The Internet Encyclopedia (Volume 3)

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506 UNIXOPERATINGSYSTEM

Figure 3: Job control.

Summary of Popular Commands

There are a number of commonly used commands on a

typical Unix operating system. For a detailed list of avail-

able commands, users should visit any number of stan-

dard texts on Unix (most notably Gilly, 1986; Hahn, 1994;

Peek, Todino, & Strang, 1998; Sobell, 1995). Online re-

sources include The Unix Reference Desk (n.d.) and Dig-

ital Unix Online Documentation., which details Digital

Unix (a BSD-flavor) but is highly useful for the standard

commands and syntax. Table 4 summarizes some of the

most popular commands.

OPERATING SYSTEM STRUCTURE

Kernel Structure

The job of an operating system is to provide an interface

between the user of the system and the hardware of the

computer. This interface is provided, at the lowest level, by

the part of the Unix operating system known as thekernel.

The kernel is a file that exists on the file system like any

other file but is read into memory when the computer is

first turned on (a process known as “booting”). When the

computer is booted, the kernel file (usually called some-

thing like “vmunix” or simply “unix”) is loaded into mem-

ory and begins executing.

The kernel provides low-level services and runs on the

computer in a protected mode, which tries to ensure that

Table 3Job Control Commands

Ctrl-z Suspend the job in the foreground

Ctrl-c Terminate (kill) the job in the foreground

fg%n Change the background job numbern

to run in the foreground

bg%n Change the stopped job numbernso that

it begins executing in the background

kill%n Terminate the job numbern

jobs Print out a list of all current jobs known to the

current shell

Table 4Popular Unix Commands

cat A filter program that prints out a file to

standard output.

cd A utility to change the current directory to

some other directory in the file system

chmod A command to change the permissions of

afile

cp A utility that copies one file to another

lp, lpr Commands to print files to a printer

ls A utility for listing the contents of a directory

on the file system

man Access the help system for information on

commands and functions

mkdir A utility for creating new directories

mv A utility to move a file from one location to

another or to rename a file

rm A utility that removes a file from the file

system.

that the operating system itself will always be available

and in a “sane” mode for users to use. Included among

these low-level services provided by the Unix kernel are

terminal handling services, signal handling, user man-

agement, process management and scheduling, memory

management, file system services, hardware interrupt ser-

vices, and interprocess communication.

These low-level services are provided to both the ker-

nel itself and to user processes by a defined set of func-

tion calls known as an API (application programming

interface). The API provided by the kernel offers func-

tions called “system calls” that can be called by a user’s

program, and these calls provide fundamental access to

services such as memory management, file system access,

and so on. Figure 4 shows an abstract view of a typical

Unix kernel layered architecture.