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UNIX Introduction

What is UNIX?

UNIX is an operating system which was first developed in the 1960s, and has been under constant development ever since. By operating system, we mean the suite of programs which make the computer work. It is a stable, multi-user, multi-tasking system for servers, desktops and laptops.

UNIX systems also have a graphical user interface (GUI) similar to Microsoft Windows which provides an easy to use environment. However, knowledge of UNIX is required for operations which aren't covered by a graphical program, or for when there is no windows interface available, for example, in a telnet session.

Types of UNIX

There are many different versions of UNIX, although they share common similarities. The most popular varieties of UNIX are Sun Solaris, GNU/Linux, and MacOS X.

Here in the School, we use Solaris on our servers and workstations, and Fedora Linux on the servers and desktop PCs.

The UNIX operating system

The UNIX operating system is made up of three parts; the kernel, the shell and the programs.

The kernel

The kernel of UNIX is the hub of the operating system: it allocates time and memory to programs and handles the filestore and communications in response to system calls.

As an illustration of the way that the shell and the kernel work together, suppose a user types rm myfile (which has the effect of removing the file myfile). The shell searches the filestore for the file containing the program rm, and then requests the kernel, through system calls, to execute the program rm on myfile. When the process rm myfile has finished running, the shell then returns the UNIX prompt % to the user, indicating that it is waiting for further commands.

The shell

The shell acts as an interface between the user and the kernel. When a user logs in, the login program checks the username and password, and then starts another program called the shell. The shell is a command line interpreter (CLI). It interprets the commands the user types in and arranges for them to be carried out. The commands are themselves programs: when they terminate, the shell gives the user another prompt (% on our systems).

The adept user can customise his/her own shell, and users can use different shells on the same machine. Staff and students in the school have the tcsh shell by default.

The tcsh shell has certain features to help the user inputting commands.

Filename Completion - By typing part of the name of a command, filename or directory and pressing the [Tab] key, the tcsh shell will complete the rest of the name automatically. If the shell finds more than one name beginning with those letters you have typed, it will beep, prompting you to type a few more letters before pressing the tab key again.

History - The shell keeps a list of the commands you have typed in. If you need to repeat a command, use the cursor keys to scroll up and down the list or type history for a list of previous commands.

Files and processes

Everything in UNIX is either a file or a process.

A process is an executing program identified by a unique PID (process identifier).

A file is a collection of data. They are created by users using text editors, running compilers etc.

Examples of files:

• a document (report, essay etc.)
• the text of a program written in some high-level programming language
• instructions comprehensible directly to the machine and incomprehensible to a casual user, for example, a collection of binary digits (an executable or binary file);
• a directory, containing information about its contents, which may be a mixture of other directories (subdirectories) and ordinary files.

The Directory Structure

All the files are grouped together in the directory structure. The file-system is arranged in a hierarchical structure, like an inverted tree. The top of the hierarchy is traditionally called root (written as a slash / )

In the diagram above, we see that the home directory of the undergraduate student "ee51vn" contains two sub-directories (docs and pics) and a file called report.doc.

The full path to the file report.doc is "/home/its/ug1/ee51vn/report.doc"

Starting an UNIX terminal

To open an UNIX terminal window, click on the "Terminal" icon from Applications/Accessories menus.

An UNIX Terminal window will then appear with a % prompt, waiting for you to start entering commands.

PUTTY DOWNLOAD LINK

http://the.earth.li/~sgtatham/putty/latest/x86/putty.exe

Introduction to the UNIX Operating System

• What is UNIX?
• Files and processes
• The Directory Structure
• Starting an UNIX terminal

 Tutorial One

• Listing files and directories
• Making Directories
• Changing to a different Directory
• The directories . and ..
• Pathnames
• More about home directories and pathnames

 Tutorial Two

• Copying Files
• Moving Files
• Removing Files and directories
• Displaying the contents of a file on the screen
• Searching the contents of a file

 Tutorial Three

•  Redirection
• Redirecting the Output
• Redirecting the Input
• Pipes

 Tutorial Four

•  Wildcards
• Filename Conventions
• Getting Help

 Tutorial Five

•  File system security (access rights)
• Changing access rights
• Processes and Jobs
• Listing suspended and background processes
• Killing a process

 Tutorial Six

Other Useful UNIX commands

 Tutorial Seven

• Compiling UNIX software packages
• Download source code
• Extracting source code
• Configuring and creating the Makefile
• Building the package
• Running the software
• Stripping unnecessary code

 Tutorial Eight

•  UNIX variables
• Environment variables
• Shell variables
• Using and setting variables

Awk command

awk -- it allows the user to manipulate files that are structured as columns of data and strings, JUST SEE THE FIRST EXAMPLE U WILL GET IDEA

Example 1:
Assume you want to process a file called file 'dimensions' that has the following content:

12 8
15 24
9 12

Assume you want to generate a file 'area' that has the same content as the file 'dimensions', but has one more column that contains the product of the two numbers on each line:

12 8 96
15 24 360
9 12 108

The following command would accomplish that:

awk '{print $0, $1*$2}' dimenions > area

 awk -- it allows the user to manipulate files that are structured as columns of data and strings. (--continued after the ads...)

Once you understand the basics of awk you will find that it is surprisingly useful. You can use it to automate things in ways you have never thought about. It can be used for data processing and for automating the application of Linux / Unix commands. It also has many spreadsheet-type functionalities.

There are two ways to run awk:

1. A simple awk command can be run from the command line.
2. More complex tasks should be written as awk programs ("scripts") to a file. Examples of each are provided below. A simple awk command is of the form

% awk 'pattern {action}' input-file > output-file

meaning: take each line of the input file; if the line contains the pattern apply the action to the line and write the resulting line to the output-file.

If the pattern is omitted, the action is applied to all lines:

% awk '{action}' input-file > output-file

By default, awk works on files that have columns of numbers or strings that are separated by white space (tabs or spaces), but the -F option can be used if the columns are separated by another character. awk refers to the first column as $1, the second column as $2, etc. The whole line referred to as $0.

Example 1:

Assume you want to process a file called file 'dimensions' that has the following content:

12 8
15 24
9 12

Assume you want to generate a file 'area' that has the same content as the file 'dimensions', but has one more column that contains the product of the two numbers on each line:

12 8 96
15 24 360
9 12 108

The following command would accomplish that:

awk '{print $0, $1*$2}' dimenions > area

The term {print $0, $1*$2} means: first print the whole line ($0), then print the product of the number in column 1 ($1) and the number in column 2 ($2).

Example 2:

If you want the output file to contain only those lines on which the first number is less than the second number, you would use the following command:

awk '$1 < $2 {print $0, $1*$2}' dimenions > area2

The contents of file area2 would then be:

15 24 360
9 12 108

Example 3:

The following command does exactly the same as the command in Example 2, but it illustrates how awk can be combined with other Unix commands:

cat dimenions | awk '$1 < $2 {print $0, $1/$2}' > area2

Used by itself, the command 'cat dimenions' simply prints the contents of file 'dimenions' to the sceen. However, if a command is followed by a '|' (called "pipe"), the contents goes as input to command to the right of the '|'.

Example 4:

Assume you have hundreds of files you want to move into a new directory and rename them by appending a .new to the filenames. Assume all the files have names that start with "data", they need to be moved to ../newdata, and need to have a '.new' appended to the name. Use the following command to accomplish this:

ls data* | awk '{print "mv "$0" ../newdata/"$0".new"}' | csh

ls data* lists the filenames. This output list is then "piped" into awk. Since there is no pattern specified, awk proceeds to print something for each line. For example, if the first two lines from 'ls data*' produced data1 and data2, respectively, then awk would print:

mv data1 ../newdata/data1.new
mv data2 ../newdata/data2.new

These are Unix commands that are executed by piping them into the "csh" command ("csh" is an operating system shell).

More complex awk scripts need to be run from a file. The syntax for such cases is:

cat file-1 | awk -f script-1.awk > file-2

where file-1 is the input file, file-2 is the output file, and script-1.awk is a file containing awk commands. awk scripts that contain more than one line need to be run from files. The following example is an awk-script that would saved in a file (e.g, script-1.awk) and executed by the above command.

Example 5:

The following awk-script prints frequency a histogram of the first column of the input file (assumed to contain numbers):

($1 > 0.1) && ($1 <= 0.2) {num_b = num_b+1}
($1 > 0.2) && ($1 <= 0.3) {num_c = num_c+1}
($1 > 0.3) && ($1 <= 0.4) {num_d = num_d+1}
($1 > 0.4) && ($1 <= 0.5) {num_e = num_e+1}
($1 > 0.5) && ($1 <= 0.6) {num_f = num_f+1}
($1 > 0.6) && ($1 <= 0.7) {num_g = num_g+1}
($1 > 0.7) {num_h = num_h+1}
END {print num_a, num_b, num_c, num_d, num_e, num_f, num_g, num_h}

Note that each line contains an instruction of the form pattern {action}. All instructions are executed sequentially. The "pattern" 'END' is satisfied when the end of the input file is reached.

Other useful awk-variables are:
· NF: number of columns;
· NR: the current line that awk is working on;
· BEGIN: satisfied before anything is read;
· length: number of characters in a line or a string;

awk also provides looping capability, a search (/) command, a substring command, and formatted printing. It provides the logical operators || (or) and && (and) that can be used in specifying patterns. You can define variables and assign values to them.