CVS—Concurrent Versions System v4.2: 1. Overview

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# 1. Overview

This chapter is for people who have never used CVS, and perhaps have never used version control software before.

If you are already familiar with CVS and are just trying to learn a particular feature or remember a certain command, you can probably skip everything here.

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## 1.1 What is CVS?

CVS is a version control system. Using it, you can record the history of your source files.

For example, bugs sometimes creep in when software is modified, and you might not detect the bug until a long time after you make the modification. With CVS, you can easily retrieve old versions to see exactly which change caused the bug. This can sometimes be a big help.

You could of course save every version of every file you have ever created. This would however waste an enormous amount of disk space. CVS stores all the versions of a file in a single file in a clever way that only stores the differences between versions.

CVS also helps you if you are part of a group of people working on the same project. It is all too easy to overwrite each others’ changes unless you are extremely careful. Some editors, like GNU Emacs, try to make sure that the same file is never modified by two people at the same time. Unfortunately, if someone is using another editor, that safeguard will not work. CVS solves this problem by insulating the different developers from each other. Every developer works in his own directory, and CVS merges the work when each developer is done.

CVS started out as a bunch of shell scripts written by Dick Grune, posted to the newsgroup `comp.sources.unix` in the volume 6 release of July, 1986. While no actual code from these shell scripts is present in the current version of CVS much of the CVS conflict resolution algorithms come from them.

In April, 1989, Brian Berliner designed and coded CVS. Jeff Polk later helped Brian with the design of the CVS module and vendor branch support.

You can get CVS in a variety of ways, including free download from the internet. For more information on downloading CVS and other CVS topics, see:

    http://www.cvshome.org/
    http://www.loria.fr/~molli/cvs-index.html

There is a mailing list, known as `info-cvs`, devoted to CVS. To subscribe or unsubscribe write to `info-cvs-request@gnu.org`. If you prefer a usenet group, the right group is `comp.software.config-mgmt` which is for CVS discussions (along with other configuration management systems). In the future, it might be possible to create a `comp.software.config-mgmt.cvs`, but probably only if there is sufficient CVS traffic on `comp.software.config-mgmt`.

You can also subscribe to the `bug-cvs` mailing list, described in more detail in Dealing with bugs in CVS or this manual. To subscribe send mail to `bug-cvs-request@gnu.org`.

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## 1.2 What is CVS not?

CVS can do a lot of things for you, but it does not try to be everything for everyone.

CVS is not a build system.

  Though the structure of your repository and modules file interact with your build system (e.g. ‘`Makefile`’s), they are essentially independent.

  CVS does not dictate how you build anything. It merely stores files for retrieval in a tree structure you devise.

  CVS does not dictate how to use disk space in the checked out working directories. If you write your ‘`Makefile`’s or scripts in every directory so they have to know the relative positions of everything else, you wind up requiring the entire repository to be checked out.

  If you modularize your work, and construct a build system that will share files (via links, mounts, `VPATH` in ‘`Makefile`’s, etc.), you can arrange your disk usage however you like.

  But you have to remember that any such system is a lot of work to construct and maintain. CVS does not address the issues involved.

  Of course, you should place the tools created to support such a build system (scripts, ‘`Makefile`’s, etc) under CVS.

  Figuring out what files need to be rebuilt when something changes is, again, something to be handled outside the scope of CVS. One traditional approach is to use `make` for building, and use some automated tool for generating the dependencies which `make` uses.

  See How your build system interacts with CVS, for more information on doing builds in conjunction with CVS.

CVS is not a substitute for management.

  Your managers and project leaders are expected to talk to you frequently enough to make certain you are aware of schedules, merge points, branch names and release dates. If they don’t, CVS can’t help.

  CVS is an instrument for making sources dance to your tune. But you are the piper and the composer. No instrument plays itself or writes its own music.

CVS is not a substitute for developer communication.

  When faced with conflicts within a single file, most developers manage to resolve them without too much effort. But a more general definition of “conflict” includes problems too difficult to solve without communication between developers.

  CVS cannot determine when simultaneous changes within a single file, or across a whole collection of files, will logically conflict with one another. Its concept of a conflict is purely textual, arising when two changes to the same base file are near enough to spook the merge (i.e. `diff3`) command.

  CVS does not claim to help at all in figuring out non-textual or distributed conflicts in program logic.

  For example: Say you change the arguments to function `X` defined in file ‘`A`’. At the same time, someone edits file ‘`B`’, adding new calls to function `X` using the old arguments. You are outside the realm of CVS’s competence.

  Acquire the habit of reading specs and talking to your peers.

CVS does not have change control

  Change control refers to a number of things. First of all it can mean bug-tracking, that is being able to keep a database of reported bugs and the status of each one (is it fixed? in what release? has the bug submitter agreed that it is fixed?). For interfacing CVS to an external bug-tracking system, see the ‘`rcsinfo`’ and ‘`verifymsg`’ files (see section Reference manual for Administrative files).

  Another aspect of change control is keeping track of the fact that changes to several files were in fact changed together as one logical change. If you check in several files in a single `cvs commit` operation, CVS then forgets that those files were checked in together, and the fact that they have the same log message is the only thing tying them together. Keeping a GNU style ‘`ChangeLog`’ can help somewhat.

  Another aspect of change control, in some systems, is the ability to keep track of the status of each change. Some changes have been written by a developer, others have been reviewed by a second developer, and so on. Generally, the way to do this with CVS is to generate a diff (using `cvs diff` or `diff`) and email it to someone who can then apply it using the `patch` utility. This is very flexible, but depends on mechanisms outside CVS to make sure nothing falls through the cracks.

CVS is not an automated testing program

  It should be possible to enforce mandatory use of a testsuite using the `commitinfo` file. I haven’t heard a lot about projects trying to do that or whether there are subtle gotchas, however.

CVS does not have a builtin process model

  Some systems provide ways to ensure that changes or releases go through various steps, with various approvals as needed. Generally, one can accomplish this with CVS but it might be a little more work. In some cases you’ll want to use the ‘`commitinfo`’, ‘`loginfo`’, ‘`rcsinfo`’, or ‘`verifymsg`’ files, to require that certain steps be performed before cvs will allow a checkin. Also consider whether features such as branches and tags can be used to perform tasks such as doing work in a development tree and then merging certain changes over to a stable tree only once they have been proven.

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## 1.3 A sample session

As a way of introducing CVS, we’ll go through a typical work-session using CVS. The first thing to understand is that CVS stores all files in a centralized repository (see section The Repository); this section assumes that a repository is set up.

Suppose you are working on a simple compiler. The source consists of a handful of C files and a ‘`Makefile`’. The compiler is called ‘`tc`’ (Trivial Compiler), and the repository is set up so that there is a module called ‘`tc`’.

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### 1.3.1 Getting the source

The first thing you must do is to get your own working copy of the source for ‘`tc`’. For this, you use the `checkout` command:

    $ cvs checkout tc

This will create a new directory called ‘`tc`’ and populate it with the source files.

    $ cd tc
    $ ls
    CVS         Makefile    backend.c   driver.c    frontend.c  parser.c

The ‘`CVS`’ directory is used internally by CVS. Normally, you should not modify or remove any of the files in it.

You start your favorite editor, hack away at ‘`backend.c`’, and a couple of hours later you have added an optimization pass to the compiler. A note to RCS and SCCS users: There is no need to lock the files that you want to edit. See section Multiple developers, for an explanation.

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### 1.3.2 Committing your changes

When you have checked that the compiler is still compilable you decide to make a new version of ‘`backend.c`’. This will store your new ‘`backend.c`’ in the repository and make it available to anyone else who is using that same repository.

    $ cvs commit backend.c

CVS starts an editor, to allow you to enter a log message. You type in “Added an optimization pass.”, save the temporary file, and exit the editor.

The environment variable `$CVSEDITOR` determines which editor is started. If `$CVSEDITOR` is not set, then if the environment variable `$EDITOR` is set, it will be used. If both `$CVSEDITOR` and `$EDITOR` are not set then there is a default which will vary with your operating system, for example `vi` for unix or `notepad` for Windows NT/95.

In addition, CVS checks the `$VISUAL` environment variable. Opinions vary on whether this behavior is desirable and whether future releases of CVS should check `$VISUAL` or ignore it. You will be OK either way if you make sure that `$VISUAL` is either unset or set to the same thing as `$EDITOR`.

When CVS starts the editor, it includes a list of files which are modified. For the CVS client, this list is based on comparing the modification time of the file against the modification time that the file had when it was last gotten or updated. Therefore, if a file’s modification time has changed but its contents have not, it will show up as modified. The simplest way to handle this is simply not to worry about it—if you proceed with the commit CVS will detect that the contents are not modified and treat it as an unmodified file. The next `update` will clue CVS in to the fact that the file is unmodified, and it will reset its stored timestamp so that the file will not show up in future editor sessions.

If you want to avoid starting an editor you can specify the log message on the command line using the ‘`-m`’ flag instead, like this:

    $ cvs commit -m "Added an optimization pass" backend.c

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### 1.3.3 Cleaning up

Before you turn to other tasks you decide to remove your working copy of tc. One acceptable way to do that is of course

    $ cd ..
    $ rm -r tc

but a better way is to use the `release` command (see section release—Indicate that a Module is no longer in use):

    $ cd ..
    $ cvs release -d tc
    M driver.c
    ? tc
    You have [1] altered files in this repository.
    Are you sure you want to release (and delete) directory `tc': n
    ** `release' aborted by user choice.

The `release` command checks that all your modifications have been committed. If history logging is enabled it also makes a note in the history file. See section The history file.

When you use the ‘`-d`’ flag with `release`, it also removes your working copy.

In the example above, the `release` command wrote a couple of lines of output. ‘`? tc`’ means that the file ‘`tc`’ is unknown to CVS. That is nothing to worry about: ‘`tc`’ is the executable compiler, and it should not be stored in the repository. See section Ignoring files via cvsignore, for information about how to make that warning go away. See section release output, for a complete explanation of all possible output from `release`.

‘`M driver.c`’ is more serious. It means that the file ‘`driver.c`’ has been modified since it was checked out.

The `release` command always finishes by telling you how many modified files you have in your working copy of the sources, and then asks you for confirmation before deleting any files or making any note in the history file.

You decide to play it safe and answer `n <RET>` when `release` asks for confirmation.

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### 1.3.4 Viewing differences

You do not remember modifying ‘`driver.c`’, so you want to see what has happened to that file.

    $ cd tc
    $ cvs diff driver.c

This command runs `diff` to compare the version of ‘`driver.c`’ that you checked out with your working copy. When you see the output you remember that you added a command line option that enabled the optimization pass. You check it in, and release the module.

    $ cvs commit -m "Added an optimization pass" driver.c
    Checking in driver.c;
    /usr/local/cvsroot/tc/driver.c,v  <--  driver.c
    new revision: 1.2; previous revision: 1.1
    done
    $ cd ..
    $ cvs release -d tc
    ? tc
    You have [0] altered files in this repository.
    Are you sure you want to release (and delete) directory `tc': y

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