(r2) Lecture2:TheLinuxOperatingSystem < Embedded

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---+ The Linux Operating System
<h2 align="center">Lecture 2</h2> <h2 align="center">Uli Raich</h2>

---++ 
---+ Starting the Linux System

This is how the screen may loop like after some programs have been started

<img alt="mainScreen.png" height="287" src="%ATTACHURL%/mainScreen.png" title="mainScreen.png" width="513" />

---++ 
---+ Linux Distributions

Linux comes in many flavors (distributions) which may be different

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...

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---+ Linux versus Windows

Linux can be downloaded and installed for free

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

---++ 
---+ Linux Command Line

When you start up a terminal window, you start a command shell with it.

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,

<br /> 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?

---++ 
---+ Most used Linux Commands

Here are a few very frequently used commands

| *Command* | *Action* |
| ls | List directory content |
| cat, less, more | Print content of a text file |
| mkdir | Create a directory |
| cd | Change working directory |
| pwd | Print current working directory |
| echo | Print variable |
| vi, nano, emacs, gedit ... | Edit a text file |
| rm, rmdir | Remove a file / directory |
| grep | Search for a string in files |
Have a look in /bin /usr/bin.<br /><br />The environment variable PATH tells the system where to look for commands.

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

and include them into the system.

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---+ Manual pages
| This is a small sample of /usr/sbin | <img alt="binaries.png" height="312" src="%ATTACHURL%/binaries.png" title="binaries.png" width="413" /> |
Each of these commands has plenty of parameters! How do I remember all this?

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

a description of the parameters.

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---+ File permissions and user groups

Typing ls -l (l means long listing format) in one of my directories,

this is what I get:

<img alt="permissions.png" height="104" src="%ATTACHURL%/permissions.png" title="permissions.png" width="582" />

The left rwx means that the file owner can<br /><br />
   * read
   * write
   * and execute
the file. The middle 3 characters have the same meaning for group members

and the last 3 characters for any user.

---++ 
---+ Changing file permissions

The chmod command allows the file owner to modify file permissions.

This can be done for<br /><br />
   * u: user who owns the file
   * g: users in the same group
   * a: any user
You may add (+) or remove (-) permissions

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?)

<img alt="chmod.png" height="181" src="%ATTACHURL%/chmod.png" title="chmod.png" width="652" />

---+ The super user

As we have seen the owner of a file / directory can do with it whatever he wants

including to refuse access to anybody but himself.<br /><br />

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.<br /><br />

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!

---+ Adding Software packages

<br />The apt command is a package management to allowing you to

add new software packages<br /><br />
   * remove them
   * upgrade them
   * find out which packages are installed
Software installation and removal can only be done by the super user

---+ Find commands on the system
| <p>To find any command click this button</p> <p>and type a command name in the search box</p> | <img alt="startButton.png" height="67" src="%ATTACHURL%/startButton.png" title="startButton.png" width="66" /> |
<img alt="ubuntuSoft.png" height="273" src="%ATTACHURL%/ubuntuSoft.png" title="ubuntuSoft.png" width="461" />

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

clicking the Ubuntu Software button

---+ stdin, stdout, stderr

Every program has 3 input/ouput channels associate with it:

stdin:      the standard input channel

stdout:   the standard output channel

stderr:      the error channels<br /><br />

stdin in usually associate with the keyboard

stdout and stderr with the terminal window

from which the application was started.<br /><br />

Any of these may be re-directed with “&lt;” for stdin,

'“&gt;” for stdout and “2&gt;” for stderr

---++ 
---+ pipes

Unix (and Linux) uses many small programs that can however be

connected to produce more complex commands.<br /><br />

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

<br />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.

<img alt="pipe.png" height="62" src="%ATTACHURL%/pipe.png" title="pipe.png" width="589" />

---++ 
---+ The home directory

Every user has his own home directory into which his personal files are stored.

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 lower from the current working directory

---++ 
---+ Change Settings

---++ 
---+ Settings

---++ 
---+ bash scripts

You can collect commands in a script file (e.g. myscript.sh)

The command sh myscript.sh sequentially executes all commands in this file.<br />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 (with the sh prefix) like any other program (how?).

Have a look at your .profile file a shell script which is executed

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

---++ 
---+ Other script files

_bash_ is a command interpreter but it is not the only one

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

-- %USERSIG{UliRaich - 2017-09-05}%

---++ Comments

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Attachments
Topic attachments
I	Attachment	History	Action	Size	Date	Who
png	binaries.png	r1	manage	65.4 K	2017-09-06 - 15:39	UnknownUser
png	chmod.png	r1	manage	26.8 K	2017-09-06 - 15:39	UnknownUser
png	mainScreen.png	r1	manage	332.6 K	2017-09-05 - 16:01	UnknownUser
png	permissions.png	r1	manage	14.4 K	2017-09-05 - 16:14	UnknownUser
png	pipe.png	r1	manage	10.5 K	2017-09-06 - 16:20	UnknownUser
png	startButton.png	r1	manage	5.5 K	2017-09-05 - 16:01	UnknownUser
png	ubuntuSoft.png	r1	manage	427.2 K	2017-09-05 - 16:14	UnknownUser
Topic revision: r2 - 2017-09-06 - uli