1 Unix basics. Part 1

Unix basics
System Software

Roman Prykhodchenko
rprikhodchenko@kture.kharkov.ua

Tuesday, February 8, 2011

Agenda

• History

• Terminology

• Unix commons


History


UNIX

• UNIX is a computer operating system originally
developed in 1969 by a group of AT&T employees
at Bell Labs.

• Today UNIX is a family of the operating systems
that correspond to the Single Unix Speciﬁcation.


Main events
• 1969 – MULTICS, the ﬁrst implementation of the
UNIX operating system for General Electric
GE-645 computer.

• 1976 – UNIX Timesharing System 6

• 1977 – Berkley Software Distribution (BSD)

• 1979 – UNIX Timesharing System 7

• 1982 – BSD 4.1 implements TCP/IP


People
Denis Ritchie Brian Kernighan Ken Thompson


Main events

• 1988 – First edition of POSIX standard

• 1989 – The ANSI C standard is published

• 1983 – Richard Stallman initiated the GNU project

• 1992 – The GNU is using the Linux kernel

• 1994 – 386 BSD is released


People
Linus Torvalds Richard Stallman


People
William Jolitz Lynne Jolitz


Terminology


*nix

• UNIX-like operating system (or just *nix) is an
operating system that behaves similar to UNIX but
does not correspond to the Single Unix
Speciﬁcation.


SUS

• Single UNIX speciﬁcation is the collective name of
a family of standards for computer operating
systems to qualify for the name "Unix"


POSIX
• POSIX (Portable Operating System Interface for
uniX)is the standard for the UNIX operating
system published by IEEE Computer Society.

• Includes:

• System API

• Shell interface

• API of the system utilities


Time-sharing

• Time-sharing operating system shares a computing
resource among many users by means of
multiprogramming and multi-tasking.


Unix commons


Key properties
• Portability

• Preemptive multitasking

• Virtual memory

• Multilevel architecture

• Support of asynchronous processes

• Device-independent I/O


Software conception
• Do only one thing and do it well.
For each task the system can perform there is a
separate program.

• Examples:
cp – copies a file
cat – prints a file’s contents

• Benefits:
- Simplicity
- Number of errors is relatively small.


Everything is a file
• Every computer’s resource is represented by a file.

• Examples:
/home/john/text.txt – John’s text file
/dev/sda – first hard drive
/proc/scsi – information about any devices
connected via a SCSI or RAID controller

• Benefits:
Device-independent I/O operations.


Multilevel architecture

• Each software type runs on an appropriate level

• Memory is not shared between different levels

• System calls for inter-level communications


Multilevel architecture

Application software, network services, utilities

System software (command interpreters, protocols...)

System calls

Kernel


Kernel

• Scheduling

• Memory management

• Interruptions processing

• Inter-process communication

• Low-level device support


Kernel

• Monolithic kernel

• Microkernel

• Nanokernel

• Exokernel

• Hybrid kernel


Kernel


Kernel
“Exokernel”
based Operating System


System calls

• Process management

• Implementation of I/O operations

• Bind user actions to drivers


Signals

• Signals are an approach of inter-process
communication (IPC)

• Signal is an asynchronous message sent to signal

• Operating system interrupts the process when it is
sent a signal


Signals

• A process can implement a handler for different
kind of signals

• Default handler kills the process

• Some signals can not be handled


Signals
• Sources of signals:
• Keyboard shortcuts
• Kernel
• Hardware exception
• Wrong system call
• I/O operations
• A process


Shell
Shell is a command-line interpreter that provides a
traditional user interface for the Unix operating system
and for Unix-like systems.

Most popular are: bash, sh, csh, zsh

We will mostly use bash.


Common syntax
$ app_name [options] [parameters]

Options begin with - or --
-o1 [value] -o2 -o3 [value]
or
-o1o2o3

Examples:
$ tar -x -j -v -f archive.tar.gz
$ tar -xjvf archive.tar.gz

Parameters are usually required.
$ rdesktop -f -u UserName 192.168.0.124


Manual pages
Man application shows a manual page for speciﬁed
application:
$ man {app_name}

Or you can use option -h to see a short help:
$ tar -h


File system hierarchy standard
/ Rood directory
/bin/ Essential command binaries
/boot/ Static bootloader
/dev/ Devices represented by files
/etc/ Host-specific configuration data
/lib/ Basic shared libraries and kernel modules
/mnt/ Temporary mount point
/opt/ Optional software
/sbin/ Basic system software
/tmp/ Temporary data
/usr/ Secondary hierarchy
/var/ Variable data


PATH
PATH is an environment variable used by shells for
searching for applications.

Applications located in directories that are included to
PATH can be launched without specifying full path.
$ echo $PATH
/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/X11R6/
bin

You can modify PATH
$ PATH=$PATH:~/bin


Navigation
cd Navigates to specified directory

ls Shows current directory’s contents

pwd Shows current directory

mkdir Creates a directory

rmdir Removes a directory

touch Modifies last change date

rm Deletes a file


Inodes
Inode is an index descriptor of a ﬁlesystem resource.

- Every resource has own inode number.
- File name is a link to inode.

Example:
$ ls -dl /usr/local
drwxr-xr-x 8 root root 240 Dec 22

/usr/local
/usr/local/.
/usr/local/bin/..
/usr/local/games/..
/usr/local/lib/..
/usr/local/sbin/..
/usr/local/share/..
/usr/local/src/..


Questions?


1 Unix basics. Part 1

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