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Go to the previous, next chapter. File HandlingThe 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 NamesMost 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
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 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 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 $ 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 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 Visiting 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 , , 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
( 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 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 ( 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 ( If you visit a nonexistent file unintentionally (because you typed the wrong file
name), use the C-x C-v command ( C-x 4 f ( C-x 5 f ( 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
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 FilesSaving a buffer in Emacs means writing its contents back into the file that was visited in the buffer.
When you wish to save the file and make your changes permanent, type C-x C-s
( 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 written) The command C-x s (
C-x C-c, the key sequence to exit Emacs, invokes 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-~ ( 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). If you wish to mark the buffer as visiting a
different file and save it right away, use C-x C-w ( 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
Backup FilesOn 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 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.
Single or Numbered BackupsIf 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
You can set If you set the environment variable For files under version control (see section Version
Control), the variable Automatic Deletion of BackupsTo 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 If Dired's . (Period) command can also be used to delete old versions. See section Deleting Files with Dired. Copying vs. RenamingBackup 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 The choice of renaming or copying is controlled by three variables. Renaming is the
default choice. If the variable Protection against Simultaneous EditingSimultaneous 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
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 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 BufferIf 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.
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; Auto-Saving: Protection Against DisastersEmacs 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-Save FilesAuto-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 When you delete a substantial part of the text in a large buffer, auto save turns off temporarily in that buffer. This is so that if you delete text accidentally, it is likely to remain present in the auto save file. To reenable auto-saving after this happens, simply save the file explicitly with C-x C-s. Using C-u 1 M-x auto-save-mode also cancels this particular state. If you want auto-saving to be done in the visited
file, set the variable A buffer's auto-save file is deleted when you save
the buffer in its visited file. To inhibit this, set
the variable When you delete a large amount of a buffer's text, auto-saving turns off 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 restart auto-saving in that buffer, save the buffer with C-x C-s, or use M-x auto-save. Controlling Auto-SavingEach time you visit a file, auto-saving is turned
on for that file's buffer if the variable 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-saving also takes place when you stop typing
for a while. The variable 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-SavesYou 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 When File Name AliasesSymbolic 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 If the variable Version ControlVersion 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 the version control system known as RCS, which is free software and available from the Free Software Foundation.
Supported Version Control SystemsVC 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. However, its concepts are rather different from those of RCS and VC; consequently, some VC commands work differently when used with CVS. Note 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 ControlWhen 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 ( A snapshot is a coherent collection of versions of the various files that make up a program. See section Snapshots. The model of checkin and checkout does not quite fit CVS. CVS has no such thing as locking; multiple users can simultaneously edit their copies of a file, and then check in their own variants of the file, but these variants must be reconciled later. See section Using VC with CVS. Editing with Version ControlThese are the commands for editing a file maintained with 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-OutWhen you want to modify a file maintained with version control, type C-x C-q
( 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.) Check-InWhen 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
You can put any file under version control by simply visiting
it, and then typing C-x v i ( When you register the file, Emacs must choose which version control system to use for
it. You can specify your choice explicitly by setting 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 Undoing Version Control Actions
If you want to discard your current set of changes and revert to the last version
checked in, use C-x v u ( 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 ( 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. The VC Mode LineWhen 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 CVSIn 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. 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 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. When you commit a change in a file, and someone else has committed another change in the meanwhile, that creates a conflict. Then C-x C-q asks you to resolve the conflict and try again. 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. You can turn off use of VC for CVS-managed files by setting the variable Log EntriesWhen you're editing an initial comment or log entry for inclusion in a master file, finish your entry by typing C-c C-c.
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: Change Logs and VCIf 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:
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: Fri May 8 21:45:00 1992 Nathaniel Bowditch * rcs2log: Ignore log messages that start with `#'. 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: Wed May 6 10:53:00 1992 Nathaniel Bowditch * vc.el (vc-do-command): Check call-process status. 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: @flushleft * 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. @end flushleft They appear like this in ChangeLog: Wed Apr 1 08:57:59 1992 Nathaniel Bowditch * vc.texinfo: Fix expansion typos. * vc.el, vc-hooks.el: Don't call expand-file-name. 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: @flushleft * 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. @end flushleft Then the text in ChangeLog looks like this: Wed Apr 1 08:57:59 1992 Nathaniel Bowditch * vc.texinfo: Fix expansion typos. * vc.el, vc-hooks.el: Don't call expand-file-name. 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
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 ( To compare two versions of a file, use the command C-x
v = ( 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 Multiple Branches of a FileOne 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 between BranchesTo 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 BranchesTo 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, lock it with C-x C-q, and make your changes. Then 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 BranchingIt 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 CommandsTo view the detailed version control status and history of a file, type C-x v l
( 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 ( 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. 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 ( Renaming VC Work Files and Master FilesWhen you rename a registered file, you must also rename its master file correspondingly
to get proper results. Use You cannot use SnapshotsA 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 and Using SnapshotsThere are two basic commands for snapshots; one makes a snapshot with a given name, the other retrieves a named snapshot.
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 CaveatsVC'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 Using Inserting Version Control HeadersSometimes 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 You can use the C-x v h command (
The default header string is $Id$ for
RCS and %W% for SCCS. You can specify other headers to insert by setting the variable 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 The variable
(("\\.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 Customizing VCEmacs 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 Normally the work file exists all the time, whether it is locked or not. If you set If VC mode does much of its work by running the shell commands for RCS, CVS and SCCS. If 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 You can specify the criterion for whether to trust the file permissions by setting the variable You can specify additional directories to search for version control programs by
setting the variable File DirectoriesThe 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.
The command to display a directory listing is C-x C-d ( 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 Comparing FilesThe command M-x diff compares two files,
displaying the differences in an Emacs buffer named *Diff*.
It works by running the 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, 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, See also section Merging Files with Emerge, for convenient facilities for merging two similar files. Miscellaneous File OperationsEmacs 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, 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 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.
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