Lecture 2

Uli Raich

First Semester 2017/2018

in how they are configured, their desktop, their software repository,

their target user community

beginners, gurus, programmers, desktop, server …

Here are some distributions:

Ubuntu (which we use), Mint, Debian, RedHat, Fedora, Kali...

The sources of the OS can be accessed freely

Linux has a window system based on X11 and has most facilities Windows offers

For certain commercial hardware there are no Linux drivers (caution!)

Writing your own Linux driver is possible but often the manufacturers do not

open the hardware specs (register meaning, initialization procedure…)

which makes this a very tedious job.

Programmers mostly use the command line interface something

that is hardly ever used in Windows.

The file system layout on disk is different (ext4 vs ntfs)

but there exist drivers on both systems

reading and writing the others files systems

We will be using bash

When you type a command, bash will check if it is a command

implemented in bash, in which case it is executed immediately,

otherwise a new process is created, the command loaded and executed

After termination of the command the process is removed again.

Either bash waits for termination of the process

or it is run in parallel (“&” after the command)

Where are the commands and how many are there?

The number of commands is unlimited since you can write your own commands

and include them into the system.

This is a small sample of /usr/sbin

Well… you don’t, but there is a man (manual) command where you can find

a description of the parameters.

this is what I get:

The left rwx means that the file owner can

• Read
• Write
• and eXecute

and the last 3 characters for any user.

This can be done for

• u: user who owns the file
• g: users in the same group
• a: any user

Example: chmod a+w Makefile (what does it do?)

You can also give octal values representing the permission bits:

chmod 655 Makefile (what will be the file permissions after the command?)

Changing the eXexute flag in a shell script to make it executable is often done:

including to refuse access to anybody but himself.

You can modify or destroy your file but you cannot modify system files

or files of other another user if he does not explicitly allow it.

There is one user who has the power to do anything: root: the super user.

Sometimes you may need this power to adapt the system to your needs:

sudo command executes a command with super user privileges.

Be careful when doing this, you have the power to destroy your system!

add new software packages

• remove them
• upgrade them
• find out which packages are installed

To find any command click this button and type a command name in the search box

Here you may try to see which software is available on Ubuntu

clicking the Ubuntu Software button

stdin: the standard input channel

stdout: the standard output channel

stderr: the error channels

stdin in usually associate with the keyboard

stdout and stderr with the terminal window

from which the application was started.

Any of these may be re-directed with “<” for stdin,

'“>” for stdout and “2>” for stderr

connected to produce more complex commands.

stdout of one command is connected to stdin to another one through a pipe.

Let’s say, I want to know how many instances of bash are running on the system

(with each terminal you get an associated shell)

In this case I can give the command ps ax, which will however print all processes

currently running. If I pipe this output into grep bash

(showing every line containing the string “bash”)

then I have constructed the command I was looking for.

This also concerns configuration files

Most configuration files are “hidden files” whose file names start with “.” To see these type

ls -a ~ Meaning: show all files in my own home directory.

cd (without parameters) takes you to your home directory

The current working directory is “.”

“..” is the directory one level up from the current working directory

(goto the parent directory of the current directory)

The command sh myscript.sh sequentially executes all commands in this file.
Actually bash is a command language with assignments,

conditional and loop statements just like any conventional programming language.

When making your script file executable by changing the permission

bits, you can run it like any other program (how?).

Have a look at your ~/.profile, a shell script which is executed

when you create a new shell (e.g. when opening a new command window).

There are other command shells like csh, tcsh …

And there are script base programming languages like python, perl …

Often the first comment in the script file tells the

system which interpreter to use:

#!/bin/sh

#!/bin/python

-- Uli Raich - 2017-09-05