If you only need to install a single executable file that depends on no other files, you can just drop it into some directory on your search path. The default search path is "/bin:/sbin:/usr/bin:/usr/sbin".
For software that requires several files, Apple offers a nice method of "drag and drop" installation. Under OS X, applications that are intended for use under the graphical interface are installed in the /Applications directory. Under this directory, each application has its own subdirectory, called an "application bundle", named with a ".app" extension (e.g., MyApp.app). Viewed from the finder, each of these app directories appears as a single file, although the directories contents may still be viewed by clicking the "gear" icon on the finder toolbar and selecting "Show Package Contents". These app directories can simply be dragged from another location (a CD, for example) and dropped into the /Applications folder. That's all that's needed to install the application. For information on the structure of an app directory, see The Structure of a Modern Bundle. Here's a partial listing of the contents of the Fugu application bundle (I've omitted a large number of language-specific files):
At the top level is a "Contents" directory containing a plist file describing the application (specifying which icon to display, for example). Under this is a "MacOS" directory, containing the application executable itself ("Fugu") and a "Resources" directory containing files used by the application (such as icons, in this case).Fugu.app
| `-- Fugu
If an application requires interaction with the user during installation (to ask for default settings, for example), if tasks need to be done to prepare the system before installing the application, or if the software creator just wants to make the installation process a little prettier, the application may be delivered in the form of a package. Apple's native package format consists of a directory, named with a ".pkg" extension, and containing all of the files necessary to install the application. Like app directories, pkg directories are shown by the finder as a single file. For more information on the structure of a pkg directory, see The Anatomy of a Package.
Packages can be installed from the command line using the installer command. (Double-clicking on a pkg directory in the graphical interface will automatically launch installer.) More information about installer here.
Since it's usually inconvenient to copy a whole directory tree over the network, the most common way of distributing software is in the form of a single disk image (dmg) file. Each such dmg file contains an HFS filesystem containing a pkg or app directory tree. Dmg files can even by "internet-enabled" (by setting a flag in the file, using the hdiutil command), causing Safari to automatically mount, unpack and delete them after they're downloaded from a web server. Dmg files can also contain license agreements that will be displayed (and must be agreed to) before the image can be mounted.
Disk images can be mounted using the "hdiutil attach" command, like this:
hdiutil attach Firefox-22.214.171.124.dmgIn the case of Firefox, the dmg contains a license agreement, which hdiutil displays. The user is prompted to accept the license, and then hdiutil proceeds to create a device associated with the disk image (e.g., /dev/disk1) and mount it under "/Volumes" (in this case "/Volumes/Firefox").
Finally, Apple provides a tool for updating software, called "softwareupdate". Softwareupdate is a command-line tool that can be used to look for available Apple software updates and/or install them. The command:
softwareupdate --listwill produce a list of available updates. Here's an example:
Software Update found the following new or updated software:The command:
QuickTime (7.1.2), 52650K [recommended] [restart]
iTunes (6.0.5), 19340K [recommended]
softwareupdate --install --allwill find all available updates and install them. If one or more updates require a reboot, you'll be asked to restart the computer after the updates are installed. With the command
softwareupdate --schedule onyou can cause softwareupdate to automatically check for updated software. The check is done once per week, and is implemented by changing the current user's ~/Library/Preferences/com.apple.scheduler.plist file. This seems to imply that the checks will only occur when this user is logged in at the graphical console, although I haven't been able to find documentation verifying this.
For more information about software distribution, see this article.