them.
M-x write-region is the inverse of M-x insert-file; it copies the contents of the region into the specified file. M-x append-to-file adds the text of the region to the end of the specified file. See section Accumulating Text.
M-x delete-file deletes the specified file, like the rm
command in the shell. If you are deleting many files in one directory, it
may be more convenient to use Dired (see section Dired, the Directory Editor).
M-x rename-file reads two file names old and new using the minibuffer, then renames file old as new. If a file named new already exists, you must confirm with yes or renaming is not done; this is because renaming causes the old meaning of the name new to be lost. If old and new are on different file systems, the file old is copied and deleted.
The similar command M-x add-name-to-file is used to add an additional name to an existing file without removing its old name. The new name must belong on the same file system that the file is on.
M-x copy-file reads the file old and writes a new file named new with the same contents. Confirmation is required if a file named new already exists, because copying has the consequence of overwriting the old contents of the file new.
M-x make-symbolic-link reads two file names target and linkname, then creates a symbolic link named linkname and pointing at target. The effect is that future attempts to open file linkname will refer to whatever file is named target at the time the opening is done, or will get an error if the name target is not in use at that time. This command does not expand the argument target, so that it allows you to specify a relative name as the target of the link.
Confirmation is required when creating the link if linkname is in use. Note that not all systems support symbolic links.
Emacs comes with a library that can automatically uncompress compressed files when you visit them, and automatically recompress them if you alter them and save them. To enable this feature, type the command M-x auto-compression-mode.
When automatic compression (which implies automatic uncompression as
well) is enabled, Emacs recognizes compressed files by their file names.
File names ending in `.gz' indicate a file compressed with
gzip. Other endings indicate other compression programs.
Automatic uncompression and compression apply to all the operations in which Emacs uses the contents of a file. This includes visiting it, saving it, inserting its contents into a buffer, loading it, and byte compiling it.
You can refer to files on other machines using a special file name syntax:
/host:filename /user@host:filename
When you do this, Emacs uses the FTP program to read and write files on the specified host. It logs in through FTP using your user name or the name user. It may ask you for a password from time to time; this is used for logging in on host.
Normally, if you do not specify a user name in a remote file name,
that means to use your own user name. But if you set the variable
ange-ftp-default-user to a string, that string is used instead.
(The Emacs package that implements FTP file access is called
ange-ftp.)
You can entirely turn off the FTP file name feature by setting the
variable file-name-handler-alist to nil.
You can quote an absolute file name to prevent special characters and syntax in it from having their special effects. The way to do this is to add `/:' at the beginning.
For example, you can quote a local file name which appears remote, to prevent it from being treated as a remote file name. Thus, if you have a directory named `/foo:' and a file named `bar' in it, you can refer to that file in Emacs as `/:/foo:/bar'.
`/:' can also prevent `~' from being treated as a special character for a user's home directory. For example, `/:/tmp/~hack' refers to a file whose name is `~hack' in directory `/tmp'.
Likewise, quoting with `/:' is one way to enter in the minibuffer a file name that contains `$'. However, the `/:' must be at the beginning of the buffer in order to quote `$'.