Version Control - are you using it?

What is version control?

• Keeping old versions of program source, configuration files, documents, etc.
• Keeping track of and manipulating these versions, through logs, labels, diffs, etc.

Why use version control?

• Encourages documentation of changes, change log
• If a change introduces a bug, debugging effort can focus on the code that changed
• If a line of development turns out to be unfruitful, easy to roll back to earlier version
• Easy to manage parallel development of different versions of a file, e.g.
  • patches to released software
  • implementing new feature independently of ongoing development
  • the local tech report version of a paper vs. the journal version
• Regulates multiple users wanting to change the same file

Systems for version control - RCS

Overview

• RCS == Revision Control System
• Version control on a single file
• All versions of a file stored as diffs in a file with ",v" appended, usually in a subdirectory named "RCS"
• Multiple users are handled by sharing a common RCS subdirectory (via symlinks)

Basic operations

• Extracts a read-only copy of the latest version of the file
• Options:

  "-r<version>"

  Extracts a read-only copy of a particular version of the file

  "-l"

  Locks the file and extracts an copy for editing

  "-d<date>"

  Extracts a read-only copy of the file as at the specified date/time

• Adds a new version of the file, prompting for a log message, and removing the checked-out version of the file
• Options:

  "-l"

  Checks in the new version, and immediately checks it out again with a lock

  "-u"

  Checks in the new version, and immediately checks it out again without a lock

  "-n<name>"

  Assign the symbolic name <name> to the checked in version

• Prints a "diff" of a file
• By default, prints changes made since last check-out, but can be used to compare any two versions

Other operations

Version numbering

• Standard version number: 1.3
• A version on a branch off 1.3 might be 1.3.2.4
• A version on a branch off 1.3.2.4 might be 1.3.2.4.1.1

Drawbacks with RCS

• Most operations only work with one file at a time
• For a collection or hierarchy of directories, each directory has to be set up/managed individually
• Only one user can work on a given file at once
• If one user forgets they have a lock on a file, it tends to annoy others (particularly if they go on holidays)

Systems for version control - CVS

Overview

• CVS == Concurrent Versions System
• Built on top of RCS
• Allows managing of an entire directory hierarchy, or a group of files and/or directories (through the module system), as a single entity
• Allows several people to edit a file simultaneously
• Uses a "central repository" (you can have more than one) where all the RCS files are stored (set with CVSROOT environment variable, or as a command-line option)
• Creates a local subdirectory "CVS" to store some administrative files

Basic operations

• Creates a private copy of the contents of <modules>
• Options:

  "-r <tag>"

  Uses versions of files with the specified tag (sticky)

  "-D <date>"

  Uses versions of files current at specified date (sticky)

• Updates a private directory from repository (e.g. if someone else has updated files)
• Attempts to merge changes if a file you've edited locally has been updated in the repository, and warns of any conflicts caused by the merge
• Options:

  "-r <tag>"

  Updates to versions of files with the specified tag (sticky)

  "-D <date>"

  Updates to versions of files current at specified date (sticky)

  "-A"

  Reset sticky tags, dates, etc.

  "-d"

  Create any new directories added to the repository

• Adds your changes to the repository (can only be done if you are up-to-date with repository)
• Options:

  "-r <revision>"

  Commit to specified branch/revision

• Marks <files> for addition to the repository on next commit

• Marks <files> for deletion from the repository (actually just moves it to an "Attic") - you must have deleted the files first

• Reverses a "cvs checkout", discarding any changes
• Options:

  "-d"

  Delete the local copy

• Displays logs for files

• Runs "diff" on two versions of files
• By default, compares local copy to the relevant version in repository
• Can compare to a different version, or compare two arbitrary versions, by using appropriate "-r" and "-D" options

• Displays status of files

Other operations

• Import files from an external source
• Can be used for creating a new module

• Like a checkout, but does not set up admin data, and cannot commit, etc.
• Options:

  "-r <tag>"

  Use versions of files with the specified tag

  "-D <date>"

  Use versions of files current at specified date

• Creates a new repository

• Associates the tag <tag> to the current versions of files
• Used to, e.g., mark which versions of files went into a distribution
• Options:

  "-b"

  Creates a branch tag - the tag is associated with a branch rather than a particular version

  "-d"

  Deletes a tag (be careful)

• Can be used to find out when each line in a file was last modified, and by whom
• Options:

  "-r <tag>"

  Annotate versions of files with the specified tag

  "-D <date>"

  Annotate versions of files current at specified date

Miscellaneous features

.cvsignore

Lists files that should be ignored by CVS (wildcards accepted)

Modules

Much more can be done with modules than has been covered here

E.g. can define commands to be run on commit, etc.

Repositories on remote machines

Access to repositories on remote machines can be arranged via

• rsh
• password authentication
• kerberos

Info pages

Where to get them

Original source

ftp://ftp.gnu.org/pub/gnu/rcs/rcs-5.7.tar.gz

ftp://ftp.gnu.org/pub/gnu/cvs/cvs-1.9.tar.gz

Closing comments

• Logs only as good as what you enter
• Best to commit one conceptual change at a time