The Internet Encyclopedia (Volume 3)

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OPERATINGSYSTEMSTRUCTURE 507

System Call Interface

Terminal Handling Network Handling I/O Handling Signal Handling Process Handling

Characater devices Routing and Network devices Caching and Disk drivers Process Management, Scheduling

HARDWARE

Figure 4: Structure of a typical Unix kernel.

Filesystem

The typical Unix file system is a hierarchical composite of

directories and files, starting with a base directory called

root. Every directory in the system, with the exception of

the root directory, belongs to some other directory. Every

regular file in the system is contained in a single directory.

Figure 5 shows a typical layout of a hierarchical Unix file

system.

The main directories and their functions on a typical

Unix system are given in Table 5.

The /home directory is significant because most users

on a Unix system are assigned a default home directory,

and this directory is generally stored under the /home

directory. Therefore, a user with the username andy would

usually have a home directory under /home/andy. The user

sally would have a home directory under /home/sally. A

user’s home directory is the current working directory im-

mediately after log-in and is the place where the user can

store personal work files.

Each file in the system is associated with a structure

known as an inode. An inode (information node) con-

tains information relative to that particular file in the

file system. The inode contains, among other things, the

filename, the current size on disk in bytes, the type of file

(regular file, directory, link, device, etc.), the number of

hard links to the file, information indicating the user and

group associated with the file, a file system unique identi-

fier, and some time stamps representing when the file was

last read, modified, and when the inode structure itself

was last changed. Most important, the inode contains a

pointer to the physical location of the file’s data on the

actual hard disk.

Every file in the file system can be defined as being

somehow derivatively off of the root directory. A file with

a pathname that begins with a / slash is called an absolute

pathname, because its location within the logical hierar-

chical structure is unambiguous. An absolute pathname

describes exactly how to navigate to a particular file on the

system. For example, the filename/etc/security/dev/audio

is an absolute pathname because it denotes a single file

on the system, unambiguously, starting from the root di-

rectory.

A relative pathname is a pathname that begins from

the current working directory. Relative pathnames often

begin with two periods (representing the parent directory

of the current directory) or adirectory name in the current

/

(root directory)

bin

(holds executables)

usr

(holds user

commands)

home

(contains home

directories)

etc tmp

bin

local

(holds local

directories)

ucb

(holds BSD

command

versions)

jmshack linda ellen

Figure 5: A typical Unix files system layout.