1. The Unix Operating System
Course code: 10CS44
UNIT-1
The Unix Operating System
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Prepared by : Thanu Kurian
Department :Computer Science
Date :

2. What is UNIX? An Operating System (OS) Mostly coded in C
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An Operating System (OS)
Mostly coded in C
Machine independence
It provides a number of facilities:
management of hardware resources
directory and file system
loading / execution / suspension of programs

3. History (Brief) 1980’s 1990’s 1969 1970’s System V release 4 TCP/IP
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History (Brief)
1969
First UNIX at Bell Labs
The MULTICS
Kernighan, Ritchie, Thompson
1970’s
Bell Labs makes UNIX freeware
Berkeley UNIX (BSD)
Bill Joy vi editor, C Shell
1980’s
System V release 4
TCP/IP
Sun Microsystems Solaris
Microsoft Xenix, SCO
MIT X-Windows
1990’s
GNU, LINUX
Stallman, Torvalds

4. Why Use UNIX? multi-tasking / multi-user lots of software
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Why Use UNIX?
multi-tasking / multi-user
lots of software
networking capability
graphical (with command line)
easy to program
portable (PCs, mainframes, super-computers)
continued

5. The UNIX architecture and Command Usage
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The UNIX architecture and Command Usage
The UNIX Architecture
Kernel and Shell:
Kernel:
is the core of the OS
kernel is loaded into memory when the system is booted and communicates directly with the hardware.
user programs access the kernel thru’ a set of functions , called system calls.
kernel also manages system’s memory, schedules processes, decides their priorities and performs other tasks.

6. Kernel – shell relationship
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user
user
user
Contd….
Kernel – shell relationship
Other s/w
shell
shell
shell cp cc
X window
Kernel ls
grep
Hardware
tar
Spreadsheets
sort
Other Compilers ps
Databases
who
sed
Text Processors
Browsers

7. Shell : Is the outer part of OS
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Contd….
Is the outer part of OS
is the interface between user and kernel
there is only one kernel running on the system, but there could be several shells in action – one for each user logged in.
acts like command interpreter- it accepts commands issued by the user, interprets these commands, and executes the appropriate programs.
Linux uses the Bash shell by default
Shell :

8. File and Process: File: Is an array of bytes Hierarchical file system
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File and Process:
File:
Is an array of bytes
Hierarchical file system
Directories and devices are members of file system
Dominant file type is text

9. is the job (program) in execution. form a hierarchical tree structure
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Process:
is the job (program) in execution.
form a hierarchical tree structure
UNIX provides tools to control processes, move them between foreground and background and kill them.
System Calls:
are the routines (functions) defined in the kernel
user to communicate with the kernel thru’ system calls
system calls are described in POSIX (Portable Operating System Interface) specification
All UNIX systems use the same system calls.
e.g. : write, open

10. Features of UNIX: 1. A Multi-user system:
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Features of UNIX:
1. A Multi-user system:
Multiple users working on a single system.
The resources (CPU,Memory and hard disk) are actually shared between all users.
2. A Multiprogramming system:
Permits multiple programs to run
this can happen in 2 ways:
i) multiple users can run separate jobs.
ii) a single user can also run multiple jobs.

11. 3. A Multitasking system:
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3. A Multitasking system:
a single user runs multiple tasks concurrently.
in this environment, one job is running in the foreground , the rest run in the background.
4. The Building-Block Approach:
“small is beautiful” is implemented thru’ pipes and filters.
by interconnecting a number of tools , we can have a large number of combinations of their usage.

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5. The UNIX Toolkit:
There are general-purpose tools, text manipulation utilities (filters), compilers and interpreters , networked applications, system administration tools and shells.
6. Pattern Matching:
The * (called as metacharacter) is a special character used by the system to indicate that it can match a number of files.
complex pattern matching can be done with the help of regular expressions.

13. 7. Programming Facility: The UNIX shell is a programming language.
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7. Programming Facility:
The UNIX shell is a programming language.
it has control structures, loops and variables.
shell scripts are the programs that can invoke the UNIX commands.
many of the system’s functions can be controlled and automated by using the shell scripts
8. Documentation:
The principal online help facility is the man command
it is the reference for commands and their configuration files.
there are several newsgroups on UNIX available on the Internet.
The FAQ (Frequently Asked Questions) – a document that addresses common problems is also available on the Internet.
articles in magazines and journals, lecture notes by universities.

14. Understanding the UNIX Command
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Understanding the UNIX Command
Locating Commands:
UNIX is command-based system.
the commands are essentially files containing programs , written in C.
the commands used by all users are located in /bin and /usr/bin.
to know the location of an executable program , use the type command.
e.g.: $ type ls
ls is /bin/ls
when we execute ls command , the shell locates this file in the /bin directory and makes arrangements to execute it.

15. this variable can be evaluated using echo command as follows:
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Contd….
The PATH variable:
The shell variable, PATH, specifies the search list of directories for locating commands.
The shell searches through these directories in order whenever it needs to find a command.
this variable can be evaluated using echo command as follows:
$ echo $PATH
/bin:/usr/bin:/usr/local/bin:/usr/ccs/bin:/usr/local/java/bin:.
Root directory

16. Environment variable:
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Environment variable:
A variable that specifies how an operating system or another program runs, or the devices that the operating system recognizes.
set of dynamic values that can affect the way running processes will behave on a computer.
A string consisting of environment information, such as a drive, path, or filename, associated with a symbolic name that can be used by MS-DOS and Windows.

17. Internal and External Commands
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Internal and External Commands
An Internal command is built into the shell, and is normally executed even if there's an external command of the same name.
e.g.: echo command
$ type echo
echo is a shell built-in
when echo is typed, the shell won’t look in its PATH to locate it (even if it’s there in /bin) , rather executes it from its own set of built-in commands.

18. External Commands exist on disk as separate files.
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External Commands exist on disk as separate files.
e.g.: ls command, exists in the /bin directory (or /usr/bin).
Command Structure:
A command is used with options and arguments.
commands and arguments must be separated by any number of
spaces or tabs (whitespace).
Options:
ls –l note
-l is an argument to ls , known as option.
An option is normally preceded by a minus sign (-).

19. ls: illegal option – z //message from ls command
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2) ls –z note
ls: illegal option – z //message from ls command
3) $ ls-l
bash:ls-l: command not found
4) ls –lat is same as ls –l –a –t to obtain the same output

20. tput clear // clear is an argument to tput cat list
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2. Filename Arguments
tput clear // clear is an argument to tput
cat list
many UNIX commands use a filename as argument, so the command can take input from the file.
many commands work with multiple filenames as arguments.
e.g.:
1) ls –lat chap01 chap02 chap03
2) cp chap01 chap02 progs // cp copies files
3) rm chap01 chap // rm removes files

21. command line is complete only after user hits [Enter].
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The command with its arguments and options is known as the command line.
command line is complete only after user hits [Enter].
The complete line is then fed to the shell as its input for interpretation and execution.

22. Flexibility of Command usage:
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Flexibility of Command usage:
a command can often be entered in more than one way.
1. Combining commands:
UNIX allows to specify more than one command in the command line.
each command has to be separated from the other by a ; (semicolon)‏
e.g.: wc note ; ls –l note
The combined output of two commands can be sent to the file newlist as follows:
e.g.: (wc note ; ls –l note ) > newlist
metacharacter

23. 2.A Command Line can be overflow or be split into multiple lines:
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2.A Command Line can be overflow or be split into multiple lines:
Terminal width is restricted to 80 characters.
The command simply overflows to the next line though it is still in a single logical line.
Sometimes it is necessary to split a long command line into multiple lines.
So the shell issues a secondary prompt , usually > , to indicate that command line is not complete.

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Example
$ echo “this is
>a three line // A second prompt(>) appears
>text message”
This is
a three line
text message

25. 3. Entering a Command before previous command has finished:
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3. Entering a Command before previous command has finished:
Subsequent commands can be entered at the keyboard (as many commands as we wish) without waiting for the prompt.
the command that has been entered remains stored in a buffer, maintained by the kernel for all keyboard input.

26. man : Browsing the manual pages online
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man : Browsing the manual pages online
UNIX offers an online help facility in the man command.
Man displays the documentation- man documentation ,of every command in the system
e.g.: man wc //help on the wc command
The entire man page for wc is displayed on the screen.
Navigation and Search:
Two navigation commands working on all systems are:
f or spacebar , advances the display by one screen of text at a time
b, which moves back one screen.

27. Understanding a man page:
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Understanding a man page:
A man page is divided into a number of compulsory and optional sections.
NAME, SYNOPSIS, DESCRIPTION are 3 common sections.
NAME presents a one-line introduction to the command.
SYNOPSIS shows the syntax- the options and arguments used by the command.
DESCRIPTION provides a detailed description.
Using man to understand man:
man man // Viewing man pages with man

28. Further Help with man –k, apropos and whatis
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Further Help with man –k, apropos and whatis
man –k: Searches a summary database and prints one-line description of the command.
Example:
$ man –k awk
awk awk(1) -pattern scanning and processing language
nawk nawk(1) -pattern scanning and processing language
apropos: lists the commands and files associated with a keyword.

29. ftp ftp(1) -file transfer program
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Example:
$ apropos FTP
ftp ftp(1) -file transfer program
ftpd in.ftpd(1m) -file transfer protocol server
ftpusers ftpusers(4) -file listing users to be disallowed
ftp login privileges
whatis: lists one-liners for a command.
$ whatis cp
cp cp(1) -copy files

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When Things Go Wrong

31. The File System The File:
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The File System
The File:
Is a container for storing information. (may be a sequence of characters)‏
UNIX file doesn’t contain the eof (end-of-file) mark.
A file’s size is not stored in the file , nor even its name.
All file attributes are kept in a separate area of the hard disk, which is accessible only to kernel.
UNIX treats directories and devices as files as well.
All physical devices like hard disk, memory, CD-ROM, printer and modem are treated as files.

32. UNIX treats shells and kernel also as files.
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UNIX treats shells and kernel also as files.
Files are divided into 3 categories:
i) Ordinary file:
also known as regular file.
contains only data as a stream of characters
ii) Directory file:
contains the file name and its associated inode number.
iii) Device file:
contains no data but the kernel uses the attributes of the file to operate the device.
all peripherals and devices are represented by files.

33. Ordinary (Regular) file:
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Ordinary (Regular) file:
Is the most common file type
ordinary file can be divided into 2 types:
i) text file
ii) binary file
Text file:
contains only printable characters which can be viewed.
text file contains lines of characters where ever line is terminated with the newline character. (linefeed or LF)‏
e.gs: C and Java program files, shell and perl scripts
ii) Binary File:
contain both printable and unprintable characters that cover the entire ASCII range(0 to 255)‏
e.gs: UNIX commands, object code and executables of C programs, Picture, sound and video files

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2. Directory File
A directory contains no data , but keeps some details of the files and subdirectories that it contains.
The UNIX file system is organized with a number of directories and subdirectories.
A directory contains the file name and not the file’s contents.
There will be an entry for each file in the directory.
Each entry has 2 components:
i) The filename
ii) A unique identification number for the file or directory, called the inode number.
When any file is created or removed, the kernel automatically updates its corresponding directory by adding or removing the entry (inode number & filename) associated with that file.

35. The attributes of every file is stored on the disk .
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3. Device File:
Device files are generally found inside a single directory structure, /dev.
The attributes of every file is stored on the disk .
The kernel identifies a device from its attributes and then uses them to operate the device.

36. In UNIX, the following characters can be used in filenames:
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Filename:
In UNIX, the following characters can be used in filenames:
Alphabetic characters and numerals
The period(.), hyphen(-) and underscore(_)‏
A file can have as many dots embedded in its name. A filename can also begin and end with a dot.
a.b.c.d.e is a valid filename.
UNIX is case-sensitive.
chap01, Chap01 and CHAP01 are 3 different filenames.

37. Parent-Child Relationship:
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Parent-Child Relationship:
The file system in UNIX is a collection of all the related files (ordinary, directory and device files) organized in a hierarchical structure.
Directories and files have parent-child relationship.
The topmost directory is called root, represented by a frontslash(/).
The root directory(/) has a number of subdirectories under it. These subdirectories have more subdirectories and files under them.
The home directory is the parent of kumar. / is the parent of home and the grandparent of kumar.
login.sql is an ordinary file under kumar directory.
The visual representation is as shown :

38. root(/)‏ UNIX File System Tree: bin var dev tmp usr home lib stand who
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root(/)‏
Contd..
UNIX File System Tree:
bin
var
dev
tmp
usr
home
lib
stand
who
cat
date
sharma
kumar
bin
sbin
include
login.sql
progs

39. The HOME Variable: The HOME Directory:
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The HOME Variable: The HOME Directory:
The home directory is created by the system when a user account is opened.
e.g.: /home/kumar
The shell variable HOME tells the home directory.
$ echo $HOME
/home/kumar
Absolute Pathname:
shows a file’s location with reference to the top, i.e., root
is simply a sequence of directory names separated by slashes

40. pwd: Checking your current directory
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pwd: Checking your current directory
The pwd (present working directory) command tells you the current directory
$ pwd
/home/kumar

41. cd: Changing the Current directory
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cd: Changing the Current directory
The cd (change directory) command changes the current directory to the directory specified as argument.
$ pwd
/home/kumar
$ cd progs //change the subdirectory to progs under current directory
/home/kumar/progs

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If we want to switch to /bin directory ,we should use the absolute path name:
$ pwd
/home/kumar/progs
$ cd /bin
/bin

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cd command can also be used without any arguments: $ pwd /home/kumar/progs $ cd // cd used without arguments reverts to the home directory /home/kumar

44. mkdir: Making Directories
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mkdir: Making Directories
Directories are created with the mkdir command.
The syntax of this command is:
mkdir name of the directory to be created
e.g.: a directory new is crated under the current directory as follows:
mkdir new
A number of subdirectories can be created with one mkdir command as follows:
mkdir new dbs doc // 3 directories created

45. e.g.: mkdir pis pis/progs pis/data //creates the directory tree
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UNIX system lets us create directory trees with just one invocation of the command.
e.g.: mkdir pis pis/progs pis/data //creates the directory tree
This creates 3 subdirectories –pis and 2 subdirectories under pis.
We can’t create a subdirectory before the creation of its parent directory.
e.g.: $ mkdir pis/data pis/progs pis
mkdir: Failed to make directory “pis/data”; No such file or directory
mkdir: Failed to make directory “pis/progs”; No such file or directory

46. Sometimes the system refuses to create a directory: $ mkdir test
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Sometimes the system refuses to create a directory:
$ mkdir test
mkdir: Failed to make directory “test”; Permission denied
This may be due to following reasons:
The directory test may already exist
there may be an ordinary file by that name in the current directory
If we try to create a directory in /bin , /etc or any other directory that houses the UNIX system files

47. rmdir: Removing Directories
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rmdir: Removing Directories
The rmdir(remove directory) command removes directories.
e.g.: rmdir doc //removes the directory doc
rmdir can also delete more than one directory in one shot.
e.g:
rmdir pis/data pis/progs pis //directories are deleted in reverse order

48. shows a file’s location with reference to the top, i.e., root
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Absolute Pathnames:
shows a file’s location with reference to the top, i.e., root
is simply a sequence of directory names separated by slashes
e.g.: /home/kumar
Suppose we are placed in /usr and want to access the file login.sql which is present in /home/kumar, we can give the pathname of the file as below:
cat /home/kumar/login.sql

49. No two files in a UNIX system can have identical absolute pathnames .
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No two files in a UNIX system can have identical absolute pathnames .
We can have 2 files with the same name , but in different directories; their pathnames will also be different.
Using the Absolute pathname for a command
Since date command resides in /bin (or /usr/bin) , we can use the absolute pathname as follows:
$ /bin/date
Mon Aug 25 09:40:45 IST 2008

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For any command that resides in the directories specified in the PATH variable, we need not use the absolute pathname.
If we want to execute programs residing in some other directory that isn’t in PATH ,the absolute pathname needs to be specified.
e.g.: to execute the program less residing in /usr/local/bin , we need to give the absolute pathname:
/usr/local/bin/less

51. A pathname which specifies the location of a file using the symbols
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Relative Pathname:
A pathname which specifies the location of a file using the symbols
. and ..
i.e (single dot) –represents the current directory
.. (two dots) –represents the parent directory
Assume that we are in /home/kumar/progs/data/text , we can use .. as an argument to cd to move to the parent directory, /home/kumar/progs/data as shown below:

52. Using . and .. in relative path names
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Using . and .. in relative path names
$ pwd
/home/kumar/progs/data/text
$ cd // moves one level up
/home/kumar/progs/data
To move to /home , we can use the relative path name as follows:
/home/kumar/pis
$ cd ../ // moves two levels up
/home

53. ls - list directory contents
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ls - list directory contents
The command to list your directories and files is ls.
With options it can provide information about the size, type of file, permissions, dates of file creation, change and access.
Syntax
ls [options] [argument]
Common Options
When no argument is used, the listing will be of the current directory. There are many very useful options for the ls command. A listing of many of them follows. When using the command, string the desired options together preceded by "-".
-a Lists all files, including those beginning with a dot (.).
-d Lists only names of directories, not the files in the directory
-F Indicates type of entry with a trailing symbol: executables with *, directories with / and symbolic links
-R Recursive list
-u Sorts filenames by last access time
t Sorts filenames by last modification time
-i Displays inode number
-l Long listing: lists the mode, link information, owner, size, last modification (time). If the file is a symbolic link, an arrow (-->) precedes the pathname of the linked-to file.

54. b block-type special file c character-type special file
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The mode field is given by the -l option and consists of 10 characters. The first character is one of the following:
CHARACTER IF ENTRY IS A
d directory
- plain file
b block-type special file
c character-type special file
l symbolic link
s socket
The next 9 characters are in 3 sets of 3 characters each. They indicate the file access permissions: the first 3 characters refer to the permissions for the user, the next three for the users in the Unix group assigned to the file, and the last 3 to the permissions for other users on the system.
Designations are as follows:
r read permission
w write permission
x execute permission
- no permission

55. Files available during system installation:
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The UNIX File System:
Files available during system installation:
/bin and /usr/bin –are the directories where all the commonly used UNIX commands are found.
/sbin and /usr/sbin – User will not be able to execute most command in these directories, only the system administrator can.
/etc – directory contains the configuration files of the system. User’s login name and password are stored in this directory.
/dev – this contains all device files. These files don’t occupy space on disk.
/lib and /usr/lib- contains all library files in binary form. C programs are linked with files in these directories.

56. usr/include - contains the standard header files used by C programs.
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usr/include - contains the standard header files used by C programs.
/usr/share/man - man pages are stored here. There are separate subdirectories here that contain the pages for each section.
The second group of files are as follows:
/tmp – the directories where users are allowed to create temporary fiels.
/var – the variable part of the file system. Contains all print jobs and outgoing and incoming mails.
/home – on many systems users are housed here