Understanding the System Concepts

A system is a combination of independent but related elements functioning together as a unified whole in order to accomplish a set of specific tasks. The term system has Greek and Latin roots and means combination. An element of a system may be a system in itself and is called a subsystem of the system. For example, a country is a system of citizens, cities, and states, and a city is a subsystem of its country. An operating system is another example of a system, and a file system is a subsystem that the operating system manages. Your Solaris system machine is a part of a network system that may contain client and server machines. In the following list we define some general and some UNIX-related system concepts that are referred to throughout this book:

• Machine. Machine is a very generic term used for any standalone computer or a computer connected to the network.

• Host. A host is a machine connected to a network and having a network (e.g., IP) address.

• Workstation. The term workstation refers to a machine that has its own storage, CPU, and memory to run the applications. This term originally developed as an alternative to dummy terminals during the era of mainframe computers. The dummy terminals did not have their own resources (storage, memory, and CPU power) to run applications on their own. So, generally speaking, a workstation can be a client or a server. However, vendors such as Microsoft and Sun use the term workstation in a more specific sense which is different from what is described here.

• Client. A client is a machine connected to the network, running a program that makes a request for a service. For example, when you are browsing the web, your machine is acting as a client, and the web browser application (e.g., Netscape or Internet Explorer) is the client program running on your machine. Obviously, a client must have a network address.

• Server. A server is a machine connected to the network, running a program that accepts and serves the requests made by the client machines on the network. A machine connected to the network that is running a web server is an example of a server.

• Process. A process is a program being executed on the machine, whereas a program is an executable file residing on a disk. In other words, a process is an executing instance of a program, which has a unique numeric identifier called process ID.

• Daemon. A daemon is a process that often runs continuously as long as the system is up. The daemons are often started when the system is hooted, and they terminate when the system is shut down. It is said that the daemons run in the background, meaning that there is no controlling terminal window for a daemon. It runs independent of a terminal or a login session. The term daemon is derived from the Greek mythology where it means an intermediary between the gods and mankind. This is an accurate description of daemons in UNIX in the sense that UNIX provides services through daemons, which act as the intermediary between services and users.

• Kernel. An operating system has a core, which consists of the defining programs for that operating system, and some utilities around it that can use the core. The core of an operating system is called the kernel. Programs (or utilities) interact with the kernel through system calls.

• Shell. A shell is a wrapper around the kernel; it protects the kernel from the user. In other words, it's a program that lets a user interact with the kernel. When a user issues a command, the command goes to the shell; the shell gets the information from the kernel and provides it to the user. So, the user interacts with the kernel through the shell. The user can put multiple commands and their logic in a file called a shell script, written according to the rules of that shell. That shell script can be executed like any other program. UNIX and Solaris support a number of shells such as C shell, Korn shell, and Bourne shell. When a user logs on to the system, a shell is activated that the user interacts with during the login session.

• Command and utilities. A command is an instruction that a user writes at a prompt in a terminal window in order to accomplish a task—for example, to see the list of Ales in a directory. A utility is a program that does something useful for a user and is not part of the kernel. A command accomplishes the task by executing a program. For that reason, commands are sometimes also called utilities. In other words, there are a number of programs other than kernel and shell that are part of UNIX. These programs can be run as commands in a shell. The shell interprets the command for the kernel.

• Multiuser and mutitasking. In the primitive days of computing, a computer could support only one user running one application at a time. However, UNIX has been a multiuser and multitasking operating system, which means it allows multiple users to log on to a machine at the same time and multiple applications to be run on the same machine concurrently, sharing resources such as CPU, memory, and file systems. Currently, most operating systems (even Windows) support multitasking and multiusers, but UNIX was the pioneer in this arena, as in many other areas of the operating systems.

• Distributed system. A distributed computer system is a collection of autonomous computers networked together that collaborate to accomplish a common task. The Internet is the largest distributed system that supports services such as the World Wide Web and email.

• Fault tolerance. A distributed system is called fault tolerant if it keeps working even if one or more of its components should fail. (The capability of a system to keep functioning when one or more of its components fails is called fault tolerance.) One common method to implement fault tolerance is through redundancy. That means there are multiple components capable of offering the given functionality; when one component fails, the other component takes over to provide the functionality of the failed component. Consequently, the user does not experience the failure.

An operating system manages resources for the users, and one of those resources is the file system. Users interact with the file system through the operating system. There are two ways to interact with the operating system: GUI and command line. Although substantial work has been done on the GUI front, UNIX traditionally has been a command-based operating system. As a system administrator, you must become familiar with the UNIX commands and know how to use those commands to interact with the file system.