Encyclopedia of Computer Science and Technology

UNIX 485Vasilakos, Athanasios, and Witold Pedrycz, eds. Ambient Intelligence,Wireless Networking, and Ubiquitous Computing. Boston:Artech House, 2006.UNIXBy the 1970s, time-sharing computer systems were in use atmany universities and engineering and research organizations.Such systems, often running on computers such asthe PDP series (see minicomputer), required a new kindof operating system that could manage the resources foreach user as well as the running of multiple programs (seemultitasking).An elaborate project called Multics had been begun inthe 1960s in an attempt to create such an operating system.However, as the project began to bog down, two of its participants,Ken Thompson and Dennis Ritchie (see Ritchie,Dennis) decided to create a simple, more practical operatingsystem for their PDP-7. The result would become UNIX,an operating system that today is a widely used alternativeto proprietary operating systems such as those from IBMand Microsoft.ArchitectureThe essential core of the UNIX system is the kernel, whichprovides facilities to organize and access files (see kerneland file), move data to and from devices, and controlthe running of programs (processes). In designing UNIX,Thompson deliberately kept the kernel small, noting that hewanted maximum flexibility for users. Since the kernel wasthe only part of the system that could not be reconfigured orreplaced by the user, he limited it to those functions that reliabilityand efficiency dictated be handled at the system level.Another way in which the UNIX kernel was kept simplewas through device independence. This meant that insteadof including specific instructions for operating particularmodels of terminal, printers, or plotters within the kernel,generic facilities were provided. These could then be interfacedwith device drivers and configuration files to controlthe particular devices.A UNIX system typically has many users, each of whommay be running a number of programs. The interface thatprocesses user commands is called the shell. It is importantto note that in UNIX a shell is just another program,so there can be (and are) many different shells reflectingvarying tastes and purposes (see shell). Traditional UNIXshells include the Bourne shell (sh), C shell (csh), and Kornshell (ksh). Modern UNIX systems can also have graphicaluser interfaces similar to those found on Windows andMacintosh personal computers (see user interface).Working with CommandsUNIX systems come with hundreds of utility programs thathave been developed over the years by researchers workingat Bell Labs and campuses such as the University of Californiaat Berkeley (UCB). These range from simple commandsfor working with files and directories (such as cd to set acurrent directory and ls to list the files in a directory) tolanguage compilers, editors, and text-processing utilities.Whatever shell is used, UNIX provides several key featuresfor constructing commands. A powerful system ofpatterns (see regular expression) can be used to find filesthat match various criteria. For a very simple example, thecommand% ls *.docwill list all files in the current directory that end in .doc.(The % represents the command prompt given by the shell.)Most earlier operating systems used special syntax torefer to devices such as the user’s terminal and the printer.UNIX, however, treats devices just like other files. Thismeans that a program can receive its input by opening a terminalfile and send its output to another file. For example:% cat > noteThis is a note.^DThe cat (short for concatenate) command adds the user’sinput to a file called note. The ^D stands for Control-D, thespecial character that marks end-of-file. Once the commandfinishes, there is a file called note on the disk, which can belisted by the ls command:% ls –l note-rw———- 1 hrh well 16 Mar 25 20:16 noteThe contents of the file can be checked by issuinganother cat command:% cat noteThis is a note.Many commands default to taking keyboard input if noinput file is specified. For example, one can type sort followedby a list of words to sort:% sortapplepearorangetangerinelemon^DOnce the input is finished, the sort command outputsthe sorted list:applelemonorangepeartangerineOne of the things that makes UNIX attractive to its usersis the ability to combine a set of commands in order to performa task. For example, suppose a user on a timesharingsystem wants to know which other users are logged on. Thewho command provides this information, but it includes alot of details that may not be of interest. Suppose one justwants the names of the current users. One way to do this isto connect the output of the who command to awk, a scriptinglanguage (see awk and scripting languages).