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Introducing Package Management

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In 1991, a young Finnish graduate student started a new personal hobby. He had acquired an Intel 386 computer and had spent a few weeks exploring it and playing early PC computer games. Eventually, however, he grew bored with the limitations of the MS-DOS environment that had come with his toy and decided that he wanted an operating system for it that he could use more productively. After exploring Minix, a feature-limited teaching operating system, he decided he needed a full-featured OS.
At that time, no full-featured PC operating systems were freely available, so he decided to write his own operating system. Today, that small hobby OS that Linus Torvalds started almost as a whim has become Linux, a significant new variant of Unix that runs millions of the world's network servers and, increasingly, desktop computers and embedded processors.
Linux has grown up, successfully making the transition from a one-man personal project to a functional, full-featured operating system used by many of the world's major corporations and deployed on millions of corporate and personal systems. Along the way, Linux has had to address many of the same issues any new operating system must face. One of these concerns is how software for Linux, and how the Linux operating system itself, should be installed. How can administrators safely remove software packages without affecting other installed packages? And how can you safely upgrade packages? Answering these questions is what this book is all about.

Installing, Removing, and Upgrading Applications

Applications for most operating systems consist of multiple files that must be copied to specific locations on the computer's file system before each application can be run. This is true for common PC operating systems such as MS-DOS or Microsoft Windows, as well as for Unix and Linux.
In the case of a Unix-like operating system such as Linux, other issues must also be considered. Unix and Linux are multiple-user systems, so they must track ownership of files. Furthermore, Unix and Linux use a system of file permissions. Administrators can grant some users access to files and can control how users may access those files, for example, allowing some users the permission to read only certain files. Administrators can deny other users access to the same files. So, installation of an application on Linux requires consideration of all these details. After files are copied into their appropriate locations, they must be granted correct permissions and correct ownerships.
Similarly, administrators occasionally need to remove installed software from the computer. Maybe the program is no longer needed; maybe it does not work correctly for the needed task, or maybe the space it is using is needed for more important programs. In addition, installed software sometimes needs to be upgraded. Perhaps a new version of the software has come out and the currently installed version needs to be replaced with the presumably improved version. In most respects, software upgrades are the same as the removal of one application (the old version), followed by installation of another application (the new version). Upgrades do, however, have additional issues. Many applications must be configured before they can be used. Ideally, the upgrade for an installed application takes the current configuration into account, preserving old configuration information and applying it to the recently installed version.
All these considerations make installation of a new application onto Unix or Linux a labor-intensive process. To further complicate matters, Unix applications have primarily been distributed as source code. To install a new application, such as the Apache Web server, you download the source code for that application—in this case, from the Apache Project's Web page (http://httpd.apache.org). Typically, the source code is provided in some sort of archive (such as the Zip archival and compression format often used in the Windows world or the tar archive format typically used in the Unix world) that you must then unpack. After unpacking this source code, you have to configure it to support the options and systems you want, compiling it to produce an executable program that can run on your particular operating system (CPU combination).
After compiling the source code, you still have to install the application by putting all of its components (executable programs, documentation, configuration files, and so forth) into the correct locations on your hard drive and setting correct permissions on all those files. You might also need to perform other steps to prepare the system for the software. In the case of Apache, for example, some space needs to be set aside for storage of Web-access logs, and a special user account needs to be created so that the Apache Web server can operate more securely. Finally, you are ready to try running the application you have spent so much time installing.
To help with all these tasks, precompiled software is becoming increasingly prevalent in the Unix and Linux communities, so you might be able to find executable (precompiled binary) copies of the application you wish to install that are appropriate for your particular machine's CPU. In that case, download an archive of the compiled application and unpack it. Then skip the compilation step, since that has already been done for you. The other steps required to install the package (copying files into correct locations, setting file permissions, and doing any needed system or application configuration) are exactly the same as the steps performed to install that application from source code. Once those steps are finished, you are ready to test your freshly installed application.
When you run your newly installed application, you might be thrilled, perhaps discovering that it is something you want to use regularly. On the other hand, you might discover that you have no use for the software you have just installed, deciding that you want to uninstall it.
Uninstallation occurs by reversing the installation steps. Remember any special steps you have performed (such as adding a user account), and undo those. Then remember all the files you have installed and where you have installed them. Manually delete them. As you can see, this can become a pretty tedious exercise.
If you like the application you have installed, you will likely find yourself wanting to upgrade it eventually. The Apache Web server, for example, like any network service, must be upgraded whenever security problems are found and fixed. If you find that you need to upgrade Apache, you need to back up your Apache configuration files and then uninstall Apache. The next step is to install a new version of Apache, applying your Apache-configuration customizations to your new installation of Apache.
All of this is a big pain. There has to be a better way. And there is.