Revision Control with Arch: Introduction to Arch

Arch was originally a set of shell scripts and wrappers around Tom Lord's hackerlabs libraries. The name of the program in those days was larch, and it was more than a little clumsy to use. The client now has been entirely rewritten in C and is called tla, which stands for Tom Lord's Arch. The interface is still not perfect, but it is good enough for regular use by a skilled developer. Packages of tla are available for most GNU/Linux distributions (see the on-line Resources).

Once you have tla installed, it's good to test it by checking out some code. Arch stores your data in a directory known as an archive. Within the archive, data is organized into nested categories: projects (the name of the work as a whole), branches (a particular thread of development or other descriptive term) and versions (a simple numerical indicator you can use to indicate how far a specific branch has progressed).

The first step to getting some code is to register a public archive so that Arch associates a name with the archive location:

$ tla register-archive http://www.lnx-bbc.org/arch

You should now see the lnx-bbc-devel@zork.net--gar archive listed when you run tla archives. If you're curious about what projects are stored in there, you can use the tla abrowse command to get a full list:

$ tla abrowse lnx-bbc-devel@zork.net--gar
lnx-bbc-devel@zork.net--gar
  lnx-bbc
    lnx-bbc--research
      lnx-bbc--research--0.0
        base-0 .. patch-10

    lnx-bbc--stable
      lnx-bbc--stable--2.1
        base-0 .. patch-29

  scripts
    scripts--gargoyle-bin
      scripts--gargoyle-bin--1.0
        base-0 .. patch-7

This listing tells us that the lnx-bbc-devel@zork.net--gar archive has two projects, lnx-bbc and scripts. The lnx-bbc project has two branches, research and stable. The lnx-bbc--research branch has only one version (0.0) and that version has had ten changes recorded in the archive. The lnx-bbc--stable branch has only one version (2.1) with 29 changesets.

Because you now have the LNX-BBC public archive registered in your local listing, you can check out a copy of the LNX-BBC stable branch:

$ tla get \
lnx-bbc-devel@zork.net--gar/lnx-bbc--stable lnxbbc

Once it finishes downloading and applying patchsets, you should have a directory named lnxbbc/ that is full of files. To simulate a change in the code, cd into lnxbbc/ and edit robots.txt to add a new comment somewhere.

Now that you have made a change, running tla what-changed should print M robots.txt to indicate that robots.txt has been modified. To get the details of the change, you can run tla what-changed --diffs, which should print out a diff file ready to be sent back to the project's development group:

--- orig/robots.txt
+++ mod/robots.txt
@@ -1,3 +1,5 @@
+# Welcome, robots!
+
 User-agent: *
 Disallow: /garchive/
 Disallow: /cgi-bin/

The drawback to this is that the diff does not indicate metadata changes. Moved files will not be listed, and new files will not be created when another developer runs this diff through patch. In order to submit a more complicated change to the project maintainers, you must generate a changeset.

In Arch, a changeset is represented as a directory tree full of bookkeeping files, patches, new files and removed files. The best contribution technique is to create a changeset directory and then tar it up for delivery:

$ tla changes -o ,,new-robot-comment
$ tar czvf my-changes.tar.gz ,,new-robot-comment/

Arch ignores files beginning with two commas, an equal sign and a few other special characters. By using a ,, at the start of our changeset directory name, we avoid the annoyance of Arch complaining that our new directory doesn't exist in the archive. It is probably good practice to use your e-mail address or some other identifier in the tarball filename and changeset directory name.

Now and then you'll want to download the latest changes to the project. This is as simple as running tla update from inside the checked-out copy.

Arch first runs tla undo to set aside your local changes before applying new changesets. Once all the patches have been applied, it runs tla redo to re-apply your local changes.

All of the tla commands introduced above require a functioning network connection to the lnx-bbc.org system that hosts the archive. For disconnected use, you need to create a local archive and then make a branch within it.