Transcript 10CS44_unit-1 - WordPress.com

Course code: 10CS44
UNIT-1
The Unix Operating System
Engineered for Tomorrow
Prepared by : Thanu Kurian
Department :Computer Science
Date : 18.01.2014
Engineered for
Tomorrow
What is UNIX?
•
•
•
•
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
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
Engineered for
Tomorrow
• 1980’s
– System V release 4
– TCP/IP
– Sun Microsystems
Solaris
– Microsoft Xenix, SCO
– MIT X-Windows
• 1990’s
– GNU, LINUX
– Stallman, Torvalds
Why Use UNIX?
•
•
•
•
•
•
Engineered for
Tomorrow
multi-tasking / multi-user
lots of software
networking capability
graphical (with command line)
easy to program
portable (PCs, mainframes,
super-computers)
continued
Engineered for
Tomorrow
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.
Engineered for
Tomorrow
user
user
user
Contd….
Kernel – shell relationship
Other s/w
shell
shell
shell
cp
X window
cc
ls
Kernel
grep
Hardware
Spreadsheets
tar
sort
ps
Databases
who
Text Processors
sed
Browsers
Other
Compilers
Engineered for
Tomorrow
Contd….
Shell :
•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
Engineered for
Tomorrow
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
Engineered for
Tomorrow
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
Engineered for
Tomorrow
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.
Engineered for
Tomorrow
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.
Engineered for
Tomorrow
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.
Engineered for
Tomorrow
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.
Engineered for
Tomorrow
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.
EngineeredContd….
for
Tomorrow
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
Engineered for
Tomorrow
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.
Engineered for
Tomorrow
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.
Engineered for
Tomorrow
 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).
1. Options:
1) ls –l note
-l is an argument to ls , known as option.
 An option is normally preceded by a minus sign (-).
Engineered for
Tomorrow
2) ls –z note
ls: illegal option – z
command
//message from ls
3) $ ls-l
bash:ls-l: command not found
4) ls –lat is same as ls –l –a –t to obtain the same
output
Engineered for
Tomorrow
2. Filename Arguments
1) 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 chap02
// rm removes files
Engineered for
Tomorrow
 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.
Engineered for
Tomorrow
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
Engineered for
Tomorrow
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.
Engineered for
Tomorrow
Example
• $ echo “this is
• >a three line
prompt(>) appears
• >text message”
• This is
• a three line
• text message
// A second
Engineered for
Tomorrow
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.
Engineered for
Tomorrow
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

The entire man page for wc is displayed on the screen.
//help on the wc command
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.
Engineered for
Tomorrow
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
Engineered for
Tomorrow
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.
Engineered for
Tomorrow
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.
Example:
$ whatis cp
cp cp(1)
-copy files
Engineered for
Tomorrow
When Things Go Wrong
Engineered for
Tomorrow
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.
Engineered for
Tomorrow
 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.
Engineered for
Tomorrow
1. Ordinary (Regular) file:
•
Is the most common file type
•
ordinary file can be divided into 2 types:
i) text file
ii) binary file
i)
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
Engineered for
Tomorrow
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.
Engineered for
Tomorrow
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.
Engineered for
Tomorrow
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.
Engineered for
Tomorrow
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 :
Engineered for
Tomorrow
root(/)
UNIX File System Tree:
bin
dev
home
Contd..
lib
stand
tmp
usr
var
cat date who
kumar
login.sql
sharma
bin include sbin
progs
38
Engineered for
Tomorrow
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
e.g.: /home/kumar
Engineered for
Tomorrow
pwd: Checking your current directory
The pwd (present working directory) command
tells you the current directory
$ pwd
/home/kumar
Engineered for
Tomorrow
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
$ pwd
/home/kumar/progs
Engineered for
Tomorrow
• If we want to switch to /bin directory ,we should
use the absolute path name:
$ pwd
/home/kumar/progs
$ cd /bin
$ pwd
/bin
Engineered for
Tomorrow
cd command can also be used without any arguments:
$ pwd
/home/kumar/progs
$ cd
// cd used without arguments reverts to the home
directory
$ pwd
/home/kumar
Engineered for
Tomorrow
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
Engineered for
Tomorrow
• 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
Engineered for
Tomorrow
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
Engineered for
Tomorrow
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
Engineered for
Tomorrow
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
48
Engineered for
Tomorrow
• 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
Engineered for
Tomorrow
• 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
Engineered for
Tomorrow
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:
Engineered for
Tomorrow
Using . and .. in relative path names
$ pwd
/home/kumar/progs/data/text
$ cd ..
// moves one level up
$ pwd
/home/kumar/progs/data
•
To move to /home , we can use the relative path name as follows:
$ pwd
/home/kumar/pis
$ cd ../..
$ pwd
/home
// moves two levels up
Engineered for
Tomorrow
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 with @
-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.
Engineered for
Tomorrow
•
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
Engineered for
Tomorrow
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.
programs are linked with files in these directories.
C
Engineered for
Tomorrow
 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