Saturday, November 06, 2010

Remastering Ubuntu Mini Remix

Ubuntu Mini Remix is a very small, very efficient Ubuntu distribution created by Fabrizio Balliano. I discovered it when I needed to create a kiosk-like boot disk image that booted the user directly into a simple command-line application. I didn't need X Windows, OpenOffice, web access, or email, I just needed to run my application, and UMR was perfect for that. The baseline UMR image is about 165 MB, and is available for i386 and for amd64.

In my previous blog post, I explained how to create a debian package. Since then I've written a shell script that takes one or more debian packages and uses them to remaster UMR. If you wanted to use the debian package from the last blog post to update UMR, you could simply type:
(cd ../make-debian-pkg/; make)
./ \
If you have multiple debian packages to be included, simply add them as additional command line arguments. The result will be a file called "customized-umr-10.10-i386.iso", located in your home directory.

For very small additions, a package may be overkill. It might make better sense to edit the script to simply add the files you need, and in my situation at work, that was the approach I ultimately chose. But if your change is more substantial, and especially if you want to include other packages that you'll rely upon, you'll want to think about using package management.

If you want your boot image to take the user directly into your application as I did, you can go into the chroot jail and replace the file /etc/skel/.bashrc with a short script that calls your application. The command would look like this:
sudo cp my-bashrc ${CHROOT_JAIL}/etc/skel/.bashrc
and you'd want to put that with the lines where the comment says "Prepare to chroot", near line 63. (Take the line number with a grain of salt as I may end up editing it at some point.)

Tuesday, November 02, 2010

Making a debian release package

I've been doing a lot with Ubuntu Linux lately, and decided it was time to find out how to put together a debian package. Ubuntu shares Debian's package management system, having descended from Debian. I've banged my way through the code so you don't have to. Here I will just discuss a few highlights. There isn't that much to it. Since this is just an example, I didn't get very creative with the name or the contents. Running make will build foo-bar_1.0_i386.deb.

First a quick look at the files.
The three files in the "usr" directory tree will be copied into the target system when you say "dpkg -i foo-bar_1.0_i386.deb". If you then say "dpkg -r foo-bar", those three files will be removed. The postinst and prerm scripts can be used to perform actions after installation and before removal respectively, but here they just print messages to the console. The real meat is in the two files control and Makefile. control specifies the package name, version number, dependencies, and other information. Makefile takes pieces of that information to build makefile variables that will be used in creating the package. There isn't a heck of a lot to say about the process other than there is a special DEBIAN directory with meta-information about the package and how it should be installed and removed.

This is the minimal possible example. You can build a deb file this way and install it on an Ubuntu machine. But there are a lot of things that could be improved and cleaned up, and put in better compliance with common practices for making these packages more maintainable. The two big areas are (1) there are recommendations about additional fields to go into control, and (2) there is a tool called lintian, a sort of lint command for deb packages, whose advice should be applied. When I build this package, the advice I get is the following:
E: foo-bar: changelog-file-missing-in-native-package
W: foo-bar: third-party-package-in-python-dir usr/lib/python2.6/
W: foo-bar: binary-without-manpage usr/bin/say-hello
W: foo-bar: maintainer-script-ignores-errors postinst
W: foo-bar: maintainer-script-ignores-errors prerm
where "E" and "W" are "Error" and "Warning". So that's not too bad.