inf322 – operating systemsozancaglayan.com/wp-content/uploads/2013/04/tp01s.pdfmanaging a...

INF322 – Operating SystemsTP01: Introduction to Linux

Ozan Çağ[email protected]

Galatasaray UniversityComputer Engineering Department

mailto:[email protected]

Fundamental Concepts

Definition of Operating System

A central software managing a

computer's resources

Accompanying softwares like command-line interpreters, GUIs,

editors, browsers, etc.

Central software that manages and allocates

computer resources like CPU, RAM,

devices, etc.

This is the definition that concerns us in this lab

+



Central software that manages and allocates computer resources like CPU, RAM, devices, etc.

Kernel



Central software that manages and allocates computer resources like CPU, RAM, devices, etc.

Kernel

[noun] the inner and usually edible part of a seed or grain or nut or fruit stone; "black walnut kernels are difficult to get out of the shell"

Kernel

● Provides a software layer to manage the limited resources of a computer,

● Process Management,

● Memory Management,

● Device Access,

● Networking,

● System Call Interface,

● ...

User space

Kernel Space

Directory Layout

Single Directory Hierarchy

● Linux maintains a single hierarchical directory structure to organize all files in the system,

● This contrasts with Windows where each storage has its own directory hierarchy:

– C:\ for the 1st disk partition

– D:\ for the 2nd disk partition

– E:\ for DVD-ROM

– ...

Single Directory Hierarchy/

bin run etc home usr

firefox group passwd ozan bob include

TP1.pdf stdio.h

Regular File

Directory

Root (/) directory

media

Pathnames

● A pathname is a string consisting of an optional initial slash (/) followed by a series of filenames separated by slashes:

/home/ozan/Documents/TP1.pdf

Path separator

Filename

Pathnames

● An absolute pathname begins with a (/) and specifies the location of a file with respect to the root directory:

– /home/ozan/Documents/TP1.pdf

Pathnames

● An absolute pathname begins with a (/) and specifies the location of a file with respect to the root directory:

– /home/ozan/Documents/TP1.pdf

● A relative pathname specifies the location of a file with respect to the current working directory:

– Documents/TP1.pdf(When the current directory is /home/ozan)


● /bin: Essential commands

● /dev: Devices

● /home: User's home directories

● /etc: System-wide configuration files

● /root: Home directory for root user

● /var: Variable files like logs.

Home Directories

● Every user in the system has an associated home directory:

– C:\Users\ozan for Win Vista & 7

– C:\Documents and Settings\ozan for Win XP

– /home/ozan under Linux

● Separating user data from system-wide data is always a good thing.

Home Directories

[ozan@DellFedora ~]$ lsCyberClient.conf Downloads git Pictures svnDesktop Dropbox Music Public TemplatesDocuments fontconfig OctaveToolboxes Setups Videos

What you will find in a typical home directory:

A typical user should only use its home directory for everything!

Users and Groups

● Each user is uniquely identified, and users may belong to groups,

● Every user has:

– A numeric ID called User ID (UID),

– A login name (username),

– A home directory under /home/username,

– A password,

– etc.

Superuser

● One special user known as superuser has special privileges in the system,

– User ID == 0

– Login name (username) == root

● Superuser has access to everything, can install new software, can remove any directories, etc.

● Home directory for superuser is /root.

Permissions

● Each file has an associated User ID (UID) and Group ID (GID) which define the owner of the file and the group to which it belongs,

● Every user has full read/write access to all files and directories in their /home directory,

● Superuser has all access to everything!

Permissions

● Every user in the system is isolated from the rest in terms of their home directories,

● User Bob cannot touch anything within:

– /home/ozan

– /home/anna

– /home/ahmet

– ...

Mounting Other Storage Devices


bin run etc home usr

firefox group passwd ozan bob include

TP1.pdf stdio.h

Regular File

Directory

Root (/) directory

media

If there is a single directory hierarchy, whereare all other disks, USB sticks, DVDs, CDs?


bin run etc

firefox group passwd

Root (/) directory

media


bin run etc


Root (/) directory

media

User ozan plugged aUSB stick


bin run etc


Root (/) directory

media

ozan

OZAN

guide.pdf boardingpass_1360935081443.pdf

User name

Disk label


bin run etc


Root (/) directory

media

ozan

OZAN


User name

Disk labelEvery storage becomes a

Sub-tree of this hierarchy!


bin run etc


Root (/) directory

media

ozan

OZAN


User name

Disk label


● All other storage devices are tied/mounted into this single directory hierarchy,

● This is done automatically by the system.

Interacting With The System

?

● Question: How come people interacted with computers at times when graphic cards were not invented?

Terminals

● Question: How come people interacted with computers at times when graphic cards were not invented?

● Answer: Using primitive terminals connected through serial port (RS-232).

Terminals (TeleTYpewriters, TTY)

Input using punch cards

Output using printer

Terminals (Early Video Type)

● Still connected through RS-232 port,

● Designed using microprocessors.

Terminals (Today)

● As many terminals as tabs in a terminal emulator,

● Each one runs its own shell.

Shell

Special-purpose program to read commands typed by a user and execute programs in

response to that commands.

Kernel

Shell



Shell Kernel

Shell



● Shell is just a program which waits for your input in a loop,

● Different shells available:

– sh, bash, csh, tcsh, zsh, etc.

Practice

● Press ALT+F2 and type gnome-terminal to launch a terminal emulator,

● Now you have a shell prompt at your order:

[ozan@DellFedora ~]$ whoamiozan[ozan@DellFedora ~]$ datePrş Şub 21 13:27:56 EET 2013[ozan@DellFedora ~]$ echo "GSU"GSU

Environment Variables

● Each process has an array of strings in key=value form called Environment List,

● A common use of environment variables is in the shell. Shell holds a list of these variables for various things,

● Now type printenv to see a list of these environment variables.

Environment Variables[ozan@DellFedora ~]$ printenv...SHELL=/bin/bashUSER=ozanPATH=/usr/local/bin:/usr/bin:/bin:/usr/local/sbin:/usr/sbinHOME=/home/ozan......

Environment Variables[ozan@DellFedora ~]$ printenv...SHELL=/bin/bashUSER=ozanPATH=/usr/local/bin:/usr/bin:/bin:/usr/local/sbin:/usr/sbinHOME=/home/ozan......[ozan@DellFedora ~]$ echo $USERozan[ozan@DellFedora ~]$ echo USERUSER

You have to put '$' before the variable so that the shell interprets is as a variable instead of a plain string!

Exercice: Let's write whoami!

● You've seen that shell has an environment variable called $USER,


● You've seen that shell has an environment variable called $USER,

● Standard C library provides:

– char *getenv(const char *name)

● Use man utility to see documentation about practically anything if a documentation exists and is installed:

[ozan@DellFedora ~]$ man getenv


GETENV(3) Linux Programmer's Manual

NAME

getenv, secure_getenv - get an environment variable

SYNOPSIS

#include <stdlib.h>

char *getenv(const char *name);

DESCRIPTION

The getenv() function searches the environment list to find the environment variable name, and returns a pointer to the corresponding value string.

RETURN VALUE

The getenv() function returns a pointer to the value in the environment or NULL if there is no match.


● Type gedit and press Enter to launch a simple text editor,

● Final source code is ~12 lines and looks something like this:


● Now, how to compile it?

– We use gcc (GNU project C and C++ compiler) to compile it:

● gcc whoami.c -o whoami

List of .c sources

Name of the output executable

When -o is not given, the resulting executable is called

a.out


● Now, how to compile it? Done.

● How to run it?



● How to run it?

– Type whoami and press Enter:

[ozan@DellFedora ~]$ whoamiozan



● How to run it?



Really?



● How to run it?


● But how does it differ from the system provided whoami?


Really?

Steps of program execution

● When you type whoami and press Enter, shell executes the whoami program and whoami prints your username.

● Questions

– Where is whoami?

– How is it located by the shell?


● whoami utility comes from a package called coreutils:

– http://www.gnu.org/software/coreutils/

● coreutils also provides cat, chmod, chown, ls, cp, dd, df, du, head, mkdir, mv, printenv, rm, etc. practically every basic tool that we daily use on a Linux system.


● coreutils is installed in every Linux distribution be it Fedora, Ubuntu or OpenSuSE, etc.

● The installation directory may differ for each distribution but usually you will find whoami under /usr/bin,

[ozan@DellFedora OS-Course]$ ls -lh /usr/bin/whoami-rwxr-xr-x 1 root root 32K Oca 24 18:43 /usr/bin/whoami

Permissions Ownership Modification Time

Filename


● Shell looks in the environment variable $PATH and adds whoami at the end of each path to see if the resulting path actually exists:

/usr/local/bin

/usr/bin

/bin

/usr/local/sbin

/usr/sbin

$PATH



/usr/local/bin

/usr/bin

/bin

/usr/local/sbin

/usr/sbin

$PATH

Does /usr/local/bin/whoamiexist?

No, continue

http://www.gnu.org/software/coreutils/



/usr/local/bin

/usr/bin

/bin

/usr/local/sbin

/usr/sbin

$PATH

Does /usr/bin/whoamiexist?

Yes, execute it!

Back to whoami


● How to run it?



Really?

Back to whoami


● How to run it?


● Shell will run the system-provided whoami found under /usr/bin.


Really? No.

Back to whoami


● How to run it?

– You have to explicitly tell your shell to run the whoami found in the current directory:

● '.' (dot) specifies current directory,● '..' specifies upper directory,● '/' is path separator.

Back to whoami


● How to run it?

– You have to explicitly tell your shell to run the whoami found in the current directory:

[ozan@DellFedora ~]$ ./whoamiozan

Finally...

● That's not how the real whoami utility is implemented,

● Run the following command:

– USER=”bill” ./whoami

● What happened and why? Discuss.

Any questions?

Ozan Çağ[email protected]

Galatasaray UniversityComputer Engineering Department

Reference bookThe Linux Programming Interface Michael Kerrisk