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CVS and RCS (Linux in a Nutshell, 3rd Edition)

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CVS and RCS (Linux in a Nutshell, 3rd Edition)



contain files from different stages of the development of the same project. Users set up and

control sandboxes using the procedures and commands found in Section 14.4, "CVS User

Reference", later in this chapter.

In a typical interaction with the version control system, a developer checks out the most

current code from the repository, makes changes, tests the results, and then commits those

changes back to the repository when they are deemed satisfactory.



14.1.1. Locking and Merging

Some systems, including RCS, use a locking model to coordinate the efforts of multiple

developers by serializing file modifications. Before making changes to a file, a developer

must not only obtain a copy of it, but he must also request and obtain a lock on it from the

system. This lock serves to prevent (really dissuade) multiple developers from working on the

same file at the same time. When the changes are committed, the developer unlocks the file,

permitting other developers to gain access to it.

The locking model is pessimistic: it assumes that conflicts must be avoided. Serialization of

file modifications through locks prevents conflicts. But it is cumbersome to have to lock files

for editing when bug-hunting. Often, developers will circumvent the lock mechanism to keep

working, which is an invitation to trouble.

Unlike RCS and SCCS, CVS uses a merging model which allows everyone to have access to

the files at all times and supports concurrent development. The merging model is optimistic: it

assumes that conflicts are not common and that when they do occur, it usually isn't difficult to

resolve them.

CVS is capable of operating under a locking model via the -L and -l options to the admin

command. Also, CVS has special commands (edit and watch) for those who want additional

development coordination support. CVS uses locks internally to prevent corruption when

multiple people are accessing the repository simultaneously, but this is different from the uservisible locks of the locking model discussed here.



14.1.2. Conflicts and Merging

In the event that two developers commit changes to the same version of a file, CVS

automatically defers the commit of the second committer's file. The second developer then

issues the cvs update command, which merges the first developer's changes into the local file.

In many cases, the changes will be in different areas of the file, and the merge is successful.

However, if both developers have made changes to the same area of the file, the second to

commit will have to resolve the conflict. This involves examination of the problematic area(s)

of the file and selection among the multiple versions or making changes that resolve the

conflict.



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CVS only detects textual conflicts, but conflict resolution is concerned with keeping the

project as a whole logically consistent. Therefore, conflict resolution sometimes involves

changing files other than the one about which CVS complained.

For example, if one developer adds a parameter to a function definition, it may be necessary

for all the calls to that function to be modified to pass the additional parameter. This is a

logical conflict, so its detection and resolution is the job of the developers (with support from

tools like compilers and debuggers); CVS won't notice the problem.

In any merge situation, whether or not there was a conflict, the second developer to commit

will often want to retest the resulting version of the project because it has changed since the

original commit. Once it passes, the developer will need to recommit the file.



14.1.3. Tagging

CVS tracks file versions by revision number, which can be used to retrieve a particular

revision from the repository. In addition, it is possible to create symbolic tags so that a group

of files (or an entire project) can be referred to by a single identifier even when the revision

numbers of the files are not the same (which is most often the case). This capability is often

used to keep track of released versions or other important project milestones.

For example, the symbolic tag hello-1_0 might refer to revision number 1.3 of hello.c and

revision number 1.1 of Makefile (symbolic tags are created with the tag and rtag commands).



14.1.4. Branching

The simplest form of development is linear, in which there is a succession of revisions to a

file, each derived from the prior revision. Many projects can get by with a completely linear

development process, but larger projects (as measured by number of files, number of

developers, and/or the size of the user community) often run into maintenance issues that

require additional capabilities. Sometimes, it is desirable to do some speculative development

while the main line of development continues uninterrupted. Other times, bugs in the currently

released version must be fixed while work on the next version is underway. In both of these

cases, the solution is to create a branch (fork) from an appropriate point in the development of

the project. If at a future point some or all of the changes on the branch are needed back on the

main line of development (or elsewhere), they can be merged in (joined).

Branches are forked with the tag -b command; they are joined with the update -j command.



13.8. Alphabetical Summary

of Commands



14.2. The CVS Utility



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Copyright © 2001 O'Reilly & QKFIN. All rights reserved.



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The CVS Utility (Linux in a Nutshell, 3rd Edition)



Linux in a Nutshell, 3rd

Edition



14.2. The CVS Utility

This section offers general background about CVS.



14.2.1. CVS Command Format

CVS commands are of the form:

cvs global_options command command_options

For example, here is a simple sequence of commands showing both kinds of options in the context of creating a repository,

importing existing files, and performing a few common operations on them:

user@localhost$

user@localhost$

user@localhost$

user@localhost$

user@localhost$

user@localhost$

user@localhost$

user@localhost$

user@localhost$

user@localhost$

user@localhost$

user@localhost$

user@localhost$

user@localhost$



cvs -d /usr/local/cvsrep init

cd ~/work/hello

cvs -d /usr/local/cvsrep import -m 'Import' hello vendor start

cd ..

mv hello hello.bak

cvs -d /usr/local/cvsrep checkout hello

cd hello

vi hello

cvs commit -m 'Fixed a typo'

cvs tag hello-1_0

cvs remove -f Makefile

cvs commit -m 'Removed old Makefile'

cvs upd -r hello-1_0

cvs upd -A



Some global options are common to both user and administrator commands, and some are specific to each of these. The

common global options are described in the next section, and the user and administrator options are described in the

Section 14.4, "CVS User Reference" and Section 14.3, "CVS Administrator Reference" sections, respectively.



14.2.2. Common Global Options

Table 14-1 lists the global options that apply to both user and administrator commands.

Table 14-1. Common Global Options

Option



Description



-b bindir



Location of external RCS programs. This option is obsolete, having been deprecated at CVS versions above

1.9.18.



-T tempdir Absolute path for temporary files. Overrides the setting of $TMPDIR.



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-v

Display version and copyright information.

--version



14.2.3. Gotchas

This section clarifies a few aspects of CVS that can sometimes cause confusion.

CVS's file orientation

While directories are supported, they are not versioned in the same way as traditional files. This is particularly

important in the early evolutionary stages of a project, when the structure may be in flux. Also, if the project is

undergoing major changes, the structure is likely to change. See later in Section 14.3.4, "Hacking the Repository".

CVS's text-orientation

There is no equivalent to diff for binary files, although CVS's support for binary files is usually sufficient. Use

admin -kb to tell CVS a file is binary.

CVS's line-orientation

Moving a segment of code from one place in a file to another is seen as one delete (from the old location) and an

unrelated add (to the new location).

CVS is not syntax-aware

As far as CVS is concerned, small formatting changes are equivalent to sweeping logic changes in the same line

ranges.

RCS anachronisms

CVS was originally built on top of RCS, but now all the RCS-related functionality is internal to CVS itself. RCS

still shows up in the name of the $RCSBIN environment variable and the description of the -b option, which are

now obsolete.



14. CVS and RCS



14.3. CVS Administrator

Reference



Copyright © 2001 O'Reilly & QKFIN. All rights reserved.



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CVS Administrator Reference (Linux in a Nutshell, 3rd Edition)



Linux in a Nutshell, 3rd

Edition



14.3. CVS Administrator Reference

This section provides details on creating and configuring repositories and performing other CVS

administrative tasks. A single computer can run multiple copies of the CVS server, and each server can serve

multiple repositories.



14.3.1. Creating a Repository

Select a directory that will contain the repository files (/usr/local/cvsrep is used in the following examples).

Use the init command to initialize the repository. Either set the $CVSROOT environment variable first:

user@localhost$ export CVSROOT=/usr/local/cvsrep

user@localhost$ cvs init

or use the -d option to specify the repository location:

user@localhost$ cvs -d /usr/local/cvsrep init

For information on importing code, see Section 14.4, "CVS User Reference", especially Section 14.4.7.11,

"import" and Section 14.4.7.1, "add" sections.



14.3.1.1. Setting up the password server

If you want users to access the repository from other computers, then configure the pserver by doing the

following as root:





Make sure there is an entry in /etc/services similar to the following:

cvspserver 2401/tcp







If you are not using tcpwrappers, then place a line like this in /etc/inetd.conf:

cvspserver stream tcp nowait root /usr/bin/cvs cvs

--allow-root=/usr/local/cvsroot pserver







Or, if you are using tcpwrappers, then use a line like this:

cvspserver stream tcp nowait root /usr/sbin/tcpd /usr/bin/cvs

--allow-root=/usr/local/cvsroot pserver







Once these changes are in place, restart inetd (or send it the appropriate signal to cause it to re-read



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inetd.conf).



14.3.2. Security Issues

The following are security issues that need to be considered when working with CVS:

























The contents of files will be transmitted in the open over the network with pserver and rsh. With

pserver, passwords are transmitted in the open as well.

When using a local repository (i.e., when CVS is not being used in client/server mode), developers

need write access to the repository, which means they can hack it.

The CVS server runs as root briefly before changing its user ID.

The ~/.cvspass file must be kept unreadable by all users except the owner to prevent passwords from

being accessible.

A user who has authority to make changes to the files in the CVSROOT module can run arbitrary

programs.

Some of the options to the admin command are very dangerous, so it is advisable to restrict its use.

This can be accomplished by creating a user group named cvsadmin. If this user group exists, then

only users in that group can run the admin command (except admin -kkflag, which is available to

everyone).



14.3.3. Repository Structure

The CVS repository is implemented as a normal directory with special contents. This section describes the

contents of the repository directory.



14.3.3.1. The CVSROOT directory

The CVSROOT directory contains the administrative files for the repository; other directories in the repository

contain the modules. The administrative files permit (and ignore) blank lines and comment lines in addition to

the lines with real configuration information on them. Comment lines start with a hash mark (`#').

Some of the administrative files contain filename patterns to match file and directory names. These patterns

are regular expressions like those used in GNU Emacs. Table 14-2 contains the special constructions used

most often.

Table 14-2. Filename Pattern Special Constructions

Construction Description



^



Match the beginning of the string.



$



Match the end of the string.



.



Match any single character.



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*



Modify the preceding construct to match zero or more repetitions.



CVS will perform a few important expansions in the contents of the administrative files before interpreting

the results. First, the typical shell syntax for referring to a home directory is ~/, which expands to the home

directory of the user running CVS; and ~user expands to the home directory of the specified user.

In addition, CVS provides a mechanism similar to the shell's environment variable expansion capability.

Constructs such as ${variable} will be replaced by the value of the named variable. Variable names start

with letters and consist entirely of letters, numbers, and underscores. Curly brackets may be omitted if the

character immediately following the variable reference is not a valid variable name character. While this

construct looks like a shell environment variable reference, the full environment is not available. Table 14-3

contains the built-in variables.

Table 14-3. Administrative File Variables

Variable



Description



CVSEDITOR

EDITOR



The editor CVS uses for log file editing.



VISUAL

CVSROOT



The repository locator in use.



USER



The name of the user (on the server, if using a remote repository) running CVS.



=var



The value of a user-defined variable named var. Values for these variables are provided by

the global -s option.



In order to edit these files, check out the CVSROOT module from the repository, edit the files, and commit

them back to the repository. You must commit the changes for them to affect CVS's behavior.

Table 14-4 describes the administrative files and their functions.

Table 14-4. CVSROOT Files

File



Description



checkoutlist



Extra files to be maintained in CVSROOT



commitinfo



Specifications for commit governors



config



Settings to affect the behavior of CVS



cvsignore



Filename patterns of files to ignore



cvswrappers Specifications for checkout and commit filters

editinfo



Specifications for log editors (obsolete)



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history



Log information for the history command



loginfo



Specify commit notifier program(s)



modules



Module definitions



notify



Notification processing specifications



passwd



A list of users and their CVS-specific passwords



rcsinfo



Template form for log messages



readers



A list of users having read-only access



taginfo



Tag processing specifications



users



Alternate user email addresses for use with notify



verifymsg



Specify log message evaluator program



writers



A list of users having read/write access



Since the editinfo file is obsolete, use the $EDITOR environment variable (or the -e option) to specify the

editor and the verifymsg file to specify an evaluator.

Each line of the taginfo file contains a filename pattern and a command line to execute when files with

matching names are tagged.



14.3.3.2. The checkoutlist file

Whenever changes to files in the CVSROOT module are committed, CVS prints the message:

cvs commit: Rebuilding administrative file database

to inform you that the checked-out copy in the repository has been updated to reflect any changes just

committed. As with any other module directory in the repository, the CVSROOT directory contains RCS (*,v)

files that retain the history of the files. But to use the files, CVS needs a copy of the latest revision. So, when

CVS prints this message, it is checking out the latest revisions of the administrative files.

If you have added files to the CVSROOT module (such as scripts to be called via entries in the loginfo file),

you will need to list them in the checkoutlist file. This makes CVS treat them the same way as it treats the

standard set of CVSROOT files.

Each line in this file consists of a filename and an optional error message that is displayed in case there is

trouble checking out the file.



14.3.3.3. The commitinfo file

Whenever a commit is being processed, CVS consults this file to determine whether or not any precommit

checking of the file is required. Each line of the file contains a directory name pattern, followed by the path of

a program to invoke when files are commited in directories with matching names.

Aside from the usual filename-pattern syntax, there are two special patterns:



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ALL

If this pattern is present in the file, then all files are passed to the specified checking program. CVS

then looks for a pattern that matches the name of each particular file and runs the additional checks

found, if any.

DEFAULT

If this pattern is present in the file, all files for which there was no pattern match are sent to the

specified checking program. The automatic match of every file to the ALL entry, if any, does not count

as a match when determining whether or not to send the file to the DEFAULT checking program.

CVS constructs the command line for the checking program by appending the full path to the directory within

the repository and the list of files being committed (this means you can specify the first few command-line

arguments to the program, if necessary). If the checking program exits with a nonzero status, the commit is

aborted.

The programs that run via this mechanism run on the server computer when a remote repository is used. Here

is an example of a commitinfo file:

ALL $CVSROOT/CVSROOT/commit-ALL.pl

DEFAULT $CVSROOT/CVSROOT/commit-DEFAULT.pl

CVSROOT$ $CVSROOT/CVSROOT/commit-CVSROOT.pl

This example assumes you will create the script files in the CVSROOT module and add them to the

checkoutlist file.



14.3.3.4. The config file

Repository configuration is specified in the config administrative file.

LockDir=dir

Directs CVS to put its lock files in the alternate directory given instead of in the repository itself,

allowing users without write access to the repository (but with write access to dir) to read from the

repository.

Version 1.10 doesn't support alternate directories for lock files and reports an error if this option is set.

Older versions of CVS (1.9 and older) don't support this option either and will not report an error. Do

not mix versions that support alternate directories for lock files with versions that don't, since lock files

in both places defeat the purpose of having them.

RCSBIN=dir

Obsolete (used in versions 1.9.12 to 1.9.18). This option used to tell CVS where to find RCS programs.

Since all RCS-related functions are now handled internally, this option does nothing.

SystemAuth=value

CVS tries to authenticate users via the CVSROOT/passwd file first, and if that fails and this option is



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set to yes, CVS tries to authenticate via the system's user database. This option is used with the

password server. The default is yes.

TopLevelAdmin=value

If this option is set to yes, an additional CVS directory is created at the top-level directory when

checkout is run. This allows the client software to detect the repository locator in that directory (see

Section 14.4.1, "Repository Locators"). The default is no.

This option is useful if you check out multiple modules to the same sandbox directory. If it is enabled,

you won't have to provide a repository locator after the first checkout; CVS infers it from the

information in the top-level CVS directory created during the first checkout.



14.3.3.5. The cvsignore file

The cvsignore administrative file contains a list of filename patterns to ignore, just like the .cvsignore files

that can appear in sandboxes and user home directories. Unlike the filename patterns in other administrative

files, these patterns are in sh syntax; they are not GNU Emacs-style regular expressions. There can be

multiple patterns on a line, separated by whitespace (consequently, the patterns themselves cannot contain

whitespace).

Table 14-5 shows the most commonly used sh-style pattern constructs.

Table 14-5. Filename Patterns for cvsignore

Construct Description



?



Any one character.



*



Any sequence of zero or more characters.



Again, diverging from the standards used by the rest of the administrative files, the cvsignore file does not

support comments.



14.3.3.6. The cvswrappers file

While the cvsignore file allows CVS to ignore certain files, the cvswrappers file allows you to give CVS

default options for commands that work with files. Lines in this file consist of a sh-style filename pattern

followed by a -k (keyword substitution mode) option and/or an -m (update method) option. The legal values

for -k are described in Table 14-19. The legal values for -m are COPY and MERGE.

If -m COPY is specified, CVS will not attempt to merge the files. Instead, it presents the user with conflicting

versions of the file, and he can choose one or the other or resolve the conflict manually.

For example, to treat all files ending in .exe as binary, add this line to the file:

*.exe -k b



14.3.3.7. The history file

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