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@c This is part of the Emacs manual., Abbrevs, This is part of the Emacs manual., Top
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@c Copyright (C) 1985-1987, 1993-1995, 1997, 1999-2018 Free Software
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@c Foundation, Inc.
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@c See file emacs.texi for copying conditions.
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@node Maintaining
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@chapter Maintaining Large Programs

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  This chapter describes Emacs features for maintaining medium- to
large-size programs and packages.  These features include:

@itemize @minus
@item
Unified interface to Support for Version Control Systems
(@acronym{VCS}) that record the history of changes to source files.

@item
A specialized mode for maintaining @file{ChangeLog} files that provide
a chronological log of program changes.

@item
@acronym{Xref}, a set of commands for displaying definitions of
symbols (a.k.a.@: ``identifiers'') and their references.

@item
@acronym{EDE}, the Emacs's own IDE.

@ifnottex
@item
A mode for merging changes to program sources made on separate
branches of development.
@end ifnottex
@end itemize

If you are maintaining a large Lisp program, then in addition to the
features described here, you may find the Emacs Lisp Regression
Testing (@acronym{ERT}) library useful (@pxref{Top,,ERT,ert, Emacs
Lisp Regression Testing}).
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@menu
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* Version Control::     Using version control systems.
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* Change Log::          Maintaining a change history for your program.
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* Xref::                Find definitions and references of any function,
                          method, struct, macro, @dots{} in your program.
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* EDE::                 An integrated development environment for Emacs.
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@ifnottex
* Emerge::              A convenient way of merging two versions of a program.
@end ifnottex
@end menu

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@node Version Control
@section Version Control
@cindex version control

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  A @dfn{version control system} is a program that can record multiple
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versions of a source file, storing information such as the creation
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time of each version, who made it, and a description of what was
changed.

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@cindex VC
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  The Emacs version control interface is called @dfn{VC}@.  VC
commands work with several different version control systems;
currently, it supports Bazaar, CVS, Git, Mercurial, Monotone, RCS,
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SCCS/CSSC, and Subversion.  Of these, the GNU project distributes CVS,
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RCS, and Bazaar.
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  VC is enabled automatically whenever you visit a file governed by a
version control system.  To disable VC entirely, set the customizable
variable @code{vc-handled-backends} to @code{nil}
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@iftex
(@pxref{Customizing VC,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{Customizing VC}).
@end ifnottex

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@findex vc-refresh-state
@findex vc-state-refresh
  To update the VC state information for the file visited in the
current buffer, use the command @code{vc-refresh-state}.  This command
is useful when you perform version control commands outside Emacs
(e.g., from the shell prompt), or if you put the buffer's file under a
different version control system, or remove it from version control
entirely.  A companion command @code{vc-state-refresh} does the same,
but does not consider switching the version control system or removal
from VC.

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@menu
* Introduction to VC::  How version control works in general.
* VC Mode Line::        How the mode line shows version control status.
* Basic VC Editing::    How to edit a file under version control.
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* Log Buffer::          Features available in log entry buffers.
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* Registering::         Putting a file under version control.
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* Old Revisions::       Examining and comparing old versions.
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* VC Change Log::       Viewing the VC Change Log.
* VC Undo::             Canceling changes before or after committing.
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* VC Ignore::           Ignore files under version control system.
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* VC Directory Mode::   Listing files managed by version control.
* Branches::            Multiple lines of development.
@ifnottex
* Miscellaneous VC::    Various other commands and features of VC.
* Customizing VC::      Variables that change VC's behavior.
@end ifnottex
@end menu

@node Introduction to VC
@subsection Introduction to Version Control

  VC allows you to use a version control system from within Emacs,
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integrating the version control operations smoothly with editing.  It
provides a uniform interface for common operations in many version
control operations.

  Some uncommon or intricate version control operations, such as
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altering repository settings, are not supported in VC@.  You should
perform such tasks outside Emacs, e.g., via the command line.
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  This section provides a general overview of version control, and
describes the version control systems that VC supports.  You can skip
this section if you are already familiar with the version control system
you want to use.

@menu
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* Why Version Control?::    Understanding the problems it addresses.
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* Version Control Systems:: Supported version control back-end systems.
* VCS Concepts::            Words and concepts related to version control.
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* VCS Merging::             How file conflicts are handled.
* VCS Changesets::          How changes are grouped.
* VCS Repositories::        Where version control repositories are stored.
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* Types of Log File::       The VCS log in contrast to the ChangeLog.
@end menu

@node Why Version Control?
@subsubsection Understanding the problems it addresses

  Version control systems provide you with three important
capabilities:

@itemize @bullet
@item
@dfn{Reversibility}: the ability to back up to a previous state if you
discover that some modification you did was a mistake or a bad idea.

@item
@dfn{Concurrency}: the ability to have many people modifying the same
collection of files knowing that conflicting modifications can be
detected and resolved.

@item
@dfn{History}: the ability to attach historical data to your data,
such as explanatory comments about the intention behind each change to
it.  Even for a programmer working solo, change histories are an
important aid to memory; for a multi-person project, they are a
vitally important form of communication among developers.
@end itemize

@node Version Control Systems
@subsubsection Supported Version Control Systems

@cindex back end (version control)
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  VC currently works with many different version control systems,
which it refers to as @dfn{back ends}:
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@itemize @bullet

@cindex SCCS
@item
SCCS was the first version control system ever built, and was long ago
superseded by more advanced ones.  VC compensates for certain features
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missing in SCCS (e.g., tag names for releases) by implementing them
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itself.  Other VC features, such as multiple branches, are simply
unavailable.  Since SCCS is non-free, we recommend avoiding it.

@cindex CSSC
@item
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CSSC is a free replacement for SCCS@.  You should use CSSC only if, for
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some reason, you cannot use a more recent and better-designed version
control system.

@cindex RCS
@item
RCS is the free version control system around which VC was initially
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built.  It is relatively primitive: it cannot be used over the
network, and works at the level of individual files.  Almost
everything you can do with RCS can be done through VC.
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@cindex CVS
@item
CVS is the free version control system that was, until recently (circa
2008), used by the majority of free software projects.  Nowadays, it
is slowly being superseded by newer systems.  CVS allows concurrent
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multi-user development either locally or over the network.  Unlike
newer systems, it lacks support for atomic commits and file
moving/renaming.  VC supports all basic editing operations under CVS.
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@cindex SVN
@cindex Subversion
@item
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Subversion (svn) is a free version control system designed to be
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similar to CVS but without its problems (e.g., it supports atomic
commits of filesets, and versioning of directories, symbolic links,
meta-data, renames, copies, and deletes).
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@cindex git
@item
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Git is a decentralized version control system originally invented by
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Linus Torvalds to support development of Linux (his kernel).  VC
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supports many common Git operations, but others, such as repository
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syncing, must be done from the command line.
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@cindex hg
@cindex Mercurial
@item
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Mercurial (hg) is a decentralized version control system broadly
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resembling Git.  VC supports most Mercurial commands, with the
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exception of repository sync operations.
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@cindex bzr
@cindex Bazaar
@item
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Bazaar (bzr) is a decentralized version control system that supports
both repository-based and decentralized versioning.  VC supports most
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basic editing operations under Bazaar.
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@cindex SRC
@cindex src
@item
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SRC (src) is RCS, reloaded---a specialized version-control system
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designed for single-file projects worked on by only one person.  It
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allows multiple files with independent version-control histories to
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exist in one directory, and is thus particularly well suited for
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maintaining small documents, scripts, and dotfiles.  While it uses RCS
for revision storage, it presents a modern user interface featuring
lockless operation and integer sequential version numbers.  VC
supports almost all SRC operations.
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@end itemize
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@node VCS Concepts
@subsubsection Concepts of Version Control

@cindex repository
@cindex registered file
   When a file is under version control, we say that it is
@dfn{registered} in the version control system.  The system has a
@dfn{repository} which stores both the file's present state and its
change history---enough to reconstruct the current version or any
earlier version.  The repository also contains other information, such
as @dfn{log entries} that describe the changes made to each file.

@cindex work file
@cindex checking out files
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  The copy of a version-controlled file that you actually edit is
called the @dfn{work file}.  You can change each work file as you
would an ordinary file.  After you are done with a set of changes, you
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may @dfn{commit} (or @dfn{check in}) the changes; this records the
changes in the repository, along with a descriptive log entry.
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@cindex working tree
  A directory tree of work files is called a @dfn{working tree}.

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@cindex revision
@cindex revision ID
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  Each commit creates a new @dfn{revision} in the repository.  The
version control system keeps track of all past revisions and the
changes that were made in each revision.  Each revision is named by a
@dfn{revision ID}, whose format depends on the version control system;
in the simplest case, it is just an integer.
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  To go beyond these basic concepts, you will need to understand three
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aspects in which version control systems differ.  As explained in the
next three sections, they can be lock-based or merge-based; file-based
or changeset-based; and centralized or decentralized.  VC handles all
these modes of operation, but it cannot hide the differences.
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@node VCS Merging
@subsubsection Merge-based vs lock-based Version Control

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  A version control system typically has some mechanism to coordinate
between users who want to change the same file.  There are two ways to
do this: merging and locking.

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@cindex merging-based version
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  In a version control system that uses merging, each user may modify
a work file at any time.  The system lets you @dfn{merge} your work
file, which may contain changes that have not been committed, with the
latest changes that others have committed.
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@cindex locking-based version
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  Older version control systems use a @dfn{locking} scheme instead.
Here, work files are normally read-only.  To edit a file, you ask the
version control system to make it writable for you by @dfn{locking}
it; only one user can lock a given file at any given time.  This
procedure is analogous to, but different from, the locking that Emacs
uses to detect simultaneous editing of ordinary files
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(@pxref{Interlocking}).  When you commit your changes, that unlocks
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the file, and the work file becomes read-only again.  Other users may
then lock the file to make their own changes.

  Both locking and merging systems can have problems when multiple
users try to modify the same file at the same time.  Locking systems
have @dfn{lock conflicts}; a user may try to check a file out and be
unable to because it is locked.  In merging systems, @dfn{merge
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conflicts} happen when you commit a change to a file that conflicts
with a change committed by someone else after your checkout.  Both
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kinds of conflict have to be resolved by human judgment and
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communication.  Experience has shown that merging is superior to
locking, both in convenience to developers and in minimizing the
number and severity of conflicts that actually occur.
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  SCCS always uses locking.  RCS is lock-based by default but can be
told to operate in a merging style.  CVS and Subversion are
merge-based by default but can be told to operate in a locking mode.
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Decentralized version control systems, such as Git and Mercurial, are
exclusively merging-based.
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  VC mode supports both locking and merging version control.  The
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terms ``commit'' and ``update'' are used in newer version control
systems; older lock-based systems use the terms ``check in'' and
``check out''.  VC hides the differences between them as much as
possible.

@node VCS Changesets
@subsubsection Changeset-based vs File-based Version Control
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@cindex file-based version control
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  On SCCS, RCS, CVS, and other early version control systems, version
control operations are @dfn{file-based}: each file has its own comment
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and revision history separate from that of all other files.  Newer
systems, beginning with Subversion, are @dfn{changeset-based}: a
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commit may include changes to several files, and the entire set of
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changes is handled as a unit.  Any comment associated with the change
does not belong to a single file, but to the changeset itself.
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@cindex changeset-based version control
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  Changeset-based version control is more flexible and powerful than
file-based version control; usually, when a change to multiple files
has to be reversed, it's good to be able to easily identify and remove
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all of it.
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@node VCS Repositories
@subsubsection Decentralized vs Centralized Repositories

@cindex centralized version control
@cindex decentralized version control
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@cindex distributed version control
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  Early version control systems were designed around a
@dfn{centralized} model in which each project has only one repository
used by all developers.  SCCS, RCS, CVS, and Subversion share this
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kind of model.  One of its drawbacks is that the repository is a choke
point for reliability and efficiency.
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  GNU Arch pioneered the concept of @dfn{distributed} or
@dfn{decentralized} version control, later implemented in Git,
Mercurial, and Bazaar.  A project may have several different
repositories, and these systems support a sort of super-merge between
repositories that tries to reconcile their change histories.  In
effect, there is one repository for each developer, and repository
merges take the place of commit operations.
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  VC helps you manage the traffic between your personal workfiles and
a repository.  Whether the repository is a single master, or one of a
network of peer repositories, is not something VC has to care about.
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@node Types of Log File
@subsubsection Types of Log File
@cindex types of log file
@cindex log File, types of
@cindex version control log

  Projects that use a version control system can have two types of log
for changes.  One is the log maintained by the version control system:
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each time you commit a change, you fill out a @dfn{log entry} for the
change (@pxref{Log Buffer}).  This is called the @dfn{version control
log}.
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  The other kind of log is the file @file{ChangeLog} (@pxref{Change
Log}).  It provides a chronological record of all changes to a large
portion of a program---typically one directory and its subdirectories.
A small program would use one @file{ChangeLog} file; a large program
may have a @file{ChangeLog} file in each major directory.
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@xref{Change Log}.  Programmers have used change logs since long
before version control systems.

  Changeset-based version systems typically maintain a changeset-based
modification log for the entire system, which makes change log files
somewhat redundant.  One advantage that they retain is that it is
sometimes useful to be able to view the transaction history of a
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single directory separately from those of other directories.  Another
advantage is that commit logs can't be fixed in many version control
systems.
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  A project maintained with version control can use just the version
control log, or it can use both kinds of logs.  It can handle some
files one way and some files the other way.  Each project has its
policy, which you should follow.

  When the policy is to use both, you typically want to write an entry
for each change just once, then put it into both logs.  You can write
the entry in @file{ChangeLog}, then copy it to the log buffer with
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@kbd{C-c C-a} when committing the change (@pxref{Log Buffer}).  Or you
can write the entry in the log buffer while committing the change, and
later use the @kbd{C-x v a} command to copy it to @file{ChangeLog}
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@iftex
(@pxref{Change Logs and VC,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{Change Logs and VC}).
@end ifnottex

@node VC Mode Line
@subsection Version Control and the Mode Line
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@cindex VC mode line indicator
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  When you visit a file that is under version control, Emacs indicates
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this on the mode line.  For example, @samp{Bzr-1223} says that Bazaar
is used for that file, and the current revision ID is 1223.
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@cindex version control status
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  The character between the back-end name and the revision ID
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indicates the @dfn{version control status} of the work file.  In a
merge-based version control system, a @samp{-} character indicates
that the work file is unmodified, and @samp{:} indicates that it has
been modified.  @samp{!} indicates that the file contains conflicts as
result of a recent merge operation (@pxref{Merging}), or that the file
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was removed from the version control.  Finally, @samp{?} means that
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the file is under version control, but is missing from the working
tree.
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  In a lock-based system, @samp{-} indicates an unlocked file, and
@samp{:} a locked file; if the file is locked by another user (for
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instance, @samp{jim}), that is displayed as @samp{RCS:jim:1.3}.
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@samp{@@} means that the file was locally added, but not yet committed
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to the master repository.
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  On a graphical display, you can move the mouse over this mode line
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indicator to pop up a tool-tip, which displays a more verbose
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description of the version control status.  Pressing @kbd{mouse-1}
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over the indicator pops up a menu of VC commands, identical to
@samp{Tools / Version Control} on the menu bar.
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@vindex auto-revert-check-vc-info
  When Auto Revert mode (@pxref{Reverting}) reverts a buffer that is
under version control, it updates the version control information in
the mode line.  However, Auto Revert mode may not properly update this
information if the version control status changes without changes to
the work file, from outside the current Emacs session.  If you set
@code{auto-revert-check-vc-info} to @code{t}, Auto Revert mode updates
the version control status information every
@code{auto-revert-interval} seconds, even if the work file itself is
unchanged.  The resulting CPU usage depends on the version control
system, but is usually not excessive.

@node Basic VC Editing
@subsection Basic Editing under Version Control

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@cindex filesets, VC
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@cindex VC filesets
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   Most VC commands operate on @dfn{VC filesets}.  A VC fileset is a
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collection of one or more files that a VC operation acts on.  When you
type VC commands in a buffer visiting a version-controlled file, the
VC fileset is simply that one file.  When you type them in a VC
Directory buffer, and some files in it are marked, the VC fileset
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consists of the marked files (@pxref{VC Directory Mode}).

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  On modern changeset-based version control systems (@pxref{VCS
Changesets}), VC commands handle multi-file VC filesets as a group.
For example, committing a multi-file VC fileset generates a single
revision, containing the changes to all those files.  On older
file-based version control systems like CVS, each file in a multi-file
VC fileset is handled individually; for example, a commit generates
one revision for each changed file.
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@table @kbd
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@item C-x v v
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Perform the next appropriate version control operation on the current
VC fileset.
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@end table

@findex vc-next-action
@kindex C-x v v
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  The principal VC command is a multi-purpose command, @kbd{C-x v v}
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(@code{vc-next-action}), which performs the most appropriate
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action on the current VC fileset: either registering it with a version
control system, or committing it, or unlocking it, or merging changes
into it.  The precise actions are described in detail in the following
subsections.  You can use @kbd{C-x v v} either in a file-visiting
buffer or in a VC Directory buffer.

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  Note that VC filesets are distinct from the named filesets used
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for viewing and visiting files in functional groups
(@pxref{Filesets}).  Unlike named filesets, VC filesets are not named
and don't persist across sessions.
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@menu
* VC With A Merging VCS::  Without locking: default mode for CVS.
* VC With A Locking VCS::  RCS in its default mode, SCCS, and optionally CVS.
* Advanced C-x v v::       Advanced features available with a prefix argument.
@end menu

@node VC With A Merging VCS
@subsubsection Basic Version Control with Merging

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  On a merging-based version control system (i.e., most modern ones;
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@pxref{VCS Merging}), @kbd{C-x v v} does the following:
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@itemize @bullet
@item
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If there is more than one file in the VC fileset and the files have
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inconsistent version control statuses, signal an error.  (Note,
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however, that a fileset is allowed to include both newly-added
files and modified files; @pxref{Registering}.)
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@item
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If none of the files in the VC fileset are registered with a version
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control system, register the VC fileset, i.e., place it under version
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control.  @xref{Registering}.  If Emacs cannot find a system to
register under, it prompts for a repository type, creates a new
repository, and registers the VC fileset with it.
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@item
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If every work file in the VC fileset is unchanged, do nothing.
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@item
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If every work file in the VC fileset has been modified, commit the
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changes.  To do this, Emacs pops up a @file{*vc-log*} buffer; type the
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desired log entry for the new revision, followed by @kbd{C-c C-c} to
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commit.  @xref{Log Buffer}.
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If committing to a shared repository, the commit may fail if the
repository that has been changed since your last update.  In that
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case, you must perform an update before trying again.  On a
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decentralized version control system, use @kbd{C-x v +}
(@pxref{Pulling / Pushing}) or @kbd{C-x v m} (@pxref{Merging}).
On a centralized version control system, type @kbd{C-x v v} again to
merge in the repository changes.
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@item
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Finally, if you are using a centralized version control system, check
if each work file in the VC fileset is up-to-date.  If any file has
been changed in the repository, offer to update it.
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@end itemize

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  These rules also apply when you use RCS in its non-locking mode,
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except that changes are not automatically merged from the repository.
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Nothing informs you if another user has committed changes in the same
file since you began editing it; when you commit your revision, his
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changes are removed (however, they remain in the repository and are
thus not irrevocably lost).  Therefore, you must verify that the
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current revision is unchanged before committing your changes.  In
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addition, locking is possible with RCS even in this mode: @kbd{C-x v
v} with an unmodified file locks the file, just as it does with RCS in
its normal locking mode (@pxref{VC With A Locking VCS}).
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@node VC With A Locking VCS
@subsubsection Basic Version Control with Locking

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  On a locking-based version control system (such as SCCS, and RCS in
its default mode), @kbd{C-x v v} does the following:
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@itemize @bullet
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@item
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If there is more than one file in the VC fileset and the files have
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inconsistent version control statuses, signal an error.
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@item
If each file in the VC fileset is not registered with a version
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control system, register the VC fileset.  @xref{Registering}.  If
Emacs cannot find a system to register under, it prompts for a
repository type, creates a new repository, and registers the VC
fileset with it.
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@item
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If each file is registered and unlocked, lock it and make it writable,
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so that you can begin to edit it.
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@item
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If each file is locked by you and contains changes, commit the
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changes.  To do this, Emacs pops up a @file{*vc-log*} buffer; type the
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desired log entry for the new revision, followed by @kbd{C-c C-c} to
commit (@pxref{Log Buffer}).
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@item
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If each file is locked by you, but you have not changed it, release
the lock and make the file read-only again.
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@item
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If each file is locked by another user, ask whether you want to
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steal the lock.  If you say yes, the file becomes locked by you,
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and a warning message is sent to the user who had formerly locked the
file.
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@end itemize

  These rules also apply when you use CVS in locking mode, except
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that CVS does not support stealing locks.
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@node Advanced C-x v v
@subsubsection Advanced Control in @kbd{C-x v v}

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@cindex revision ID in version control
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  When you give a prefix argument to @code{vc-next-action} (@kbd{C-u
C-x v v}), it still performs the next logical version control
operation, but accepts additional arguments to specify precisely how
to do the operation.

@itemize @bullet
@item
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@cindex specific version control system
You can specify the name of a version control system.  This is useful
if the fileset can be managed by more than one version control system,
and Emacs fails to detect the correct one.
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@item
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Otherwise, if using CVS or RCS, you can specify a revision ID.
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If the fileset is modified (or locked), this makes Emacs commit with
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that revision ID@.  You can create a new branch by supplying an
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appropriate revision ID (@pxref{Branches}).
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If the fileset is unmodified (and unlocked), this checks the specified
revision into the working tree.  You can also specify a revision on
another branch by giving its revision or branch ID (@pxref{Switching
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Branches}).  An empty argument (i.e., @kbd{C-u C-x v v @key{RET}})
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checks out the latest (head) revision on the current branch.
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This is silently ignored on a decentralized version control system.
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Those systems do not let you specify your own revision IDs, nor do
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they use the concept of checking out individual files.
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@end itemize

@node Log Buffer
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@subsection Features of the Log Entry Buffer

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@cindex C-c C-c @r{(Log Edit mode)}
@findex log-edit-done
  When you tell VC to commit a change, it pops up a buffer named
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@file{*vc-log*}.  In this buffer, you should write a @dfn{log entry}
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describing the changes you have made (@pxref{Why Version Control?}).
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After you are done, type @kbd{C-c C-c} (@code{log-edit-done}) to exit
the buffer and commit the change, together with your log entry.
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@cindex Log Edit mode
@cindex mode, Log Edit
@vindex vc-log-mode-hook
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@c FIXME: Mention log-edit-mode-hook here?  --xfq
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  The major mode for the @file{*vc-log*} buffer is Log Edit mode, a
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variant of Text mode (@pxref{Text Mode}).  On entering Log Edit mode,
Emacs runs the hooks @code{text-mode-hook} and @code{vc-log-mode-hook}
(@pxref{Hooks}).

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  In the @file{*vc-log*} buffer, you can write one or more @dfn{header
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lines}, specifying additional information to be supplied to the
version control system.  Each header line must occupy a single line at
the top of the buffer; the first line that is not a header line is
treated as the start of the log entry.  For example, the following
header line states that the present change was not written by you, but
by another developer:
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@smallexample
Author: J. R. Hacker <jrh@@example.com>
@end smallexample
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@noindent
Apart from the @samp{Author} header, Emacs recognizes the headers
@samp{Date} (a manually-specified commit time) and @samp{Fixes} (a
reference to a bug fixed by the change).  Not all version control
systems recognize all headers: Bazaar recognizes all three headers,
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while Git, Mercurial, and Monotone recognize only @samp{Author} and
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@samp{Date}.  If you specify a header for a system that does not
support it, the header is treated as part of the log entry.
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@kindex C-c C-f @r{(Log Edit mode)}
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@findex log-edit-show-files
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@kindex C-c C-d @r{(Log Edit mode)}
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@findex log-edit-show-diff
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  While in the @file{*vc-log*} buffer, the current VC fileset is
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considered to be the fileset that will be committed if you type
@w{@kbd{C-c C-c}}.  To view a list of the files in the VC fileset,
type @w{@kbd{C-c C-f}} (@code{log-edit-show-files}).  To view a diff
of changes between the VC fileset and the version from which you
started editing (@pxref{Old Revisions}), type @kbd{C-c C-d}
(@code{log-edit-show-diff}).
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@kindex C-c C-a @r{(Log Edit mode)}
@findex log-edit-insert-changelog
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  If the VC fileset includes one or more @file{ChangeLog} files
(@pxref{Change Log}), type @kbd{C-c C-a}
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(@code{log-edit-insert-changelog}) to pull the relevant entries into
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the @file{*vc-log*} buffer.  If the topmost item in each
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@file{ChangeLog} was made under your user name on the current date,
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this command searches that item for entries matching the file(s) to be
committed, and inserts them.
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@ifnottex
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If you are using CVS or RCS, see @ref{Change Logs and VC}, for the
opposite way of working---generating ChangeLog entries from the Log
Edit buffer.
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@end ifnottex

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  To abort a commit, just @emph{don't} type @kbd{C-c C-c} in that
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buffer.  You can switch buffers and do other editing.  As long as you
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don't try to make another commit, the entry you were editing remains
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in the @file{*vc-log*} buffer, and you can go back to that buffer at
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any time to complete the commit.

@kindex M-n @r{(Log Edit mode)}
@kindex M-p @r{(Log Edit mode)}
@kindex M-s @r{(Log Edit mode)}
@kindex M-r @r{(Log Edit mode)}
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  You can also browse the history of previous log entries to duplicate
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a commit comment.  This can be useful when you want to make several
commits with similar comments.  The commands @kbd{M-n}, @kbd{M-p},
@kbd{M-s} and @kbd{M-r} for doing this work just like the minibuffer
history commands (@pxref{Minibuffer History}), except that they are
used outside the minibuffer.
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@node Registering
@subsection Registering a File for Version Control

@table @kbd
@item C-x v i
Register the visited file for version control.
@end table

@kindex C-x v i
@findex vc-register
  The command @kbd{C-x v i} (@code{vc-register}) @dfn{registers} each
file in the current VC fileset, placing it under version control.
This is essentially equivalent to the action of @kbd{C-x v v} on an
unregistered VC fileset (@pxref{Basic VC Editing}), except that if the
VC fileset is already registered, @kbd{C-x v i} signals an error
whereas @kbd{C-x v v} performs some other action.

  To register a file, Emacs must choose a version control system.  For
a multi-file VC fileset, the VC Directory buffer specifies the system
to use (@pxref{VC Directory Mode}).  For a single-file VC fileset, if
the file's directory already contains files registered in a version
control system, or if the directory is part of a directory tree
controlled by a version control system, Emacs chooses that system.  In
the event that more than one version control system is applicable,
Emacs uses the one that appears first in the variable
@iftex
@code{vc-handled-backends}.
@end iftex
@ifnottex
@code{vc-handled-backends} (@pxref{Customizing VC}).
@end ifnottex
If Emacs cannot find a version control system to register the file
under, it prompts for a repository type, creates a new repository, and
registers the file into that repository.

  On most version control systems, registering a file with @kbd{C-x v
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i} or @kbd{C-x v v} adds it to the working tree but not to the
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repository.  Such files are labeled as @samp{added} in the VC
Directory buffer, and show a revision ID of @samp{@@@@} in the mode
line.  To make the registration take effect in the repository, you
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must perform a commit (@pxref{Basic VC Editing}).  Note that a single
commit can include both file additions and edits to existing files.
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  On a locking-based version control system (@pxref{VCS Merging}),
registering a file leaves it unlocked and read-only.  Type @kbd{C-x v
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v} to start editing it.
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@node Old Revisions
@subsection Examining And Comparing Old Revisions

@table @kbd
@item C-x v =
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Compare the work files in the current VC fileset with the versions you
started from (@code{vc-diff}).  With a prefix argument, prompt for two
revisions of the current VC fileset and compare them.  You can also
call this command from a Dired buffer (@pxref{Dired}).

@ifnottex
@item M-x vc-ediff
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Like @kbd{C-x v =}, but using Ediff.  @xref{Top,, Ediff, ediff, The
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Ediff Manual}.
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@end ifnottex
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@item C-x v D
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Compare the entire working tree to the revision you started from
(@code{vc-root-diff}).  With a prefix argument, prompt for two
revisions and compare their trees.
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@item C-x v ~
Prompt for a revision of the current file, and visit it in a separate
buffer (@code{vc-revision-other-window}).
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@item C-x v g
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Display an annotated version of the current file: for each line, show
the latest revision in which it was modified (@code{vc-annotate}).
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@end table

@findex vc-diff
@kindex C-x v =
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  @kbd{C-x v =} (@code{vc-diff}) displays a @dfn{diff} which compares
each work file in the current VC fileset to the version(s) from which
you started editing.  The diff is displayed in another window, in a
Diff mode buffer (@pxref{Diff Mode}) named @file{*vc-diff*}.  The
usual Diff mode commands are available in this buffer.  In particular,
the @kbd{g} (@code{revert-buffer}) command performs the file
comparison again, generating a new diff.
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@kindex C-u C-x v =
  To compare two arbitrary revisions of the current VC fileset, call
@code{vc-diff} with a prefix argument: @kbd{C-u C-x v =}.  This
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prompts for two revision IDs (@pxref{VCS Concepts}), and displays a
diff between those versions of the fileset.  This will not work
reliably for multi-file VC filesets, if the version control system is
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file-based rather than changeset-based (e.g., CVS), since then
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revision IDs for different files would not be related in any
meaningful way.

  Instead of the revision ID, some version control systems let you
specify revisions in other formats.  For instance, under Bazaar you
can enter @samp{date:yesterday} for the argument to @kbd{C-u C-x v =}
(and related commands) to specify the first revision committed after
yesterday.  See the documentation of the version control system for
details.

  If you invoke @kbd{C-x v =} or @kbd{C-u C-x v =} from a Dired buffer
(@pxref{Dired}), the file listed on the current line is treated as the
current VC fileset.

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@ifnottex
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@findex vc-ediff
  @kbd{M-x vc-ediff} works like @kbd{C-x v =}, except that it uses an
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Ediff session.  @xref{Top,, Ediff, ediff, The Ediff Manual}.
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@end ifnottex

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@findex vc-root-diff
@kindex C-x v D
  @kbd{C-x v D} (@code{vc-root-diff}) is similar to @kbd{C-x v =}, but
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it displays the changes in the entire current working tree (i.e., the
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working tree containing the current VC fileset).  If you invoke this
command from a Dired buffer, it applies to the working tree containing
the directory.
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@vindex vc-diff-switches
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  You can customize the @command{diff} options that @kbd{C-x v =} and
@kbd{C-x v D} use for generating diffs.  The options used are taken
from the first non-@code{nil} value amongst the variables
@code{vc-@var{backend}-diff-switches}, @code{vc-diff-switches}, and
@code{diff-switches} (@pxref{Comparing Files}), in that order.  Here,
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@var{backend} stands for the relevant version control system,
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e.g., @code{bzr} for Bazaar.  Since @code{nil} means to check the
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next variable in the sequence, either of the first two may use the
value @code{t} to mean no switches at all.  Most of the
@code{vc-@var{backend}-diff-switches} variables default to @code{nil},
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but some default to @code{t}; these are for version control systems
whose @code{diff} implementations do not accept common diff options,
such as Subversion.
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@findex vc-revision-other-window
@kindex C-x v ~
  To directly examine an older version of a file, visit the work file
and type @kbd{C-x v ~ @var{revision} @key{RET}}
(@code{vc-revision-other-window}).  This retrieves the file version
corresponding to @var{revision}, saves it to
@file{@var{filename}.~@var{revision}~}, and visits it in a separate
window.
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@findex vc-annotate
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@vindex vc-annotate-background-mode
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@kindex C-x v g
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  Many version control systems allow you to view files @dfn{annotated}
with per-line revision information, by typing @kbd{C-x v g}
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(@code{vc-annotate}).  This creates a new ``annotate'' buffer
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displaying the file's text, with each line colored to show how old it
is.  Red text is new, blue is old, and intermediate colors indicate
intermediate ages.  By default, the color is scaled over the full
range of ages, such that the oldest changes are blue, and the newest
changes are red.  If the variable @code{vc-annotate-background-mode}
is non-@code{nil}, the colors expressing the age of each line are
applied to the background color, leaving the foreground at its default
color.
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  When you give a prefix argument to this command, Emacs reads two
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arguments using the minibuffer: the revision to display and annotate
(instead of the current file contents), and the time span in days the
color range should cover.
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  From the ``annotate'' buffer, these and other color scaling options are
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available from the @samp{VC-Annotate} menu.  In this buffer, you can
also use the following keys to browse the annotations of past revisions,
view diffs, or view log entries:

@table @kbd
@item p
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Annotate the previous revision, i.e., the revision before the one
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currently annotated.  A numeric prefix argument is a repeat count, so
@kbd{C-u 10 p} would take you back 10 revisions.
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@item n
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Annotate the next revision, i.e., the revision after the one
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currently annotated.  A numeric prefix argument is a repeat count.
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@item j
Annotate the revision indicated by the current line.

@item a
Annotate the revision before the one indicated by the current line.
This is useful to see the state the file was in before the change on
the current line was made.

@item f
Show in a buffer the file revision indicated by the current line.

@item d
Display the diff between the current line's revision and the previous
revision.  This is useful to see what the current line's revision
actually changed in the file.

@item D
Display the diff between the current line's revision and the previous
revision for all files in the changeset (for VC systems that support
changesets).  This is useful to see what the current line's revision
actually changed in the tree.

@item l
Show the log of the current line's revision.  This is useful to see
the author's description of the changes in the revision on the current
line.

@item w
Annotate the working revision--the one you are editing.  If you used
@kbd{p} and @kbd{n} to browse to other revisions, use this key to
return to your working revision.

@item v
Toggle the annotation visibility.  This is useful for looking just at
the file contents without distraction from the annotations.
@end table

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@node VC Change Log
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@subsection VC Change Log
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@table @kbd
@item C-x v l
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Display the change history for the current fileset
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(@code{vc-print-log}).

@item C-x v L
Display the change history for the current repository
(@code{vc-print-root-log}).
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@item C-x v I
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Display the changes that a ``pull'' operation will retrieve
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(@code{vc-log-incoming}).

@item C-x v O
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Display the changes that will be sent by the next ``push'' operation
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(@code{vc-log-outgoing}).
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@end table

@kindex C-x v l
@findex vc-print-log
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  @kbd{C-x v l} (@code{vc-print-log}) displays a buffer named
@file{*vc-change-log*}, showing the history of changes made to the
current file, including who made the changes, the dates, and the log
entry for each change (these are the same log entries you would enter
via the @file{*vc-log*} buffer; @pxref{Log Buffer}).  Point is
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centered at the revision of the file currently being visited.  With a
prefix argument, the command prompts for the revision to center on,
and the maximum number of revisions to display.

  If you call @kbd{C-x v l} from a VC Directory buffer (@pxref{VC
Directory Mode}) or a Dired buffer (@pxref{Dired}), it applies to the
file listed on the current line.
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@findex vc-print-root-log
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@findex log-view-toggle-entry-display
  @kbd{C-x v L} (@code{vc-print-root-log}) displays a
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@file{*vc-change-log*} buffer showing the history of the entire
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version-controlled directory tree (RCS, SCCS, and CVS do not support
this feature).  With a prefix argument, the command prompts for the
maximum number of revisions to display.

  The @kbd{C-x v L} history is shown in a compact form, usually
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showing only the first line of each log entry.  However, you can type
@key{RET} (@code{log-view-toggle-entry-display}) in the
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@file{*vc-change-log*} buffer to reveal the entire log entry for the
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revision at point.  A second @key{RET} hides it again.
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  On a decentralized version control system, the @kbd{C-x v I}
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(@code{vc-log-incoming}) command displays a log buffer showing the
changes that will be applied, the next time you run the version
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control system's pull command to get new revisions from another
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repository (@pxref{Pulling / Pushing}).  This other repository is the default
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one from which changes are pulled, as defined by the version control
system; with a prefix argument, @code{vc-log-incoming} prompts for a
specific repository.  Similarly, @kbd{C-x v O}
(@code{vc-log-outgoing}) shows the changes that will be sent to
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another repository, the next time you run the push command; with a
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prefix argument, it prompts for a specific destination repository.
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  In the @file{*vc-change-log*} buffer, you can use the following keys
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to move between the logs of revisions and of files, and to examine and
compare past revisions (@pxref{Old Revisions}):
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@table @kbd
@item p
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Move to the previous revision entry.  (Revision entries in the log