An Introduction to the arch Version Control System

arch is a software version control system similar to CVS and Subversion that was started by programmer Tom Lord to meet his version control needs. This article introduces some of arch's key features and presents a brief tutorial on using version 1.0pre11.

One reason for arch's creation was to overcome some weaknesses in existing version control systems, such as the lack of atomic commits, the inability to keep track of file renames and difficulties when working on different branches of a project.

arch also provides support for easily and intelligently merging code from several different branches (e.g., stable, development, feature-test) of a project. Projects and revisions stored in arch have globally unique names, which allows branch and merge operations to span network boundaries.

arch was started in November 2001 and since then it has gone from a concept to having early adopters and distributed repositories. arch uses many small specialized tools to get the job done and is written using a combination of C code, awk and shell scripts.

First, download arch from www.regexps.com and untar the source file.

arch's configure script frowns upon building arch in the same directory as the source code, so first you must create a build directory under arch's src directory. From the build directory, run ../configure, make and make install to get a working copy of arch.

If you don't specify an installation prefix for the configure script, arch's executables will be put in a directory called /install under the build directory. To control where install puts files use the --prefix= option when you run the configure script. If arch's bin directory isn't in your path, you can either add arch's bin directory to your PATH variable or copy the files from the =install/bin directory to your normal bin (e.g., /usr/local/bin). If you move the other arch directories (include, lib, libexec) arch won't work.

For shared archives, arch can be configured to send e-mails when project changes occur. If sendmail isn't in your path, you have to use the --with-sendmail=sendmail location option with the configure script to tell arch where it is.

While arch is the project name, larch is the program that runs all of arch's commands. You can get a list of arch commands by typing larch --help-commands and command-specific help by typing larch command --help.

The first thing to do is set up your user ID so arch can correctly identify your changes. An arch ID consists of two parts: a free form part (i.e., your name) and a unique part (i.e., your e-mail address). To create your user ID use the following command:

larch my-id "John Doe <johndoe@somewhere.com>"

Next, you will want to create an archive for arch to use. arch archives can be located on your machine or accessed over a network using FTP. To create an archive on your local machine use the following command:

larch make-archive name location

An archive's name should be globally unique so the arch manual suggests that you use your e-mail address as the base name of the archive followed by a pair of dashes and then the archive name. An example of this type of archive name would be johndoe@somewhere.com--archive. If you are creating a shared archive, make sure that other users have write permissions to it.

Location specifies the directory to create for the archive.

To access shared archives or archives over a network you need to tell arch where they are with larch register-archive name location.

Here the archive's name should be provided by its maintainer. The archive's location can be something local, such as /usr/local/arch/{archive}, or an FTP site such as ftp://somewhere.com/pub/archive.

The arch archive is organized along development paths that are made up of successive revisions of your source code files. A development path is identified by a combination of user-specified and arch-generated names. The user-specified names describe a project's category, branch and version, while the arch-generated names identify each revision of code put into the archive.

One choice for the archive category would be your application's name. The following command will create a new category in the archive called myprogram:

larch make-category myprogram

A branch name can be used to differentiate between sets of code such as development code and released code. For example, you could make a development branch with larch make-branch myprogram--development.

If you don't want to use a branch name for your code, issue the make-branch command with just the category name with nothing following it.

Finally, each project in the archive has a version number made up of digits in the format of major.minor. The archive version number is not a revision number; it does not change each time you check in changed code. A version number can be equivalent to your application's version numbers (e.g., myprogram 1.0, myprogram 1.5, myprogram 2.0, etc.). Set the version number with larch make-version myprogram--development--1.0