Unix - part 1

invented - USA - Bell Labs - 1970 (written mainly in C)
1991 - Linus Torvalds wrote part of LINUX (Unix on PC)

Key Concepts

multi-user
group working
pipes
powerful shell scripting
software tools

O S Structure


    +---------------+
    |  shell        |
    |  +---------+  |
    |  | kernel  |  |
    |  |         |  |
    |  +---------+  |
    +---------------+

The kernel deals with

scheduling processes (switching between programs)
low-level IO
interrupts

The shell sits on top of the kernel, and uses it. Shell provides

file system (names, folders..)
command-line interpreting
shell scripting

On top of all this, there are several GUI systems we can run, which give it windows, mouse facility.

Linux

Many components for a PC version of Unix were created before Linus arrived.
But he wrote the kernel, resulting in a PC open-source version of Unix for PC was available - Linux.

Filing System

Allows 'proper' sharing of files. Each file can have 3 categories of protection

read/write .......(unsafe)
read-only .......(safer)
execute .........(even safer)

There are also 3 categories of user

you
'friends'
everyone

We can combine these to produce a range of levels of protection for each file and/ or folder, e.g:

file1: everyone can execute only
file2: friends and you can read (you can choose your friends)
file3: only you can do everything to a file.

I.O. redirection and pipes

We can use > to redirect the output of a program into a file
If we type the ls command , it displays our file names, as in:


ls                      (we type)
fred.java               (displayed by the ls command)
test.txt

Now we type:


ls > myfiles

nothing is displayed - what would have been displayed is placed in the new file named 'myfiles'

Now we will use the wc (word count) tool, which counts the number of lines in its input data.
We use < to send a data file to it.

wc < myfiles

We can also send the output of one program to the input of another program, without using a file. We use the pipe |

ls | wc

Both programs appear to run at the same time, in RAM. A pipe is a queue of characters held in RAM.

Data flow:
                            characters in transit
                 ls ------>---------------------->--------- wc
                      queue back             queue front

behind the scenes, the kernel switches between ls and wc.
Q: when must ls and/or wc wait?

Summary

Had advanced facities early
command-line important
I/O redirection and pipes taken up by DOS
tool-building by connecting together existing programs
Linux is a PC version of Unix.