2. Overcoming the Installation Obstacles
None of the tasks you must perform to install, upgrade, or uninstall applications are especially difficult. However, these steps quickly become daunting when you consider all the files that must be managed. A full Red Hat Linux 7.3 installation provides around 3,000 executable commands and over 160,000 total files (some other Linux distributions are even larger!). Obviously, managing all these files by hand, although theoretically possible, is not technically feasible. On a smaller scale, even management of single applications is not practical. The Postfix e-mail server application, for example, consists of around 275 files scattered in a dozen or so different directories. Imagine trying to remember and manually remove all of those files (and only those files) to uninstall Postfix from your system!
All the steps needed to manage software on Unix or Linux systems are hardly unique to Unix; all operating systems have similar procedures that must be followed to make software usable on the system. For this reason, many approaches have been adopted toward software installation, uninstallation, and upgrading.
2.1. Application-level utilities
Some operating systems, such as MS-DOS, have supplied absolutely no built-in tools for software management. Installation of applications on such systems occurs in one of two ways: software is installed manually, using file-copy utilities to put all the application files in the appropriate places on the system, or software is installed using a custom-written installation application (as is usually the case for MS-DOS applications).
Once installed, software can be uninstalled in one of two ways: you can manually delete each file installed for the application (assuming you can even remember them all), or the application might come with a custom uninstallation utility that can be run to remove the application. Upgrading an already installed application on such a system uses a similar procedure. If the application comes with an installation utility capable of handling application upgrades, you can use the utility to perform the upgrade. Otherwise, the software must be manually upgraded using the procedure described previously.
Current versions of Windows, such as Windows XP, have a central database of installed applications.
