Module1-UNIX architecture; FEATURES OF UNIX OS

18CS56- UNIX PROGRAMMING
Module-1

Introduction about Operating system
 An operating system is system software that manages
computer hardware and software resources and
provides common services for computer programs. It
acts as an interface between the user and the
computer hardware.
 Following are some of important functions of an
operating System.
• Memory Management
• Processor Management
• Device Management

cont..
• File Management
• Security
• Control over system performance
• Job accounting
• Error detecting aids
• Coordination between other software and users

UNIX OS
• Unix is a computer Operating System which is capable
of handling activities from multiple users at the same
time.
• The development of Unix started around 1969 at AT&T
Bell Labs by Ken Thompson and Dennis
• The main components of UNIX OS are kernel and shell.
The computer programs that allocate the system
resources and coordinate all the details of the
computer's internal is called the kernel.

Cont..
• Users communicate with the kernel through a program
known as the shell.
• The shell is a command line interpreter; it translates
commands entered by the user and converts them into
a language that is understood by the kernel.

History of UNIX
• The UNIX operating system was born in the late 1960s. It
originally began as a one man project led by Ken Thompson of
Bell Labs, and has since grown to become the most widely used
operating system.
• In the time since UNIX was first developed, it has gone through
many different generations and even mutations.
– Some differ substantially from the original version, like
Berkeley Software Distribution (BSD) or Linux.
– Others, still contain major portions that are based on the
original source code.

cont..
 There are various Unix variants available in the market.
Solaris Unix, AIX, HP Unix and BSD are a few examples.
Linux is also a flavor of Unix which is freely available.

UNIX ARCHITECTURE
 The heart of Fig is the hardware. This is surrounded by
the operating system. UNIX architecture comprises of
two major components viz., the shell and the kernel.

Cont..
 The kernel interacts with the machine’s hardware and
the shell with the user.
 The heart of the operating system is often called the
kernel.
 The kernel normally contains essential features such as
the scheduler, file management etc. Users and
programs cannot communicate directly with the kernel
normally.
 However there is a mechanism by which we can
communicate – this is via system calls. Here is a basic
block diagram of a Unix system

Cont..
(i) Division of labor: kernel and shell
Kernel − The kernel is the heart of the operating system.
It is a collection of routines written in C.
 It is loaded into memory when the system is
booted and communicates directly with the
hardware. It interacts with the hardware and most of
the tasks like memory management, task scheduling
and file management.
Shell − The shell performs the role of command
interpreter. When you type in a command at your
terminal, the shell interprets the command and calls
the program that you want.

Cont..
 Even though there’s only one kernel running on the
system, there could be several shells in action, one for
each user who’s logged in.
 The shell uses standard syntax for all commands. C
Shell, Bourne Shell and Korn Shell are the most famous
shells which are available with most of the Unix
variants.
(ii) The File and Process
 All the data of Unix is organized into files. All files are
then organized into directories.
 These directories are further organized into a tree-like
structure called the file system.

Cont..
 A file is an array of bytes that stores information. It is
also related to another file in the sense that both
belong to a single hierarchical directory structure.
 A process is the second abstraction UNIX provides. It
can be treated as a time image of an executable file.
 Like files, processes also belong to a hierarchical
structure.
(iii) System Calls
• User programs that need to access the
hardware use the services of the kernel via use
of system calls.

Cont..
 These are similar to function calls, but remove control
from the user process All UNIX flavors use same system
calls. Eg:
• exit: exits a process
• write: writes to a file
• read: reads from a file
• fork: creates a new process

FEATURES OF UNIX OS
• Several features of UNIX have made it popular. Some
of them are:
• Portable : UNIX can be installed on many hardware
platforms. Its widespread use can be traced to the
decision to develop it using the C language.
• Multiuser: The UNIX design allows multiple users to
concurrently share hardware and software
• Multitasking: UNIX allows a user to run more than one
program at a time. In fact more than one program can
be running in the background while a user is working
foreground.

Cont..
• Networking: While UNIX was developed to be an
interactive, multiuser, multitasking system, networking
is also incorporated into the heart of the operating
system.
• Access to another system uses a standard
communications protocol known as Transmission
Control Protocol/Internet Protocol (TCP/IP).
• Organized File System: UNIX has a very organized file
and directory system that allows users to organize and
maintain files.

Cont..
• Device Independence: UNIX treats input/output
devices like ordinary files. The source or destination for
file input and output is easily controlled through a UNIX
design feature called redirection.
• Building block approach: UNIX uses a building-block
approach in the design of some of its tools and lets you
develop complex command routines by connecting
these tools. For example we can use pipe to connect ls
and wc and count number of files in your directory.
• Programming facilities

Cont..
• UNIX toolkit and documentation.
• Utilities: UNIX provides a rich library of utilities that can
be use to increase user productivity.

POSIX AND SINGLE UNIX SPECIFICATION
• POSIX (Portable Operating System Interface) is a set of
standard operating system interfaces based on
the Unix operating system.
• It is designed for achieving portability. It is a family of
standards specified by the IEEE Computer Society for
maintaining compatibility between operating systems.
• Beginning in 1998, a joint working group known as the
Austin Group began to develop the combined standard
that would be known as the Single UNIX Specification
Version 3 and as POSIX:2001 (formally: IEEE Std 1003.1-
2001).

Cont..
• It was released on January 30, 2002. The main two
interfaces are POSIX.1 and POSIX.2.
• POSIX.1 is the standard for an application program
interface in the C language. POSIX.2 is the
standard shell and utility interface.
• PSIX.4 is another important interface for thread
management.

Cont..
• Single Unix specification: In December 2008, the Austin
Group published a new major revision, known as
POSIX:2008 (formally: IEEE Std 1003.1-2008). This is the
core of the Single UNIX Specification, Version 4.
• The Single UNIX Specification is an industry standard
description of the C language program and user command
interfaces for a standard Unix operating system.
• The “write once, adopt everywhere” approach of this
development ensure that a program developed in one
POSIX compliant Unix operating system would run in a
somewhat different POSIX compliant Unix operating
system.

GENERAL FEATURES OF UNIX COMMANDS/ COMMAND
STRUCTURE
• A command is a program that tells the Unix system to
do something. It has the form:
Command name [options] [arguments]
• where command is the command name that can take a
set of optional options and one or more optional
arguments.
• An argument indicates on what the command is to
perform its action, usually a file or series of files. An
option modifies the command, changing the way it
performs.

Cont..
Commands:
• Commands are case sensitive.
 i.e command and Command are not the same.
 Commands, options and arguments have to be
separated by spaces or tabs to enable the shell to
interpret them as words.
 A contiguous string of spaces and tabs together is
called a whitespace.

Cont..
 The shell compresses multiple occurrences of whitespace into a
single whitespace.
• $ cp file1 file2
(Will copy file 1 to file 2)
 To change the name of a file, use the mv command. Following is
the basic syntax
$ mv old_file new_file

Cont..
Options
• Options are generally preceded by a hyphen (-), and for most
commands, more than one option can be strung together, in the
form:
command -[option][option][option]
• Example: $ ls –l
• There must not be any whitespaces between – and l.
• Options are also arguments, but given a special name because
they are predetermined. Options can be normally combined
with only one – sign. i.e., instead of using
• $ ls –l –a –t we can as well use,
• $ ls –lat

Cont..
Filename Arguments
• Many UNIX commands use a filename as argument so that the
command can take input from the file.
• If a command uses a filename as argument, it will usually be the
last argument, after all options.
Example: cp file1 file2 file3 dest_dir
rm file1 file2 file3
• The command with its options and arguments is known as the
command line, which is considered as complete after [Enter] key
is pressed, so that the entire line is fed to the shell as its input
for interpretation and execution.

Cont..
Exceptions
• Some commands in UNIX like pwd do not take any options and
arguments. Some commands like who may or may not be
specified with arguments.
• The ls command can run without arguments (ls), with only
options (ls –l), with only filenames (ls f1 f2), or using a
combination of both (ls –l f1 f2).
• Some commands compulsorily take options (cut). Some
commands like grep, sed can take an expression as an argument,
or a set of instructions as argument.

Cont..
Cut command:
The cut command extracts a given number of characters or
columns from a file.
Syntax: cut [options] [file]
Example: $ cut -c 5-10 file1
This command will extract characters 5 to 10 from each line.
grep command in Unix
The grep filter searches a file for a particular pattern of characters,
and displays all lines that contain that pattern. The pattern that
is searched in the file is referred to as the regular expression
grep [options] pattern [files]

Cont..
Example: $grep -i "UNix" geekfile.txt
The -i option enables to search for a string case insensitively in the
give file. It matches the words like “UNIX”, “Unix”, “unix”.
Sed Command
SED is a powerful text stream editor. Can do insertion, deletion,
search and replace(substitution).
Syntax:
sed OPTIONS... [SCRIPT] [INPUTFILE...]
Example:
$sed 's/unix/linux/' geekfile.txt
Replacing or substituting string : Sed command is mostly used to
replace the text in a file. The below simple sed command
replaces the word “unix” with “linux” in the file.

Understanding of some basic commands such as echo, printf, ls,
who, date, passwd, cal
cal: The calendar
• The cal command is a command line utility for displaying a
calendar in the terminal. It can be used to print a single month,
many months or an entire year. The syntax is::
cal [-mjy] [[month] year]
• Everything within rectangular box is optional.
Tag Description
-m Display monday as the first day of the week.
-j
Display julian dates (days one-based, numbered from
January 1).
-y Display a calendar for the current year.

cont..
To display feb 2015 calendar
$ cal 2 2015
To display complete year calendar.
$ cal –y
date: Displaying the system date
The date command displays the current date and time. It can also
be used to display or calculate a date in a format you specify
The syntax is:
date [OPTION]... [+%FORMAT]
• To Print current system date and time:
$date
output:
Sun Jan 8 21:38:15 IST 2017

cont..
• To print only the month:
$date +%m
output:
08
• To print only the month name:
$date +%h
output:
Aug

cont..
• echo: Displaying a Message
 echo is a fundamental command found in most operating
systems that offer a command line.
 It is frequently used in scripts, batch files, and as part of
individual commands; anywhere you may need to insert text.
 Many command shells such as bash, ksh and csh implement
echo as a built-in command.
Escape sequence used by echo and printf

cont..
Example-1:
To print string "Hello, World!" on console
$ echo "Hello, World!"
output:
Hello, World!
Example-2:
To print value of x, where x=10.
$ echo “$x”
output:
10
Example-3:
Use option ‘b‘ – backspace with backslash interpretor ‘-e‘
removes all the spaces in between.
$ echo -e 'Here bthe bspaces bare bbackspaced.'
output:
Herethespacesarebackspaced.

cont..
Example-4:
Use option ‘n‘ – New line with backspace interpretor ‘-e‘ treats
new line from where it is used.
$ echo -e 'Here nthe nspaces nare nnewlined.‘
output:
Here
the
spaces
are
newlined.
printf: An alternate to echo
It format and print data
printf FORMAT [ARGUMENT]...

cont..
$ printf "%dn" 5
5
$ printf "%fn" 5
5.000000
$ printf "There are %d customers with purchases over %d.n" 50 20000
There are 50 customers with purchases over 20000.
who: who are you
The who command prints information about all users who are currently
logged in.
Who
Displays the username, line, and time of all currently logged-in
sessions.

cont..
Passwd: Changing your password
The passwd command changes passwords for user accounts. A normal
user may only change the password for his/her own account, while the
superuser may change the password for any account.
passwd also changes the account or associated password validity
period.
Example-1:
Change your own password:
$ passwd
output:
$ passwd
Changing password for ubuntu.
(current) UNIX password:
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully

cont..
Ls: listing files
ls List information about the FILEs (the current directory by default).
Sort entries alphabetically with uppercase having precedence over
lower..
ls [OPTION]... [FILE]..
To list all files of current directory:
$ ls

cont..
Example:
To display all information about files/directories:
$ ls -l
Example output format of ls -l:
-rw-r--r-- 1 root root 209 Mar 30 17:41 printcap
To display hidden files:
$ ls –a
To display files recursively:
$ ls -R /etc/network
To display file inode number:
$ ls -i /etc/wgetrc

cont..
Combining Commands:
Instead of executing commands on separate lines, where each
command is processed and executed before the next could be
entered, UNIX allows you to specify more than one command in
the single command line.
 Each command has to be separated from the other by a ;
(semicolon).
 wc sample.txt ; ls –l sample.txt
You can even group several commands together so that their
combined output is redirected to a file.
(wc sample.txt ; ls –l sample.txt) > newfile
When a command line contains a semicolon, the shell
understands that the command on each side of it needs to be
processed separately. Here ; is known as a metacharacter.

cont..
A command line can overflow or be split into multiple
lines:
When a command overflows into the next line or needs to be split
into multiple lines, just press enter, so that the secondary prompt
(normally >) is displayed and you can enter the remaining part of
the command on the next line.
Ex:
$ echo “This is a two-line
>text message”
TYPE COMMAND AND LOCATING COMMANDS
You can find the location of an executable program using type
command:
$ type ls
ls is /bin/ls
This means that when you execute ls command, the shell locates
this file in /bin directory and makes arrangements to execute it.

cont..
The Path: Locating Commands
UNIX obtains the list of directories that has to be searched from
of an environment variable – PATH
If you evaluate the value of PATH, you’ll find a directory list
separated by colons:
$ echo $PATH
/bin:/usr/bin:/usr/X11R6/bin:/usr/local/bin:.

INTERNAL AND EXTERNAL COMMANDS
Internal commands:
Some commands are implemented as part of the shell itself
rather than separate executable files.
Such commands that are built-in are called internal commands.
The type command itself is a shell built-in.
 Whether or not you are able to execute it depends on the shell
you use. Ex: cd, source, fg.
External commands:
 These are not built into the shell. These are executables
present in a separate file.
When an external command has to be executed, a new process
has to be spawned and the command gets executed.

Cont..
For example, when you execute the "cat" command, which
usually is at /usr/bin, the executable /usr/bin/cat gets executed.
Since ls is a file having an independent existence in the /bin
directory (or /usr/bin), it is called an external command.
Most commands are external in nature. Ex: ls, cat
$ type cd
cd is a shell builtin
$ type cat
cat is /bin/cat
For the internal commands, the type command will clearly say its
shell built-in, however for the external commands, it gives the
path of the command from where it is executed

Cont..
root: THE SYSTEM ADMINISTRATOR’S LOGIN
The UNIX system provides a special login name for the exclusive
use of the administrator called root. Its password is generally set
at the time of installation.
The prompt of root is #, unlike $ or %used by unprivileged users.
Once you log in as root, you are placed in root’s home directory.
Depending on the system, this directory could be / or /root.root’s
PATH list is different from other users.
It doesn’t contain the current directory, since it’s possible that
root might execute programs of other users’.

Cont..
su COMMAND: AQUIRING SUPERUSER STATUS
su can be used to change the ownership of a session to any user,
it is most commonly employed to change the ownership from an
ordinary user to the root (i.e., administrative) user, thereby
providing access to all parts of and all commands on the
computer or system.
For this reason, it is often referred (although somewhat
inaccurately) as the superuser command. It is also sometimes
called the switch user command
su is also used to change a login session's owner (i.e., the user
who originally created that session by logging on to the system)
without the owner having to first log out of that session.
Syntax
The syntax for the su command is:
su [option] [user] [shell_args]

Cont..
$ su
Password: ****** //root’s password
# pwd
/home/student //Prompt changes, but directory doesn’t.
Creating a User’s Environment:
su, when used with a -, recreates the user’s environment without
taking the root password.
su –student //No password required.
It will create a separate profile for student and runs a separate
sub shell.

Unix - User Administration
There are three types of accounts on a Unix system
Root account: A superuser can run any commands without any
restriction. This user should be assumed as a system
administrator.
System accounts: These accounts are usually needed for some
specific function on your system, and any modifications to them
could adversely affect the system. Eg: mail accounts and sshd
accounts.
User accounts:User accounts provide interactive access to the
system for users and groups of users.
General users are typically assigned to these accounts and
usually have limited access to critical system files and directories.

Cont..
Unix supports a concept of Group Account which logically groups
a number of accounts.
Every account would be a part of another group account. A Unix
group plays important role in handling file permissions and
process management.
groupadd: Adding a group
We need to create groups before creating any account otherwise
we can make use of the existing groups in our system.
following is the syntax to create a new group account −
 groupadd [-g gid [-o]] [-r] [-f] groupname
Following example creates a developers group with default
values, which is very much acceptable for most of the
administrators.
$ groupadd developers

Cont..
S.No. Option & Description
1 -g GID
The numerical value of the group's ID
2 -o
This option permits to add group with non-unique GID
3 -r
This flag instructs groupadd to add a system account
4 -f
This option causes to just exit with success status, if the specified group
already exists. With -g, if the specified GID already exists, other (unique) GID
is chosen
5 Groupname
Actual group name to be created

Cont..
Modify a Group
To modify a group, use the groupmod
To change the developers_2 group name to developer, type −
$ groupmod -n developer developer_2
Delete a Group
To delete an existing group, all you need is the groupdel
command and the group name. To delete the financial group,
the command is −
$ groupdel developer

Unix files
File is a container for storing information. It is a sequence of
characters. UNIX files doesn’t contain the eof(end-of-file)
mark.
 All file attributes are kept in a separate are of the hard disk,
only accessible to kernel.
Basic file types/categories
Ordinary (Regular) File
A large majority of the files found on UNIX system is ordinary
files. Ordinary files contain ASCII (human-readable) text,
executable program binaries, program data, and more..
An ordinary file itself can be divided into two:
Text File
Binary File

Cont..
A text file contains only printable characters and you can
view and edit them. All C and Java program sources, shell
scripts are text files.
Every line of a text file is terminated with the newline
character.
A binary file, on the other hand, contains both printable and
nonprintable characters that cover the entire ASCII range. The
object code and executables that you produce by compiling C
programs are binary files.
Sound and video files are also binary files.

Cont..
Directory File
A directory contains no data, but keeps details of the
files and subdirectories that it contains.
A directory file contains one entry for every file and
subdirectory that it houses. Each entry has two components
namely, the filename
A unique identification number of the file or directory
(called the inode number).
When you create or remove a file, the kernel automatically
updates its corresponding directory by adding or removing
the entry (filename and inode number) associated with the file.

Cont..
Device File
All the operations on the devices are performed by reading
or writing the file representing the device.
 It is advantageous to treat devices as files as some of the
commands used to access an ordinary file can be used with
device files as well.
 Device filenames are found in a single directory structure,
/dev. A device file is not really a stream of characters. It is the
attributes of the file that entirely govern the operation of the
device.
The kernel identifies a device from its attributes and uses
them to operate the device.

Cont..
Special Files:
Links: A link is a tool used for having multiple filenames that
reference a single file on a physical disk. They appear in a file
system just like an ordinary file or a directory.
(Domain) sockets: a special file type, similar to TCP/IP
sockets, providing inter−process networking protected by the
file system's access control.
Named pipes: act more or less like sockets and form a way
for processes to communicate with each other, without using
network socket semantics.

Naming files
On a UNIX system, a filename can consist of up to 255
characters.
 Files may or may not have extensions and can consist of
practically any ASCII character except the / and the Null
character.
You are permitted to use control characters or other
nonprintable characters in a filename.
However, you should avoid using these characters while
naming a file.
It is recommended that only the following characters be used
in filenames:
-Alphabets and numerals.
-The period (.), hyphen (-) and underscore (_).

Cont..
UNIX imposes no restrictions on the extension. In all cases, it
is the application that imposes that restriction.
Eg. A C Compiler expects C program filenames to end with .c,
Oracle requires SQL scripts to have .sql extension.
Hidden Files: have names that begin with a dot (.) For
example:
.cshrc .login .mailrc .mwmrc
Uniqueness: as children in a family, no two files with the
same parent directory can have the same name. Files located
in separate directories can have identical names.

Cont..
A file can have as many dots embedded in its name. A filename
can also begin with or end with a dot.
UNIX is case sensitive; cap01, Chap01 and CHAP01 are three
different filenames that can coexist in the same directory.
Reserved Filenames:
/ - the root directory (slash)
. - current directory (period)
.. - parent directory (double period)
~ - your home directory (tilde)
The HOME variable and home directory
When you log onto the system, UNIX automatically places you in
a directory called the home directory.
The shell variable HOME indicates the home directory of the
user.
E.g.,$ echo $HOME/home/kumar

Cont..
Unix File Organization or Parent Child Relationship in
INUX
Unix organizes files in a treelike hierarchical structure, with the
root directory, indicated by a forward slash (/), at the top of the
tree.
The root directory has a number of subdirectories under it.
These subdirectories in turn have more subdirectories and other
files under them.
For instance, bin and usr are two directories under root. Every
file apart from root must have a parent. In the parent child
relationship, the parent is always a directory.

Cont..
The unix file system can be grouped into two categories:
The files available at installation:
/bin and /usr/bin: Common programs, shared by the system, the
system administrator and the users.
/sbin and /usr/sbin- Programs for use by the system and the
system administrator
/dev -Contains references to all the CPU peripheral hardware,
which are represented as files with special properties.
/etc-Most important system configuration files are in /etc, this
directory contains data similar to those in the Control Panel in
Windows

Cont..
/lib and /usr/lib-Library files, includes files for all kinds of
programs needed by the system and the users.
/usr/share/man-Man files
/usr- Programs, libraries, documentation etc. for all user-related
programs
The temporary files:
/tmp- Temporary space for use by the system, cleaned upon
reboot, so don't use this for saving any work!
/var-Storage for all variable files and temporary files created by
users, such as log files, the mail queue, the print spooler area,
space for temporary storage of files downloaded from the internet,
or to keep an image of a CD before burning it.
/home-Home directories of the common users.

Absolute and relative paths
Absolute Path Name
An absolute pathname traverses the file system hierarchy tree
from the very top, always starting at the topmost ROOT directory
of the file system hierarchy.
The topmost root directory is signaled by the leading “slash”
character (/) at the start of an absolute pathname.
Eg: /home/kumar/login.sql
No two files in a UNIX system can have identical absolute path
names. You can have two files with the same name, but in
different directories; their pathnames will also be different.
Thus, the files /home/kumar/progs/c2f.pl can coexist with
/home/kumar/safe/c2f.pl.

Cont..
Using absolute path names for a command
If you execute programs residing in some other directory that is
not in PATH, then you need to specify the absolute path name.
For example, to execute the command less residing in
/usr/local/bin you need to enter the absolute path name
/usr/local/bin/less
Relative pathname
A relative pathname uses the current directory as point of
reference and specifies the path relative to it.
Eg: progs/scripts

Cont..
Using . and .. in relative path name
. (Single dot) represents the current directory
.. (Double dot) represents the parent directory
cd .. ---Moves one level up
cd../.. ---Moves two levels up
$ pwd
/home/kumar/ progs
$ cd ..
$ pwd
/home/kumar
$ pwd
/home/kumar/ progs
$ cd ../..
$ pwd
/home

Cont..
A filename can begin with a dot
cp ../sharma/.profile
To refer to a file in the parent directory of the current directory,
use “../” followed by the name of the file.
Any command that used current directory as argument can also
work with a single dot.
Eg: cp ../sharma/.profile .
This copies the file .profile to the current directory (.)

Directory Commands
pwd - print working directory
At any time you can determine where you are in the file system
hierarchy with the pwd, print working directory, command,
E.g.,:
$ pwd
/home/student/src
cd - change directory
You can change to a new directory with the cd, change directory,
command. cd will accept both absolute and relative path names.
Syntax: cd [directory]
cd changes to user's home directory
cd / changes directory to the system's root
cd .. goes up one directory level
cd ../.. goes up two directory levels

Cont..
cd /full/path/name/from/root changes directory to
absolute path named
cd path/from/current/location changes directory to
path relative to current location
Examples
$ pwd
/home/kumar
$ cd progs //progs must be in current directory
$ pwd
/home/kumar/progs

Cont..
mkdir - make a directory
You extend your home hierarchy by making sub-directories
underneath it. This is done with the mkdir , make directory,
command.
You can specify either the full or relative path of the directory.
Syntax: mkdir directoryname
Examples
mkdir patch
Creates a directory patch under current directory
mkdir patch dbs doc
Creates three directories under current directory

Cont..
The system may refuse to create a directory due to the following
reasons:
1. The directory already exists.
2. There may be an ordinary file by the same name in the current
directory.
3. The permissions set for the current directory don’t permit the
creation of files and directories by the user.
rmdir - remove directory
A directory needs to be empty before you can remove it. If it’s
not, you need to remove the files first. Also, you can’t remove a
directory if it is your present working directory; you must first
change out of that directory.

Cont..
You cannot remove a subdirectory unless you are placed in a
directory which is hierarchically above the one you have chosen
to remove.
Examples
rmdir patch
Delete the directory called patch. Directory must be empty
rmdir pis pis/progs pis/data
Shows error as pisnis not empty. However rmdir silently deletes
the lower level subdirectories Progs and data. The correct order
for deleting subdirectory is:
rmdir pis/data pis/progs pis

The PATH environment variable
The PATH environment variable:
Environmental variables are used to provide information to the
programs you use.
A command runs in UNIX by executing a disk file. When you
specify a command like date , the system will locate the
associated file from a list of directories specified in the PATH
variable and then executes it.
The PATH variable normally includes the current directory also.
Whenever you enter any UNIX command, you are actually
specifying the name of an executable file located somewhere on
the system.

Cont..
The system goes through the following steps in order to
determine which program to execute:
1.Built in commands (such as cd and history) are executed within
the shell.
2. If an absolute path name (such as /bin/ls) or a relative path
name (such as ./myprog), the system executes the program
from the specified directory.
3. Otherwise the PATH variable is used.

FILE RELATED COMMANDS
cat: displaying and creating files
Cat command is used to display the contents of a small file on the
terminal.
Eg:
$ cat cprogram.c
# include <stdioh>
void main ()
{
Printf(“hello”);
}
As like other files cat accepts more than one filename as
arguments
$ cat ch1 ch2
It contains the contents of chapter1
It contains the contents of chapter2

Cont..
In this the contents of the second files are shown immediately
after the first file without any header information. So cat
concatenates two files- hence its name.
cat options
Displaying Nonprinting Characters (-v)
Nonprinting ASCII characters can be displayed with –v option.
Numbering Lines (-n)
-n option numbers lines. This numbering option helps
programmer in debugging programs.
Using cat to create a file
Cat is also useful for creating a file. Enter the command cat,
followed by > character and the filename.
Eg:
$ cat > new
This is a new file which contains some text, just to
Add some contents to the file new
[ctrl-d]
$_

Cont..
cp: COPYING A File
The cp command copies a file or a group of files. It creates an
exact image of the file on the disk with a different name.
The syntax takes two filename to be specified in the command
line.
When both are ordinary files, first file is copied to second.
$ cp csa csb
If the destination file (csb) doesn’t exist, it will first be created
before copying takes place .If not it will simply be overwritten
without any warning from the system.
To copy a file ‘new’ from /home/user1 to your current directory,
use the following command:
cp /home/user1/new . - destination is the current
directory

Cont..
cp command can be used to copy more than one file with a
single invocation of the command.
In this case the last filename must be a directory.
Ex: To copy the file ch1,chh2,ch3 to the module , use cp as
$ cp ch1 ch2 ch3 module
Interactive Copying(-i) :
The –I option warns the user before overwriting the destination
file,
If unit 1 exists, cp prompts for response
$ cp -i ch1 unit1
$ cp: overwrite unit1 (yes/no)? Y
A y at this prompt overwrites the file, any other response leaves
it uncopied.

Cont..
Copying directory structure (-R) :
It performs recursive behavior command can descend a directory
and examine all files in its subdirectories.
-R : behaves recursively to copy an entire directory structure
$ cp -R usp newusp
$ cp -R class newclass
If the newclass/newuspdoesn’t exist, cp creates it along with the
associated subdirectories.
rm: deleting files
The rm command deletes one or more files.
Ex: Following command deletes three files:
$ rm mod1 mod2 mod3
Can remove two chapters from usp directory without having to cd
Ex: rm usp/marks ds/marks

Cont..
To remove all file in a directory use *
$ rm *
Removes all files from that directory
Interactive Deletion (-i) :
Ask the user confirmation before removing each file:
$ rm -i ch1 ch2
rm: remove ch1 (yes/no)? ? y
rm: remove ch1 (yes/no)? ? n [Enter]
A ‘y’ removes the file (ch1) any other response like n or any other
key leave the file undeleted.
Recursive deletion (-r or -R):
It performs a recursive search for all directories and files within
these subdirectories. At each stage it deletes everything it finds.
$ rm -r *
Works as rmdir It deletes all files in the current directory and all
its subdirectories.

Cont..
Forcing Removal (-f):
rm prompts for removal if a file is write-protected.
The –f option soverrides this minor protection and forces removal.
$rm -rf*
mv: renaming files
The mv command renames (moves) files. The main two functions
are:
It renames a file(or directory)
It moves a group of files to different directory
It doesn't create a copy of the file; it merely renames it. No
additional space is consumed on disk during renaming.
Ex: To rename the file csb as csa we can use the following
command
$ mv csb csa
If the destination file doesn’t exist in the current directory, it will
be created. Or else it will just rename the specified file in mv
command.

Cont..
A group of files can be moved to a directory.
Ex: Moves three files ch1,ch2,ch3 to the directory module
$mv ch1 ch2 ch3 module
Can also used to rename directory
$ mv rename newname
mv replaces the filename in the existing directory entry with the
new name. It doesn't create a copy of the file; it renames it
more : paging output
To view the file ch1, we can use more command along with the
filename, it is used for display
^d used as an interrupt key
^e indicates the end of file
It displays the contents of ch1 on the screen, one page at a time.
If the file contents is more it will show the filename and
percentage of the file that has been viewed:
----More--- (15%)

Cont..
Navigation
f or Spacebar: to scroll forward a page at a time
b to move back one page
The repeat features
The repeat factor: We can use the repeat factors with the
navigation keys Use the repeat factor as a command prefix simply
repeats the command that many times.
Use 10f for scrolling forward 10 pages and 5b for scrolling back 5
pages
wc: counting lines,words and characters
wc command performs Word counting including counting of lines
and characters in a specified file. It takes one or more filename as
arguments and displays a four columnar output.
$ wc ofile
4 20 97 ofile

Cont..
Line: Any group of characters not containing a newline
Word: group of characters not containing a space, tab or newline
Character: smallest unit of information, and includes a space, tab
and newline
wc offers 3 options to make a specific count.
–l option counts only number of lines, -w and –c options count
words and characters, respectively.
$ wc -l ofile
4 ofile
$ wc -w ofile
20 ofile
Multiple filenames,
wc produces a line for each file, as well as a total count.
$ wc -c ofile file

Cont..
97 ofile
15 file
112 total
od: displaying data in octal
od command displays the contents of executable files in a ASCII
octal value.
-b option, displays this value for each character separately.
Each line displays 16 bytes of data in octal, preceded by the offset
in the file of the first byte in the line.
$ od –b file
o0000000 164 150 151 163 040 146 151 154 145 040 151 163
040 141 156 040
0000020 145 170 141 155 160 154 145 040 146 157 162 040
157 144 040 143

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