UNIX Lecture Two

As it appeared when it was first sent via email.

Greetings Everyone, 

Before I begin with today's lecture I would like to address 
a few things brought up by a few of your comrades. 
I received a few emails asking me to make the lectures shorter, 
and I also received a few emails asking me to make the lectures 
longer. For now, I do not think that the lectures will be either 
shorter or longer. Please understand that while some of you are 
advanced in your knowledge, others are not so advanced, and what 
may seem short to you may be long for others. Also, there are 
time constraints to consider, for me and for you. While some 
people will have time to read, for example, 20 pages, others 
will only have time for 3 or 4 pages. So for now I will leave 
the length as it is, mostly determined by the topic, sub topic, 
or idea that is being presented. If time permits I will add some 
supplemental material someplace on the Russian News Network website. 

Also, if anyone has any questions or comments that you think might 
be of interest to at least some other people, send me an email. 
Depending on the number and nature of the questions, I will post 
the questions and the answers someplace on the website mentioned 
above. If the questions are few in number, I will answer them here, 
during lectures (or in a supplemental mini lecture that I'll email 
to everyone). 

(Lecture continued after this important 
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(This week’s lecture continued…) 


Today's lecture.... 

I would like to start by talking about the UNIX System Layers. 

The UNIX Operating System consists of the following layers: 

CORE 

The core refers to the hardware, nothing more. 

KERNEL 

The kernel is a collection of programs that handle the hardware 
resources (I talked a bit about the kernel last time). 

UTILITIES 

These are a collection of specialized programs that are 
independent of the hardware. 

SHELL 

This is the outermost layer. The shell establishes the interface 
between the user and the rest of the system. The shell is what 
interprets your commands for UNIX systems, meaning that when 
you type a command, the shell tells the system what you want it 
to do. For now keep in mind that the shell is the command 
interpreter for UNIX systems. 

There are three shells that are commonly available on UNIX 
systems. They are as follows: 

The Bourne Shell 

This was the first major shell created for UNIX . It is named after 
its creator (S.R. Bourne) and is the standard version available 
on version 7 UNIX systems. However, it is also available on almost 
all UNIX systems. This shell is denoted by: 
"sh" 
and has the American dollar sign as the prompt symbol: "$" 

The second shell is the C Shell 

This one was created by William N. Joy. It is commonly used in 
the BSD based systems. This shell is denoted by: 
"ksh" 
and has the percent sign as the prompt symbol: "%" 

The next shell is the Korn Shell 

The Korn shell is also named after its creator, David Korn (AT&T Labs). 
This shell is the new shell, which is a superset of the Bourne 
shell. It is denoted by: 
"ksh" 
and has the American dollar sign as the prompt symbol: "$". 

SOme major shell features.... 

1) Analyses of the Command Line and Run Command 

What this means is that it starts the command that you indicate 
you want performed, and passes the remainder of the command line 
as "arguments" to the command (Those of you who have some knowledge 
of Calculus or advanced Algebra will probably feel comfortable with 
the "argument" concept). The shell runs each command as a separate 
job. Such a job can be run in the foreground. The shell runs each 
command as a separate job. Such a job can be run in the foreground 
(meaning that you are specifically waiting for the job to be 
completed) or it can be run in the background (out of site), so that 
the shell returns immediately and the user can continue typing in 
more commands. 

2) Input and Output Redirection 

The shell redirects the standard input (meaning data that is to 
be processed), standard output (the result, typically displayed on 
the monitor for viewing, etc) and standard error (this is any 
unwanted output, usually sent to a file that you, as the user, specify) 
defined by the user. 
stdin (standard input) is represented by a 0 (zero). 
stdout (standard output) is represented by a 1 (one). 
stderr (standard error) is represented by a 2 (two). 

3) Pipes 

This is a tool that is used for the implementation of inter process 
communication. The shell can arrange for the standard output of one 
command to be used as the standard input of the next command line. 
In this manner, multiple commands can be connected, resulting in a 
connection that is called a "pipeline". 

4) Meta Character Expansion 

The shell is very gracious, and allows for meta character usage. Some 
of the more common meta characters are the question mark ("?" - this 
matches any single character in a file name, used when performing 
searches), the asterisk ("*" - this matches any string of characters, 
including the empty strings, in a file name) and the two "square 
brackets", i.e., "[" and "]". The shell performs the meta character 
substitution first before taking any further action. 


5) Set Path 

What this refers to is the fact that the shell provides a way for 
the user to set the search path to be searched by "search" commands. 
(I know this may sound like a strange sentence but it really means 
what it says). 

6) Programming Language 

The shell can be used as a programming language. To do this, all 
that is needed is to place a collection of shell commands in a file 
(along with control structures - more on these later), change 
permissions on the file (more on permissions later) and the filename 
can be used just like any shell command. This new command (which the 
filename now is) runs the shell commands in the file. Thus, you are 
effectively running a customized program that you created. 

Some Special Features of the C Shell... 

1) History Facility 

The history mechanism maintains a list of previously used commands. 
Indicating the number of the command on the list can re issue any 
command from the "history" list. It is also possible to change parts 
of a command from the history list before executing it. 

2) Job Control 

Jobs that are running can be put on hold. These jobs are not terminated 
completely, only stopped. This means you will have the shell prompt 
to do other things, and you can reactivate the stopped jobs whenever 
you wish. When a stopped job resumes, it continues from the point at 
which it was stopped. 

3) Alias Mechanism 

"Alias" is a tool that allows users to assign their own names for 
frequently used and/or long commands. The syntax of C shell commands 
resembles the C programming language. For comparison, the Bourne 
shell commands resemble something called Algol-68. 

Special Features of the Korn Shell.... 

1) Superset of the Bourne Shell 

What this means is that the Korn shell has all the features available 
in the Bourne shell, with a few more added on. 

2) Includes the Best Features of the C Shell 

The Korn shell includes all the useful features of the C shell, with 
a slightly different syntax. This means that the Korn shell has a 
"history" facility", "job control" and the "alias" mechanism. In 
addition to these things the Korn shell also has: 

A) Command Line Editing 

The Korn shell allows the user to display the previous commands 
on the command line, make changes, and then execute the resulting 
new command. 

B) Easy Integer Arithmetic 

Handling integer arithmetic in the Korn shell is more convenient 
than in the Bourne shell. In the Bourne shell it is necessary to 
use a command with complicated syntax, while in the Korn shell it is 
as simple as issuing commands in the usual way, with arithmetic 
operators and the assignment symbol. 

C) Function 

The Korn shell supports memory resident functions as well as 
functions in shell programming. 

These are the basic ideas behind the "shell" concept. I thought 
it necessary to introduce these ideas to you early on. 

Now, a little about login in to a UNIX System.... 

To gain access tot a UNIX system you need a user account. This 
means that you must have a "user id" and a "password" to login 
to a UNIX system. Now I can hear some of you saying, "but there 
are many operating systems that require a user name and password!" 
Yes, that's true, I am merely pointing out that UNIX has this 
feature. 

When you first turn on you computer and the operating system has 
finished loading, you will be prompted with a "login:" prompt (this 
prompt may vary from version to version). At this point type in your 
user id. 
Aside: It is generally not advisable to log in as "root", since 
you can potentially damage the operating system if you are not 
100% sure that you know what you are doing. 
For our purposes, however, we will be login in as "root" anyway. 
The system will next ask you for a password. Type in whatever 
password you assigned for the "root" account during setup. If you 
do not remember the password, there is no way for you to gain access 
to the system and you will have to reinstall the operating system. 
If you are using one more popular Linux distributions (the free ones), 
then all you have to do is insert the cd, boot from the cd-rom and 
choose "upgrade" when "setup" asks you what type of installation you 
want. Choosing the upgrade option is usually the fastest option 
(about 15 minutes) and it is also the easiest. Once logged in you 
would be ready to execute commands, etc. 

We will work some more on this next time. 

A word of caution... 

Be sure that you are permitted to install UNIX/Linux on your computer 
before you install anything. What I means is: if you are at work or 
at school, you probably are NOT permitted to install things on work 
and school computers. If you already have a UNIX or Linux system 
installed (as are found at many engineering schools), it should be 
alright for you to practice commands and write shell programs, but 
if you are not sure, get permission please. Above, when referring to 
login as "root" and reinstalling the system in the event you forgot 
your password, I mean do that ONLY IF the computer is yours (ie, 
you own it). Also, I advise you to get a FREE version. There is no 
need to pay anything, especially for learning purposes. With a free 
version, if for some reason you completely mess up the system and you 
cannot reinstall it, if your cd melts, breaks, gets lost, whatever, 
you can get another copy of the system for free with no financial loss 
to you. If you are not sure as to where to get a free copy, email me. 
I am reluctant to suggest any particular site to you, since it would 
be more beneficial to you if you performed a search yourself. The 
information would then be more "valuable" to you and you would be 
more eager to learn. If anyone has some suggestions that they would 
like to share with your fellows, please email me. 
Finally, although it would be useful to have access to a UNIX/Linux 
system, it is not necessarily a requirement (basically, I leave it 
up to you). 

(Lecture continued after this important 
sponsor break…) 

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(This week’s lecture continued…) 


I think that will be all for this lecture. If you found some of the 
things in this lecture confusing, there is no need to worry. You 
are not required (nor is it expected) to understand everything the 
first time. Few people understand things the first time they encounter 
them. 

A proverb for you: "The appetite comes during the meal." 

Best Wishes and Health to You, 
Vladimir Polyakov 

P.S. Many people have not yet confirmed their desire to receive this 
newsletter/lecture via the email I told you about (called "UNIX 
Lecture" with the link inside). Please do so at your earliest 
convenience, it would save me much time and make the mailing of these 
lectures much easier and efficient. 

P.P.S. And a special thanks to the Russian News Network for their 
help in this endeavor. 
http://www.russiannewsnetwork.com