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What is UNIX?

UNIX is an operating system. The job of an operating system is to orchestrate the various parts of the computer -- the processor, the on-board memory, the disk drives, keyboards, video monitors, etc. -- to perform useful tasks. The operating system is the master controller of the computer, the glue that holds together all the components of the system, including the administrators, programmers, and users. When you want the computer to do something for you, like start a program, copy a file, or display the contents of a directory, it is the operating system that must perform those tasks for you.

More than anything else, the operating system gives the computer its recognizable characteristics. It would be difficult to distinguish between two completely different computers, if they were running the same operating system. Conversely, two identical computers, running different operating systems, would appear completely different to the user.

UNIX was created in the late 1960s, in an effort to provide a multiuser, multitasking system for use by programmers. The philosophy behind the design of UNIX was to provide simple, yet powerful utilities that could be pieced together in a flexible manner to perform a wide variety of tasks.

The UNIX operating system comprises three parts: The kernel, the standard utility programs, and the system configuration files.

The kernel

The kernel is the core of the UNIX operating system. Basically, the kernel is a large program that is loaded into memory when the machine is turned on, and it controls the allocation of hardware resources from that point forward. The kernel knows what hardware resources are available (like the processor(s), the on-board memory, the disk drives, network interfaces, etc.), and it has the necessary programs to talk to all the devices connected to it.

The standard utility programs

These programs include simple utilities like cp, which copies files, and complex utilities, like the shell that allows you to issue commands to the operating system.

The system configuration files

The system configuration files are read by the kernel, and some of the standard utilities. The UNIX kernel and the utilities are flexible programs, and certain aspects of their behavior can be controlled by changing the standard configuration files. One example of a system configuration file is the filesystem table "fstab" , which tells the kernel where to find all the files on the disk drives. Another example is the system log configuration file "syslog.conf", which tells the kernel how to record the various kinds of events and errors it may encounter.

UNIX File System

UNIX has been around for many decades, making it the oldest of all file systems used on PC hardware. UNIX file systems are also probably the most different from the other file systems used on PC, both internally and externally (referring to how the user accesses the file system). While most Windows users are accustomed to the Explorer-type interface for managing files and folders, UNIX files are usually managed with discrete text commands--similar to how DOS works (in fact, many principles of the FAT file system are based on UNIX.). There are graphical UNIX shells as well, of course, but many UNIX users (myself included) never use them.

The more important differences, however, are internal. UNIX file systems are designed not for easy use, but for robustness, security and flexibility. UNIX file systems offer the following features, and have for many years:

• Excellent expandability, and support for large storage devices.
• Directory-level and file-level security and access controls, including the ability to control which users or groups of users can read, write or execute a file.
• Very good performance and efficient operation.
• The ability to create "flexible" file systems containing many different devices, to combine devices and present them as a single file system, or to remotely mount other storage devices for local use.
• Facilities for effectively dealing with many users and programs in a multitasking environment, while requiring a minimum of administration.
• Ways to create special constructs such as logically linked files.
• Reliability and robustness features such as journaling and support for RAID.

 

ISAM stands for Indexed Sequential Access Method. ISAM was originally developed by IBM and antedates the concept of the relational database. ISAM stores data sequentially in a file. This data is typically records for a database, though the term has grown beyond its original database-specific meaning.

The records are composed of fixed length fields. This allows for sequential reading and writing of records to the database. In order to speed access to the records, ISAM adds an index to the file which allows for random access of any record simply by looking up some key field of the record in the index, just as a human might look up a page number in a book by looking for a word in the index at the back of the book.

Introduction to Unix File System

A diagram of the Unix file system looks like an inverted tree.

Shaded boxes represent the absolute pathname /home/jane.

Root directory:At the top of this inverted tree is the root directory, the source of all the files and directories in the file system. The root directory is identified with a / (slash).

Parents and children:The inverted tree structure of the Unix file system resembles a family tree. Indeed, Unix references files and directories with family names. For example, in the above illustration the directory home is the parent of the directory jane. The directory jane is the child of the directory home.

Pathnames:In Unix, you can specify the location of a file within the file structure with either an absolute or relative pathname.

The full name of a directory is called its absolute pathname, which shows its unique location within the Unix structure. In the above illustration, if your userid is jane, then the absolute pathname for your directory is:

/home/jane

The pathname /home/jane tells you that the root directory (indicated by the leading / in the pathname) contains the subdirectory home, which contains the subdirectory jane.

Unix also supports relative pathnames. Relative pathnames tell the location of a file with respect to the current working directory (see below). Relative pathnames do not begin with a / (the root directory), but with . (dot), .. (dot dot) or ~ (tilde), where . stands for the current directory, .. stands for the parent directory, and ~ represents your home directory.

In the above illustration, if you are currently working in the directory named jane, the relative pathname from jane to julia is:

../julia

This indicates a directory julia under the current directory's parent.

Home directory

When you first log in to your Unix account, you will be in your home directory. This directory belongs to you, and you control the files and subdirectories created and stored here.

Working directory

The working directory is the directory in which you are currently doing work. When you log in, your home directory will be your working directory. If you change directories, then the directory into which you have moved becomes your working directory.

Unix commands

There are MANY commands available for you in a UNIX shell account. A list of them follows (in no particular order). Remember, you can always read the manpage on each command for more information.

 

ls ................. show directory, in alphabetical order

logout ............. logs off system

mkdir .............. make a directory

rmdir .............. remove directory 

rm -r . . . . . . .  delete folders with files

rm ................. remove files

cd ................. change current directory

cp . . . . . . . . . copy from a source to destination

cat . . . . . . . .  displays the content of the file

more (file) ........ views a file, pausing every screenful

sz ................. send a file (to you) using zmodem

rz ................. recieve a file (to the unix system) using zmodem

passwd ............. change your password

pwd . . . . . . . .  present working directory

pg . . . . . . . .   displays the content of a file page wise

chfn ............... change your "Real Name" as seen on finger

chsh ............... change the shell you log into

grep ............... search for a string in a file

tail ............... show the last few lines of a file

who ................ shows who is logged into the local system

w .................. shows who is logged on and what they're doing

wc . . . . . . . . . word count a given file

finger (emailaddr).. shows more information about a user

df ................. shows disk space available on the system

du ................. shows how much disk space is being used up by folders

chmod .............. changes permissions on a file

bc ................. a simple calculator

make ............... compiles source code

gcc (file.c) ....... compiles C source into a file named 'a.out'

gzip ............... best compression for UNIX files

zip ................ zip for IBM files

tar ................ combines multiple files into one or vice-versa

lharc, lzh, lha .... un-arc'ers, may not be on your system