A system administrator is someone who keeps the system useful and convenient for its users. To maintain a Mac OS X system, you must work with a variety of files and programs. Much of the work you do as the system administrator requires you to use sudo or to log in as root. The root user, called Superuser, has extensive systemwide powers that normal users do not have. Superuser can read from and write to any file and can execute programs that ordinary users are not permitted to execute.
When you bring up the system in single-user mode, only the text console is functional and not all the filesystems are mounted. When the system is in single-user mode, you can back up files and use fsck or diskutil to check the integrity of filesystems before you mount them. You can then bring the system into multiuser mode. With the system running in multiuser mode, you can still perform many administration tasks, such as adding users and printers.
System operation includes numerous functions: booting up, running startup scripts, entering single-user mode, entering multiuser mode, bringing the system down, and knowing what to do when the system crashes. The system administrator must also be familiar with NetInfo, a hierarchical database that stores user and system information that is frequently shared among systems on a local network. NetInfo is similar in scope and function to NIS.
A Mac OS X filesystem contains many important directories. They include /usr/bin, which stores most of the Mac OS X utilities, and /dev, which stores device files, many of which represent a physical piece of hardware.
Setting up a server requires an understanding of configuration files, including property lists, and lookupd, which determines what a system uses as the source of certain informationNetInfo, DNS, local files, or a combinationand in what order. The launchd superserver is a daemon that launches other daemons that provide services. A chroot jail enables you to secure a server by defining the root directory on a per-process basis, thereby limiting access to the filesystem. DHCP enables client systems to retrieve network configuration information each time they connect to the network, leading to greater system efficiency and better network resource management.
Another tool useful to system administrators is Linux-PAM, which allows you to maintain fine-grained control over who can access the system, how they can access it, and what they can do. The fink program can install third-party software packages automatically.