• Is a general purpose device
• that can read, process and write numbers
  • (and things that can be represented by numbers)
• to and from the memory
• following a program stored also in the memory
  • many simple steps
• Changing the program changes the purpose of the machine

For technical reasons modern computers handle memory in bytes

• memory big enough to hold a single character
  • like W or @
• Or a small integer number (0 to 255)

Numbers with decimals can be represented using scientific notation 1.8466 * 10¹⁹

In the computer we write 1.8466E19

Notice the meaning of E inside a number

We have a fixed number of digits

Not all numbers are represented exactly

For example \[\frac{1}{3}=0.33333333\cdots\] cannot not be represented exactly with 10 digits

• Sound is transformed into electricity by a microphone.
• The voltage is measured 44100 times each second
• Each sample is stored as a number in a CD

Two steps: sampling (in time) and discretization (in voltage)

Besides the data itself, files have metadata

That is, data about the data. For example

• Files have a name
• Files have a modification date, maybe other dates too
• Files have a size
• Files have permissions

The names of the files are “words”: a series of letters, numbers and some symbols

Technically, a filenames is a String or list of characters

Maximum length of a filename is 250 characters

Avoid /, :, +, |, <, *, >, " and '

Use letters (A-Z, a-z), numbers (0-9), ., -,   and _

In some systems small caps and BIG CAPS are not equivalent. Be systematic and coherent

If the filename includes ., the text after it is called extension

In Microsoft Windows (c) extensions are usually 3 letters

• EXE, JPG, DOC, XLS, TXT, CSV
• These are hints on how to interpret the file

At low level there is only one type of file

For us, it is useful to separate in two:

Text Files

each byte is a character, we can read it

Binary Files

bytes are grouped in binary numbers, representing images, sounds, etc.

Among binary files we have EXE files, which are programs for Windows

• They are universal
• They are easy to read and write from a program

Also

• They do not have any style like bold or italic
• They are like books without figures

Microsoft Word files (doc or docx) are NOT text files

• Binary files are hard to read
  • unless you have the correct program
• Text files can be read by humans
  • Each byte is a letter
• Text files can be read by computers
  • Data must be recyclable
  • The output of one program is the input of another program

UNIX is an operating system

• invented in 1969 at Bell Labs (AT&T)
• developed in the 1970s
• under constant development ever since
• multi-user
• multi-tasking
• system for servers, desktops and laptops.

UNIX systems also have a graphical user interface (GUI)

• similar to Microsoft Windows
• easy to use environment

However, command line interface is also used

• for operations which aren’t covered by a graphical program
• when there is no windows interface available
  • for example, using a cloud server

On 1991, Finish student Linus Torvalds decided to write a Kernel that did the same as commercial UNIX Kernel did

Doing the same but without using any of the original code

This way he could avoid copyright issues

He made this program free to download and modify by anybody

This project is one of the best examples of on-line collaboration enabled by the Internet

No company is owner of the system. Instead, thousands of voluntaries help with small parts

Instead of starting from 0, you find something 90% as you want, you add 10%, and give it back

Then someone else adds another 10%, and so, and so

Some travelers come to a village, carrying nothing more than an empty cooking pot.

Upon their arrival, the villagers are unwilling to share any of their food stores with the hungry travelers.

Then the travelers go to a stream and fill the pot with water, drop a large stone in it, and place it over a fire.

One of the villagers becomes curious and asks what they are doing.

The travelers answer that they are making “stone soup”,

which tastes wonderful and which they would be delighted to share with the villager,

although it still needs a little bit of garnish, which they are missing, to improve the flavor.

The villager, who anticipates enjoying a share of the soup, does not mind parting with a few carrots, so these are added to the soup.

Another villager walks by, inquiring about the pot, and the travelers again mention their stone soup which has not yet reached its full potential.

The villager hands them a little bit of seasoning.

More and more villagers walk by, each adding another ingredient.

Finally, the stone (being inedible) is removed from the pot,

and a delicious and nourishing pot of soup is enjoyed by travelers and villagers alike.

Although the travelers have thus tricked the villagers into sharing their food with them, they have successfully transformed it into a tasty and nutritious meal which they share with the donors.

Science has always been about sharing. Scientific knowledge is for everybody.

That is why UNIX is good for academia, and vice-versa

Big companies did not like Open Source. It was against their business

But today even IBM and Microsoft support Open Source

To handle open source projects with hundreds of people around the world, Linus Torvalds and others developed a system called Git in 2005

It is one of the important tools in modern projects

In 2008 a website called GitHub was created, to store open source projects

Most of scientific programs are distributed through GitHub

On June 4, 2018, Microsoft announced its trying to buy GitHub for USD 7.5 billion

Linux is really only the kernel

Most of the programs, including the shell, were developed by GNU project at MIT

The selection of which programs include in a system is called a distribution

The best way to use Linux is to replace Microsoft Windows™ completely

But that can be painful if you have used windows for a long time

Some tools allow you to split your disk in two halves

• One for Microsoft Windows™
• One for Linux

If you cannot install Linux in your computer, you can use one of the servers in our University

I will send to each one a personal username and password

You are legally responsible for your account. Be careful

To connect to the server you need only one program: PuTTY

Set of programs which make the computer work.

The UNIX operating system is made up of three parts:

• the kernel
• the shell
• the programs

The kernel of UNIX is the hub of the operating system:

• it manages time and memory for programs
• handles the disk and secondary storage
• handles communications and networking

The kernel wraps the different hardware. No other software can interact with the hardware.

Programs have to ask to the kernel to access the hardware

When you first login, your current working directory is your home directory

To find out what is in your home directory, type

$ ls

The ls command ( lowercase L and lowercase S ) lists the contents of your current working directory.

In that case, the UNIX prompt will be returned

ls does not show all the files but only those ones whose name does not begin with a dot .

Files beginning with a dot . are known as hidden files and usually contain important program configuration information.

They are hidden because you should not change them unless you are very familiar with UNIX!!!

ls is an example of a command which can take options: -a is an example of an option.

The options change the behaviour of the command.

There are online manual pages that tell you which options a particular command can take, and how each option modifies the behaviour of the command

This class is a derived work from http://www.ee.surrey.ac.uk/Teaching/Unix/

M.Stonebank@surrey.ac.uk, © 9th October 2000

Images from: archive.computerhistory.org, staff.salisbury.edu/~rafantini