Automating Tasks with Aap

Many people use a Makefile and shell scripts to automate tasks where a change to a file requires an action to be taken. You edit a file and then invoke make in the hope that all actions necessary to effect the changes are done. Often you need to tweak the Makefile to get it right and end up using touch to work around a missing dependency. As a result, when other people look at your carefully tuned Makefile, they have a hard time understanding how it works.

These tasks can be done more easily and reliably with Aap than with make. For example, Aap has built-in Internet support. Downloading and uploading is taken care of without the need to specify the commands or to keep timestamp files. Reliability is achieved by figuring out dependencies automatically and using signatures instead of timestamps. With Aap it is simpler to specify the work you want done, and you make fewer mistakes. You still can fall back on using shell commands where you want them. This article presents two examples of Aap in action, maintaining a Web site and building a program. Aap can do much more, of course, but these subjects should be sufficient to get you started.

To use Aap you need Python version 1.5 or later. In the unlikely event that you do not have Python on your system, download it from Python.org, or install it from your Linux distribution CD or update system.

Installing Aap can be done in four simple steps. Start by downloading the latest Aap zip archive (see the on-line Resources section). Then, unpack the archive in a temporary directory (unzip aap-1.053.zip). If you are root, run ./aap install. If you are a normal user, install the archive in your home directory with ./aap install PREFIX=$HOME. For more information about downloading and installing Aap, see Resources. Aap is distributed as open-source software under the GNU GPL.

With the current Aap software in place, let's look at using it in a basic example task. You have designed a simple Web site with HTML files and images. The files are on your local computer, and you need to upload them to the Web server. Listing 1 shows the Aap script, called a recipe, that does this.

# The list of files to be uploaded.
Files = index.html
        info.html
        download.html
        images/*.png

# The publish attribute tells where to upload to.
:attr {publish = scp://my.server.net/html/%file%}
        $Files

# When executed without a target: publish the files.
all : publish

Store the recipe under the name main.aap. This file is what Makefile is to make—the default file to be executed. Running aap without arguments executes the main.aap recipe in the current directory.

Comments in an Aap recipe start with # and continue until the end of the line, as in a Makefile or shell script. The first effective line in the recipe is an assignment; the list of files to be uploaded is assigned to the Files variable. There are no back slashes nor punctuation to mark the end of the assignment. Aap recognizes command continuation by the amount of indentation used. This may appear strange at first, but you quickly get used to it. This method avoids the usual mistakes with punctuation and enforces a layout that is easy to read. Aap allows you to use either tabs or spaces to indent lines.

The :attr line is an Aap command. All Aap commands start with a colon to make them easy to recognize. This command adds the publish attribute to its arguments. The publish attribute tells Aap where to upload the files when they are published. The method used here is scp://, secure copy. Other supported methods are rsync:// and ftp://. The last argument of :attr is $Files, the value of the Files variable. The attribute is attached to each item in $Files.

When you run Aap without an argument, it updates the target all. The final line specifies that the default target all depends on publish. This is a special target, which tells Aap to upload all items that have a publish attribute.

You now can edit the HTML files, add pictures and view them locally. Once you are satisfied, execute aap. Aap figures out which files have changed and uploads them. Signatures (checksums) are used; thus, if you restore an old version of a file it still works properly. If you were using make, you would have to touch the restored file to update its timestamp.

If you want to try this example, but you don't have a server to upload to, you can use the publish attribute file:/tmp/html/%file%. With it, Aap creates the /tmp/html directory, if necessary.

A word of warning: Aap does not delete files on the server no longer in use. Then again, neither does make. You have to delete these files manually. Hopefully, automatic deletion will be added to Aap soon.