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# File Handling

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.

- File Names: How to type and edit file name arguments.
- Visiting: Visiting a file prepares Emacs to edit the file.
- Saving: Saving makes your changes permanent.
- Reverting: Reverting cancels all the changes not saved.
- Auto Save: Auto Save periodically protects against loss of data.
- File Aliases: Handling multiple names for one file.
- Version Control: Version control systems (RCS, CVS and SCCS).
- Directories: Creating, deleting and listing file directories.
- Comparing Files: Finding where two files differ.
- Misc File Ops: Other things you can do on files.
- Compressed Files: Accessing compressed files.

## File Names

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.

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.

You can turn off the FTP file name feature by setting the variable `file-name-handler-alist` to `nil`.

``$'` in a file name is used to substitute environment variables. For example, if you have used the shell command ``export FOO=rms/hacks'` to set up an environment variable named `FOO`, then you can use ``/u/$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 ``$'` at the same time as variable substitution is performed for single ``$'`. 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.

## Visiting Files

`C-x C-f`
  Visit a file (`find-file`).
`C-x C-r`
  Visit a file for viewing, without allowing changes to it (`find-file-read-only`).
`C-x C-v`
  Visit a different file instead of the one visited last (`find-alternate-file`).
`C-x 4 C-f`
  Visit a file, in another window (`find-file-other-window`). Don't change the selected window.
`C-x 5 C-f`
  Visit a file, in a new frame (`find-file-other-frame`). Don't change the selected frame.
`M-x auto-compression-mode`
  Toggle automatic uncompression and recompression for compressed files.

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.

`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.

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.

The command `M-x auto-compression-mode` toggles a mode in which visiting a compressed file automatically uncompresses it. (Editing the file and saving it automatically recompresses it.)

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 Files

Saving a buffer in Emacs means writing its contents back into the file that was visited in the buffer.

`C-x C-s`
  Save the current buffer in its visited file (`save-buffer`).
`C-x s`
  Save any or all buffers in their visited files (`save-some-buffers`).
`M-~`
  Forget that the current buffer has been changed (`not-modified`).
`C-x C-w`
  Save the current buffer in a specified file (`write-file`).
`M-x set-visited-file-name`
  Change file the name under which the current buffer will be saved.

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`:

`y`
  Save this buffer and ask about the rest of the buffers.
`n`
  Don't save this buffer, but ask about the rest of the buffers.
`!`
  Save this buffer and all the rest with no more questions.
``RET``
  Terminate `save-some-buffers` without any more saving.
`.`
  Save this buffer, then exit `save-some-buffers` without even asking about other buffers.
`C-r`
  View the buffer that you are currently being asked about. When you exit View mode, you get back to `save-some-buffers`, which asks the question again.
`C-h`
  Display a help message about these options.

`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 with 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 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.

- Backup: How Emacs saves the old version of your file.
- Interlocking: How Emacs protects against simultaneous editing of one file by two users.

### Backup Files

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.

- Names: How backup files are named; choosing single or numbered backup files.
- Deletion: Emacs deletes excess numbered backups.
- Copying: Backups can be made by copying or renaming.

#### Single or Numbered Backups

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`
  Make numbered backups.
`nil`
  Make numbered backups for files that have numbered backups already. Otherwise, make single backups.
`never`
  Do not in any case make numbered backups; always make single backups.

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 under version control (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 Editing with Version Control.

#### Automatic Deletion of Backups

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.

#### Copying vs. Renaming

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 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.

### Protection against Simultaneous Editing

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.

For the sake of systems where that is not possible, and in case someone else proceeds to change the file despite the warning, Emacs also checks when the file is saved, and issues a second warning if you are about to overwrite a file containing another user's changes. You can prevent loss of the other user's work by taking the proper corrective action at that time.

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 writing another file in a directory reserved for this purpose.) The lock is removed 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:

`s`
  Steal the lock. Whoever was already changing the file loses the lock, and you gain the lock.
`p`
  Proceed. Go ahead and edit the file despite its being locked by someone else.
`q`
  Quit. This causes an error (`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. On these systems, 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.

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.

Simultaneous editing checks are also made when you visit with `C-x C-f` a file that is already visited and when you start to modify a file. This is not strictly necessary, but it can cause you to find out about the collision earlier, when perhaps correction takes less work.

## Reverting a Buffer

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 randomly with `C-x b` cannot be reverted; `revert-buffer` reports an error when asked to do so.

## Auto-Saving: Protection Against Disasters

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.

- Files: The file where auto-saved changes are actually made until you save the file.
- Control: Controlling when and how often to auto-save.
- Recover: Recovering text from auto-save files.

### Auto-Save Files

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.

### Controlling Auto-Saving

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`.

### Recovering Data from Auto-Saves

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 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.

## File Name Aliases

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

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 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.

- Version Systems: Supported version control back end systems.
- VC Concepts: Basic version control information; checking files in and out.
- Editing with VC: Commands for editing a file maintained with version control.
- Log Entries: Logging your changes.
- Change Logs and VC: Generating a change log file from log entries.
- Old Versions: Examining and comparing old versions.
- Branches: Selecting a branch to put your changes in, and creating a new branch.
- Status in VC: Commands to view the VC status of files and look at log entries.
- Renaming and VC: A command to rename both the source and master file correctly.
- Snapshots: How to make and use snapshots, a set of file versions that can be treated as a unit.
- Version Headers: Inserting version control headers into working files.
- Customizing VC: Variables to change VC's behavior.

### Supported Version Control Systems

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. See section Using VC with CVS.

SCCS is a proprietary but widely used version control system. In terms of capabilities, it is the weakest of the 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.

### Concepts of Version Control

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, so that you can reconstruct from it either the current version or any specified earlier version. Usually the master file also records a log entry for each version describing 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.

To examine a file, you check it out. This extracts a version of the source file (typically, the most recent) from the master file. If you want to edit the file, you must check it out locked. Only one user can do this at a time for any given source file. (This kind of locking is completely unrelated to the locking that Emacs uses to detect simultaneous editing of a file.)

When you are done with your editing, you must check in the new version. This records the new version in the master file, and unlocks the source file so that other people can lock it and thus modify it.

Checkin and checkout are the basic operations of version control. You can do both of them with a single Emacs command: `C-x C-q` (`vc-toggle-read-only`).

There are variants of this basic pattern, though. CVS, for example, has no such thing as locking, and therefore you can normally edit files right away, without having to check them out first. See section Using VC with CVS. With RCS, you can optionally select non-strict locking for a particular source file; then you can edit the file in Emacs without explicitly locking it.

A snapshot is a coherent collection of versions of the various files that make up a program. See section Snapshots.

### Editing with Version Control

These are the commands for editing a file maintained with version control:

`C-x C-q`
`C-x v v`
  Check the visited file in or out.
`C-x v u`
  Revert the buffer and the file to the last checked in version.
`C-x v c`
  Remove the last-entered change from the master for the visited file. This undoes your last check-in.
`C-x v i`
  Register the visited file for version control.

(`C-x v` is the prefix key for version control commands; all of these commands except for `C-x C-q` start with `C-x v`.)

- Check-Out: Checking out a file so you can edit it.
- Check-In: After you edit, you check in your changes to make a new version.
- Version Control Undo: Canceling changes before or after checkin.
- Registering: How to start using version control for a file.
- VC Mode Line: Mode line indicates version and lock status.
- CVS and VC: Checkout and checkin work differently in CVS.

#### Check-Out

When you want to modify a file maintained with version control, type `C-x C-q` (`vc-toggle-read-only`). This checks out the file, and tells RCS or SCCS to lock the file. This means making the file writable for you (but not for anyone else).

If you specify a prefix argument (`C-u C-x C-q`) for checkout, Emacs asks you for a version number, and checks out that version unlocked. This lets you move to old versions, or existing branches of the file (see section Multiple Branches of a File). You can then start editing the selected version by typing `C-x C-q` again. (If you edit an old version of a file this way, checking it in again creates a new branch.)

Under CVS, you normally don't need to check out files explicitly. CVS does not have locking; multiple users can edit their copies of a file whenever they want. (If two users make conflicting changes, they need to reconcile their changes when checking them in.) We therefore say that an implicit check-out happens when you make the first change in the file.

CVS has an alternative mode in which explicit check-out is required. And RCS has an alternative mode called non-strict locking in which explicit check-out is not required. Selecting these modes is done outside of VC, but once you have selected them, VC obeys them. With RCS, you can select non-strict locking for a particular file using the ``rcs -U'` command. See section Using VC with CVS, for an explanation of how to do this with CVS.

#### Check-In

When you are finished editing the file, type `C-x C-q` again. When used on a file that is checked out, this command checks the file in. But check-in does not start immediately; first, you must enter the log entry---a description of the changes in the new version. `C-x C-q` pops up a buffer for you to enter this in. When you are finished typing in the log entry, type `C-c C-c` to terminate it; this is when actual check-in takes place. See section Log Entries.

With RCS and SCCS, a checked-out file is also locked, which means it is writable for you, but not for anyone else. As long as you own the lock on the file, nobody else can modify it, and nobody can check in any changes to that particular version. Checking in your changes unlocks the file, so that other users can lock it and modify it.

CVS, on the contrary, doesn't have a concept of locking. The working files are always modifiable, allowing concurrent development, with possible conflicts being resolved at check-in time. See section Using VC with CVS.

To specify the version number for the new version, type `C-u C-x C-q` to check in a file. Then Emacs asks you for the new version number in the minibuffer. This can be used to create a new branch of the file (see section Multiple Branches of a File), or to increment the file's major version number.

It is not impossible to lock a file that someone else has locked. If you try to check out a file that is locked, `C-x C-q` asks you whether you want to "steal the lock." If you say yes, the file becomes locked by you, but a message is sent to the person who had formerly locked the file, to inform him of what has happened. The mode line indicates that a file is locked by someone else by displaying the login name of that person, before the version number.

#### Registering a File for Version Control

`C-x v i`
  Register the visited file for version control.

You can put any file under version control by simply visiting it, and then typing `C-x v i` (`vc-register`). After `C-x v i`, the file is unlocked and read-only. Type `C-x C-q` if you wish to start editing it.

When you 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.

After registering a file with CVS, you must subsequently commit the initial version by typing `C-x C-q`. See section Using VC with CVS.

The initial version number for a newly registered file is 1.1, by default. To specify a different number, give `C-x v i` a numeric argument; then it reads the initial version number using the minibuffer.

If `vc-initial-comment` is non-`nil`, `C-x v i` reads an initial comment (much like a log entry) to describe the purpose of this source file.

#### Undoing Version Control Actions

`C-x v u`
  Revert the buffer and the file to the last checked in version.
`C-x v c`
  Remove the last-entered change from the master for the visited file. This undoes your last checkin.

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 cancels your last check-out, leaving the file unlocked. If you want to make a different set of changes, you must first check the file out 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 use to unlock a file if you lock it and then decide not to change it.

You can cancel a change after checking it in, with `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 say `no`, then 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 checkin 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 throw away 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 this back end.

#### The VC Mode Line

When you visit a file that is under version control, the mode line indicates the current status of the file: the name of the version control back end system, the locking state, and the version.

The locking state is displayed as a single character, which can be either ``-'` or ``:'`. ``-'` means the file is not locked or not modified by you. Once you lock the file, the state indicator changes to ``:'`. If the file is locked by someone else, that user's name appears after the version number.

For example, ``RCS-1.3'` means you are looking at RCS version 1.3, which is not locked. ``RCS:1.3'` means that you have locked the file, and possibly already changed it. ``RCS:jim:1.3'` means that the file is locked by jim.

#### Using VC with CVS

In CVS, files are never locked. Two users can check out the same file at the same time; each user has a separate copy and can edit it. Work files are always writable; once you have one, you need not type a VC command to start editing the file. You can edit it at any time.

When using RCS and SCCS, you normally use `C-x C-q` twice for each change; once before the change, for checkout, and once after, for checkin. With CVS, it's different: you normally use `C-x C-q` just once for each change, to commit the change when it is done. The work file remains writable, so you can begin editing again with no special commands.

One way to understand this is that VC does an implicit check-out when you save the modified file for the first time. VC indicates this on the mode line: the status indicator changes from ``-'` to ``:'` as soon as you save a modified version, telling you that you are not in sync with the repository anymore (see section The VC Mode Line). The file stays "checked out" until you check it back in, even if you kill the buffer and visit the file again.

If, instead, you would like to use explicit check-out with CVS, set the `CVSREAD` environment variable to some value. (It does not matter what value you use.) CVS then makes your work files read-only by default, and VC requires you to check them out explicitly with `C-x C-q`. When setting `CVSREAD` for the first time, make sure to check out all your modules anew, so that the file protections are set correctly.

VC does not provide a way to check out a working copy of an existing file in the repository. You have to use the CVS shell commands to do that. Once you have a work file, you can start using VC for that file.

CVS terminology speaks of committing a change rather than checking it in. But in practical terms they work the same way: Emacs asks you to type in a log entry, and you finish it with `C-c C-c`.

When you try to commit a change in a file, but someone else has committed another change in the meanwhile, that creates a conflict. VC detects this situation and offers to merge your changes and those of the other user, creating a new local version of the file, which you can then commit to the repository. This works smoothly if the changes are in different parts of the file, although it is wise to check the resulting file for semantic consistency.

However, if you and the other user changed the same parts of the file, the conflict cannot be resolved automatically. In this case, CVS inserts both variants of the conflicting regions into your working file, and puts so-called conflict markers around them. They indicate how the region looks in the respective user's version. You must resolve the conflict manually, for example by choosing one of the two variants and deleting the other one (and the conflict markers). Then you can commit the resulting file into the repository. The example below shows how a conflict region looks; the file is called ``name'` and the current repository version with user B's changes in it is 1.11.

    <<<<<<< name
      User A's version
    =======
      User B's version
    >>>>>>> 1.11

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 managed, and the VC commands are not available for them. You must do all CVS operations manually.

### Log Entries

When you're editing an initial comment or log entry for inclusion in a master file, finish your entry by typing `C-c C-c`.

`C-c C-c`
  Finish the comment edit normally (`vc-finish-logentry`). This finishes check-in.

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 its 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.

### Change Logs and VC

If you use RCS 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:

`C-x v a`
  Visit the current directory's change log file and create new entries for versions checked in since the most recent entry in the change log file (`vc-update-change-log`). This command works with RCS only; it does not work with CVS or 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 checkins 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'`.

### Examining And Comparing Old Versions

`C-u C-x C-q version `RET``
  Select version version as the current work file version.
`C-x v ~ version `RET``
  Examine version version of the visited file, in a buffer of its own.
`C-x v =`
  Compare the current buffer contents with the latest checked-in version of the file.
`C-u C-x v = file `RET` oldvers `RET` newvers `RET``
  Compare the specified two versions of file.

There are two ways to work with an old version of a file. You can make the old version your current work file, for example if you want to reproduce a former stage of development, or if you want to create a branch from the old version (see section Multiple Branches of a File). To do this, visit the file and type `C-u C-x C-q version `RET``. (This works only with RCS.)

If you want only to examine an old version, without changing your work file, 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.

To compare two versions of a file, use 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 work file, this command compares the two specified versions of all registered files in that directory and its subdirectories. You can also specify a snapshot name (see section Snapshots) instead of one or both version numbers.

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).

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`.

### Multiple Branches of a File

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, and switch between existing branches. Note, however, that branches are supported only with RCS.

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 may start an independent branch. A branch starting at version 1.2 would have version number 1.2.1.1. 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, and so on.

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 branches in the example above have branch numbers 1.2.1 and 1.2.2.

A version which is the last in its branch is called a head version.

- Switching Branches: How to get to another existing branch.
- Creating Branches: How to start a new branch.
- Multi-User Branching: Multiple users working at multiple branches in parallel.

#### Switching between Branches

To switch between branches, type `C-u C-x C-q` and specify the version number you want to select. This version is then checked out unlocked (write-protected), so you can examine it before really checking it out. Switching branches in this way is allowed only when the file is not locked.

You may omit the minor version number, thus giving only the branch number; this takes you to the highest version on the indicated 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.

#### Creating New Branches

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 checkouts and checkins create new versions on that branch. To leave the branch, you must explicitly select a different version with `C-u C-x C-q` for checkout.

#### Multi-User Branching

It is sometimes useful for multiple developers to work simultaneously on different branches of a file. This 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 versions checked out, 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.

### VC Status Commands

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.

When you are working on a large program, it's often useful to find all the files that are currently locked, or all the files maintained in version control at all. You can use `C-x v d` (`vc-directory`) to show all the locked files in or beneath a certain directory. This includes all files that are locked by any user. `C-u C-x v d` lists all files in or beneath the specified directory that are maintained with version control.

The list of files is displayed as a buffer that uses an augmented Dired mode. The names of the users locking various files are shown (in parentheses) in place of the owner and group. (With CVS, a more detailed status is shown for each file.) All the normal Dired commands work in this buffer. Most interactive VC commands work also, and apply to the file name on the current line.

The `C-x v v` command (`vc-next-action`), when used in the augmented Dired buffer, operates on all the marked files (or the file on the current line). If it operates on more than one file, it handles each file according to its current state; thus, it may check out one file and check in another (because it is already checked out). If it has to check in any files, it reads a single log entry, then uses that text for all the files being checked in. This can be convenient for registering or checking in several files at once, as part of the same change.

### Renaming VC Work Files and Master Files

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.

### Snapshots

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.

- Making Snapshots: The snapshot facilities.
- Snapshot Caveats: Things to be careful of when using snapshots.

#### Making and Using Snapshots

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``
  Define the last saved versions of every registered file in or under the current directory as a snapshot named name (`vc-create-snapshot`).
`C-x v r name `RET``
  Check out all registered files at or below the current directory level using whatever versions correspond to the snapshot name (`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.

#### Snapshot Caveats

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.

### Inserting Version Control Headers

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.

`C-x v h`
  Insert headers in a file for use with your version-control system.

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.

### Customizing VC

There are many ways of customizing VC. The variables that control its behavior fall into three categories, described in the following sections.

- VC Workfile Handling: Various options concerning working files.
- VC Status Retrieval: How VC finds the version control status of a file, and how to customize this.
- VC Command Execution: Which commands VC should run, and how.

#### VC Workfile Handling

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 checking it out, 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.

#### VC Status Retrieval

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 the backend system is 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 may 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.

#### VC Command Execution

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.

## File Directories

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.

`C-x C-d dir-or-pattern `RET``
  Display a brief directory listing (`list-directory`).
`C-u C-x C-d dir-or-pattern `RET``
  Display a verbose directory listing.
`M-x make-directory `RET` dirname `RET``
  Create a new directory named dirname.
`M-x delete-directory `RET` dirname `RET``
  Delete the directory named dirname. It must be empty, or you get an error.

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).

## Comparing Files

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.

## Miscellaneous File Operations

Emacs has commands for performing many other operations on files. All operate on one file; they do not accept wild card 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 old and linkname, then creates a symbolic link named linkname and pointing at old. The effect is that future attempts to open file linkname will refer to whatever file is named old at the time the opening is done, or will get an error if the name old is not in use at that time. This command does not expand the argument filename, 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.

## Accessing Compressed Files

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.

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