UNIX and LINUX part 10

SYSTEM DOCUMENTATION
There are many different documents produced by AT and T and other UNIX developers. The primary documents for each UNIX System are the Reference Manuals and Guides. These documents provide you with specific details on the UNIX System you are using. One other document that most users ignore but that is extremely important is the Release Notes for UNIX and all related software. Release Notes provide insight to changes, quirks, problems, and compatibility issues relating to the new release of the software. It will be to your benefit to locate a copy and discover what your release of UNIX holds in store for you.
The User's Reference Manual and the User's Guide describe the basic utilities and software of the UNIX System. The Reference Manual describes each utility that is available on the UNIX System. The Guide provides an overview and tutorials on the more common and complex UNIX utilities.
Illustrated UNIX addresses both of these areas to provide a tutorial and a reference manual in one book. It demystifies the guru language of the standard reference manuals and organizes the tutorials into a useful sequence that can later be easily referenced.

UNIX and LINUX part 8

FILE SYSTEM
UNIX uses a hierarchical structure to store and maintain files. This structure is referred to as a tree structure from its resemblance of an upside down tree. The UNIX file system starts with one master directory called "root." Since the tree is upside down, the "root" directory is at the top. The root directory, symbolized by a single "/", has multiple directories under it.
This tree structure allows maximum flexibility for storing information to resemble its logical or real life structure. Multiple users may be grouped by corporate department. As an individual user you may group your data by project or subject. The options are almost limitless and the result is a very flexible and productive environment.
The file system is used by the UNIX operating system to store and control system and user files, directories, and special files. It allows a logical and simple method to maintain large quantities of files with little effort.
The file system operations are performed internally by the UNIX System but there are many utilities that can be used to manage and manipulate the information stored within a file system.
The UNIX operating system has three concepts of files within the file system, the directory, ordinary file, and special file. When using the term file we are usually referring to an ordinary file.
Characteristics
The following list describes the characteristics of the UNIX file system.
Hierarchical structure - Allows grouping of related information and efficient manipulation of these groups.
Dynamic file growth - Files grow as needed, no advanced decisions have to be made about file sizes. Disk space is not wasted since only the amount required to store the current contents of the file is used.
Structureless files - There is no internal structure imposed on the contents of a file. Any structure desired by the user can be used.
Security - Unauthorized users can be restricted from accessing a file.
Device independence - Input and Output from a device is processed as if it were in a file. Therefore, programs that process data in a file can also process data to and from a device.
Limitations
File sizes cannot exceed 4 gigabytes (2^32). As files increase over 10,000 bytes in size, the access time required to retrieve the data increases. This is caused by indirection to locate the block of data.
As the number of entries in a directory increases, the access time to view or manipulate the directory increases. In-core (file system information stored in RAM) file system information may be lost if the system crashes.
DIRECTORIES
A directory is a storage area where any combination of files and subdirectories can be managed. You can think of a directory as a filing cabinet drawer, used to store related file folders. Each directory has a name and each file within the directory has a filename.
Directories are used to store multiple files according to the needs of you, the user. Some people organize their directories according to subjects. Others may organize them based on projects or time schedules. The point is to utilize the freedom provided by the file system to organize them as you see necessary.
The UNIX System is distributed with an established set of directories. There are many system directories and possibly locally created system directories. Check the vendor's Release Notes, Install Procedures, or System Administrator's Guide for a full list or ask your system administrator.
The / (root) and /usr file system must be mounted for the system to be fully functional. The root file system contains the essential files and utilities. The /usr file system contains more general files and utilities. On smaller systems the /usr directory is not a file system but is part of the / file system.
Limitations
As with a physical file cabinet drawer you can only put a specific number of file folders in the drawer. A directory has a physical limit of files in the thousands.
NOTE:
It is advisable to limit the number of files in a directory to under 100. Some systems can actually handle up to 500 files in one directory but 100 is still a good limit. There are two major reasons for this limit. The first is performance. If more than 100 files exist in a directory, the UNIX System encounters high overhead to perform various file system functions in that directory. The second reason is user sanity, referred to by some people as "organization." If you try to keep too many files in one directory, you begin to lose track of where you stored your data.
The directory name can be a maximum of 256 characters, consisting of any character, including a white space. Some characters may have to be escaped or quoted to perform this trickery, but it is possible, though not often desired. Some systems allow up to 1024 characters in the filename.
ORDINARY FILES
Ordinary files are used to store data. A file is like a folder in a filing cabinet; many different types of information may be stored in the file. Files may be ASCII text, compiled programs, or free formatted application controlled data. Files may grow as needed without intervention from the user. This makes it easy for the user and saves space on the physical storage device.
Internal Structure
An ordinary file has no internal structure defined. The data may be stored in any format desired. The UNIX System views files as a stream of bytes, it reads and writes them byte-by-byte or block-by-block, not interpreting the data.
UNIX identifies files by a unique number called an inode number. Inode numbers are maintained in a directory file along with the related filename. If you refer to a file by its filename, the system uses the related inode number to retrieve the physical data.

UNIX and LINUX part 7

Module 3
SYSTEM OVERVIEW
DESCRIPTION
This module provides a general overview of UNIX and the UNIX User Utilities described in this book. It specifically covers the following information:
General overview
The UNIX Kernel (the operating system)
Multitasking
Multiuser
Portability
Job control
File System
Directories multi user
Ordinary Files
Special Files
Fifo (pipe)
Device Drivers
The UNIX shell
System Documentation
GENERAL OVERVIEW
The UNIX operating system has rapidly grown in popularity in recent years. Every major computer manufacturer offers a version of UNIX for their hardware, a claim only UNIX can make. It has been ported to over 150 different types of computers, ranging from micros to supercomputers.
The philosophy of UNIX is to modularize problems and build on existing software solutions. Each program should be designed to solve one problem, and complex problems should be solved by combining multiple programs. This basic idea of UNIX is often referred to as the "building block" approach. Elegantly designed as a simple yet extremely powerful general purpose operating system, UNIX has become a standard in the computer industry.
The UNIX System has introduced many new features to the computer world. Because of its rapid growth in popularity it has become the center of much praise and controversy. It is often referred to as a programmer's "dream" and a user's "nightmare." UNIX is a tremendous solution to many problems, but there are definite weaknesses that are being rectified as new releases are developed. The user interfaces that were once foreign to UNIX are now a reality. There are many software packages available today for UNIX.
THE UNIX KERNEL
The UNIX kernel is the operating system program of UNIX. It is a C and assembly language program that provides the low-level functions of the "UNIX environment." It controls the computer hardware and provides an interface for all other UNIX programs to use the hardware. This concept allows the higher level programs to be hardware independent. When a program needs service to a hardware device, it calls on the kernel to perform the function.
The kernel provides the user the ability to interface with the computer hardware and peripherals. When you log in to UNIX you communicate with the kernel via a shell program. Seldom does a user interact directly with the kernel.
MULTITASKING
Multitasking refers to an operating system that executes multiple tasks simultaneously. UNIX refers to a task as a process. UNIX can be configured to allow the user to run from one to as many processes as the operating system can handle. The standard is usually a maximum of 25 processes or tasks per user login. The theory of a multitasking system is to approach 100 percent in computer resource utilization while increasing user productivity. Of course, at some point, one or more resources are over used and productivity begins to fall instead of continuing to rise.
A user can run several commands in background while executing another command in foreground. Background refers to commands that are detached from your terminal. Foreground is the execution of a command attached to your terminal. You must wait for the current foreground command to complete before you can execute another foreground command. Once a command is entered, the system breaks the command down into separate processes if possible. Each process equates to one program or utility. Internal ksh commands do not always generate (spawn) a new process.
MULTIUSER
Multiuser refers to an operating system that allows multiple users to use the system simultaneously. UNIX currently runs on systems that support from one to over 2000 users. The theory of a multiuser system is to approach 100 percent in computer resource utilization while reducing the cost per user. This is done by having multiple users share system resources. A single user cannot use the printer, disk, memory, or cpu 100 percent of the time. But multiple users can increase use of these devices and resources by having an operating system that manages the resources for them. This reduces the cost per user of the computer as more users share the system.
Computers with enough resources to share among multiple users are prime targets for multiuser operating systems. The main requirem2ents are enough RAM memory, disk storage, terminal ports, and cpu power to handle the operating system and each user's computing needs. Even "old" micros have this capacity - the popular Intel 80286 or the Motorola 68010 can run 1 to 8 users with relatively good response to each user. The key to good response on UNIX is not just the cpu speed, but a fast access time on your disk drives. The faster your disk drives (low access time) are, the better UNIX responds. It is also very important to have fast I/O to disk and terminal ports.