Unix Shell Programming

Ability Enhancement Course

AEC410
ABILITY ENHANCEMENT COURSE ASSESSMENT :

• The assessment of the Ability Enhancement Course shall comprise of

Continuous Internal Evaluation (CIE) of 50 marks and a Semester End
Examination (SEE) of 50 marks.

• The CIE will comprise two tests of 30 marks each having Part-A with MCQs
and Part-B with descriptive Questions. Another 20 marks of CIE will be
assessed through quizzes, Assignments, Presentations, etc.

• The SEE will comprise Part-A with MCQs and Part-B with descriptive
Questions
Unit 1
• The architecture of UNIX
• Features of UNIX
• Introduction to UNIX file system
• vi editor
• File handling utilities and security by file permissions
• Basic UNIX commands (PATH, man, echo, printf, script, passwd, uname,
who, date, stty, pwd, cd, mkdir, rmdir, ls, cp, mv, rm, cat, more, wc, lp,
od, tar, gzip).
What is Unix?
• The Unix operating system is a set of programs that act as a link between the computer and
the user.

• The computer programs that allocate the system resources and coordinate all the details of
the computer's internals are called the operating system or the kernel.

• 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.

• Unix was originally developed in 1969 by a group of AT&T employees Ken Thompson,
Dennis Ritchie, Douglas McIlroy, and Joe Ossanna at Bell Labs.

• 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
• Kernel: The kernel is the heart of the operating system. It interacts with the
hardware and performs most of the tasks like memory management, task
scheduling, and file management.
It contains two basic parts of the OS:
Process Control and Resource Management: It performs the following tasks
 Process Management
 File Management
 Device Management
 I/O Management
 Memory Management
• Shell: The shell is the utility that processes your requests. When you type in a
command at your terminal, the shell interprets the command and calls the
program that you want. 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.
Unix Architecture
• Commands and Utilities: There are various commands and utilities which
you can make use of in your day-to-day activities. cp, mv, cat and grep,
etc. are a few examples of commands and utilities. There are over 250
standard commands plus numerous others provided through 3rd party
software. All the commands come along with various options.
• Files and Directories: 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.
• *Utility: is a standard UNIX program that provides a support process for
users. Three common utilities are Text editors, Search Programs, and sort
programs.
Features:

• Portable
• Multiuser
• Multitasking
• Networking
• Organized File System
• Device Independence
• Utilities
• Services
Introduction to UNIX File System:
• Filenames:

• Length:14 -255 characters

• Avoid special characters : > or < must not be used

• Use Dividers: _ ,. ,-

• Use Extensions

• Never start a filename with a period. Filenames that start with a period are hidden
files in UNIX.
Wildcards:
• ?: Any Single character.
• […]: Single character from the set
• *: zero or more characters
Ex: c? c?t c??t ?a?
Matches: c1,ca,cat,c12t, bat, car etc….
EX: f[aoei]d f[a-d]t f[A-z][0-9] f[A-Za-z][0-9]
Matches: fad, fod, fed, fid, fat, fa3, f^2 etc….
Does not match: fud, faa….
* : every file
f*: all files which begins with f
*f : all files which ends in f
*.* every file whose name has a period.
Wildcards with echo command:
$ echo f?t
f1t fat fbt fgt fwt
File types
Listing Files

• To list the files and directories stored in the current directory, use the
following command:
• $ls
• The command ls supports the -l option which would help you to get more
information about the listed files –
• $ls –l
 In the ls -l listing example, every file line begins with a d, -, or l. These
characters indicate the type of file that's listed

First Column: Represents the file type and the permission given on the file.
Second Column: Represents the number of memory blocks taken by the file or directory.
Third Column: Represents the owner of the file. This is the Unix user who created this file.
Fourth Column: Represents the group of the owner. Every Unix user will have an associated group.
Fifth Column: Represents the file size in bytes.
Sixth Column: Represents the date and the time when this file was created or modified for the last time.
Seventh Column: Represents the file or the directory name.
Hidden Files
An invisible file is one, the first character of which
is the dot or the period character (.). Unix
programs (including the shell) use most of these
files to store configuration information.
Some common examples of hidden files include
the files −
.profile − The Bourne shell ( sh) initialization script
.kshrc − The Korn shell ( ksh) initialization script
.cshrc − The C shell ( csh) initialization script
.rhosts − The remote shell configuration file

To list the invisible files, specify the -a option to ls –

Display Content of a File
• Display contents of the file:

• Display the contents with the line number

• wc command to get a count of the total number of lines,

words, and characters contained in a file.
Copying Files
To make a copy of a file use the cp command. The basic syntax of the
command is −

Renaming Files
To change the name of a file, use the mv command. Following is the
basic syntax

Deleting Files
vi Editors
• An editor is a utility that facilitates the editing task- creation /Modification of
text files.

• Editors are of 2 types:

• Line Editors
• Screen Editors

• Line Editors: changes are applied to a line or a group of lines. They are useful
when you want to make global changes over a group of lines. Ex: if extra space
is to be added at the start of the line, all the lines can be selected at once,
instead of adding to each line.
• Screen Editors: Presents a whole screen of text at a time. Each line of the text in
context with other lines can be viewed at once.
Starting the vi Editor
 2 Modes:
• Command mode − This mode enables you to
perform administrative tasks such as saving
files, executing commands, moving the cursor,
cutting and pasting lines or words, and finding
and replacing. In this mode, whatever you type
is interpreted as a command.
• Insert mode − This mode enables you to insert
text into the file. Everything that's typed in this
mode is interpreted as input and finally, it is put
in the file.
You can move around the file with the
arrow keys, or use h - j - k -
l keys. h-l for left-right, j-k for
down-up.

Once you are done editing you can

press the esc key to exit insert mode,
and go back to command mode.
Editing Files
• To edit the file, you need to be in the insert mode. There are
many ways to enter insert mode from the command mode −
Deleting Characters
Vi save and Exit Command

• :w - saves the contents of the buffer without quitting vi

• :w filename – Write contents of buffer to new file and continues.
• ZZ – saves the contents of the buffer and exit
• :wq - saves the contents of the buffer and exit
• :q - Exit from vi if only buffer is unchanged
• :q! – Exit from vi without saving.
Adding text

Deleting text

Moving
Cursor

Join
Undo

Scrolling

Saving
and
Quitting
Basic UNIX Commands
PATH
• PATH is an environment variable used by the shell to determine
where to look for executable programs.

• PATH is interpreted as a list of directory names separated by a colon.

When any command is typed on the command line, the SHELL looks
up into each PATH directory in the list in turn until the executable file
with the command name is encountered

• $PATH
 man
• man command in Linux
is used to display the
user manual of any
command that we can
run on the terminal
 echo
• Echo is a Unix/Linux
command tool used
for displaying lines
of text or string
which are passed
as arguments on
the command line.
This is one of the
basic command in
linux and most
commonly used in
shell scripts.
printf
 In Unix and Unix-like operating systems, printf is a shell builtin that formats and prints data
 %s specifier: It is basically a string specifier for string output.
 %b specifier: It is same as string specifier but it allows us to interpret escape sequences with
an argument.
 %d specifier: It is an integer specifier for showing the integral values
 %f specifier: It is used for output of floating point values.
script
• script command in Linux is used to make typescript or record all the
terminal activities. After executing the script command it starts
recording everything printed on the screen including the inputs and
outputs until exit. By default, all the terminal information is saved
in the file typescript, if no argument is given. The script is mostly
used when we want to capture the output of a command or a set
of commands while installing a program or the logs generated on
the terminal while compiling open source codes,
etc. script command uses two files i.e. one for the terminal output
and other for the timing information.
passwd
• The passwd command changes passwords for user accounts. A
normal user may only change the password for their own account,
while the superuser may change the password for any account.
passwd also changes the account or associated password validity
period.
• Options
• passwd -S <username>
• The -S option displays the status of user account password settings.

• For example:
• # passwd -S evans
• evans PS 2020-09-07 0 99999 7 -1 (Password set, SHA512 crypt.)
• The output above shows the account evans was created on 7th
September 2020 and has a password set with SHA512 encryption.
• passwd -u <username>
This option will unlock the password. This option works for an account
that already has the password locked.
$passwd -u user2
• passwd -l <username>
$passwd -l user1
• passwd -d <username>
This is a quick way to delete a password for an account.
• passwd -e <username>
This is a quick way to expire a password for an account. The user will
be forced to change the password during the next login attempt.
uname
• The command ‘uname‘ displays the information about the system.
• uname [OPTION]

1. -a option: It prints all the system information in the following order: Kernel
name, network node hostname, kernel release date, kernel version, machine
hardware name, hardware platform, operating system
 who
• The standard Unix command who displays a list of users who
are currently logged into the computer.

• Who -m -H (Command to display the hostname and user associated with the
input/output devices)
• who -a (To display all details of currently logged in users )
• Who -all
• who -p -h (all active processes that are spawned by the NIT process)
• who -d -H (complete list of all dead processes)
date
• date command is
used to display the Options with Examples:
system date and
time. date 1. $date: current date and time, including the abbreviated day name,
command is also
used to set date and abbreviated month name, day of the month, the time separated by
time of the system. colons, the time zone name, and the year.
By default the date
command displays Tue Oct 10 22:55:01 PDT 2017
the date in the time
zone on which 2. -u Option: Displays the time in GMT(Greenwich Mean
Unix/linux operating
system is Time)/UTC(Coordinated Universal Time )time zone.
configured. You
must be the super- 3. Using –date option for displaying past dates:
user (root) to $date --date="2 year ago"
change the date and
time.
tty and stty
• tty command writes the name of your terminal to standard output
• stty displays or changes the characteristics of the terminal.
pwd
• The pwd command writes to standard output the full path name of
your current directory (from the root directory). All directories are
separated by a / (slash). The root directory is represented by the first
/, and the last directory name is your current directory.

• $pwd
• (Print Working Directory)
cd
• cd command known as change directory command.
• cd /: this command is used to change directory to the root directory,
The root directory is the first directory in your file system hierarchy.
• cd ~ : this command is used to change directory to the home directory.
• cd .. : this command is used to move to the parent directory of current
directory, or the directory one level up from the current directory. “..”
represents parent directory.
mkdir
• mkdir command allows the user to create directories (also referred to
as folders in some operating systems ). This command can create
multiple directories at once as well as set the permissions for the
directories. It is important to note that the user executing this
command must have enough permissions to create a directory in the
parent directory, or he/she may receive a “permission denied” error.
more
• more command is used to view the text files in the command prompt,
displaying one screen at a time in case the file is large (For example log files).
The more command also allows the user do scroll up and down through the
page.
• more [-options] [-num] [+/pattern] [+linenum] [file_name]
• [-options]: any option that you want to use in order to change the way the file is
displayed. Choose any one from the followings: (-d, -l, -f, -p, -c, -s, -u)
• [-num]: type the number of lines that you want to display per screen.
• [+/pattern]: replace the pattern with any string that you want to find in the text
file.
• [+linenum]: use the line number from where you want to start displaying the
text content.
• [file_name]: name of the file containing the text that you want to display on the
screen.
Options:
• -d : Use this command in order to help the user to navigate. It displays
“[Press space to continue, ‘q’ to quit.]” and displays “[Press ‘h’ for
instructions.]” when wrong key is pressed.
• -f : This option does not wrap the long lines and displays them as such.
• -p : This option clears the screen and then displays the text.
• -c : This command is used to display the pages on the same area by
overlapping the previously displayed text.
• -s : This option squeezes multiple blank lines into one single blank line.
• -u : This option omits the underlines.
rmdir: Removes the directory from the system. The directory must be empty
before you can remove it.
Use the ls -al command to check whether the directory is empty.
ls: Used to list files and directories. The contents of your current working directory.
cp: This command is used to copy files or groups of files or directories. It creates
an exact image of a file on a disk.
mv: Moves files and directories from one directory to another or renames a file or
directory.
rm: Use the rm command to remove files you no longer need. The rm command
removes the entries for a specified file, group of files, or certain select files from a
list within a directory.
cat: To print the content of a file onto the standard output. concatenation of files
which combines multiple files into a single file.
wc
• wc stands for word count. As the name implies, it is mainly used for counting purpose. It is used
to find out number of lines, word count, byte and characters count in the files specified in the
file arguments.
• By default it displays four-columnar output.
• First column shows number of lines present in a file specified, second column shows number of
words present in the file, third column shows number of characters present in file and fourth
column itself is the file name which are given as argument.
• Syntax: wc [OPTION]... [FILE]...
• -l: This option prints the number of lines present in a file. With this option wc command
displays two-columnar output, 1st column shows number of lines present in a file and 2nd itself
represent the file name.
• -w: This option prints the number of words present in a file. With this option wc command
displays two-columnar output, 1st column shows number of words present in a file and 2nd is
the file name.
• -c: This option displays count of bytes present in a file. With this option it display two-columnar
output, 1st column shows number of bytes present in a file and 2nd is the file name.
• -m: Using -m option ‘wc’ command displays count of characters from a file.
• lp: submits files for printing or alters a pending job..
Tag Description
-- Marks the end of options; use this to print a file whose name begins with a dash (-).
-E Forces encryption when connecting to the server.
-U username Specifies the username to use when connecting to the server.

This option is provided for backwards-compatibility only. On systems that support it, this option forces the print file to be copied to
-C
the spool directory before printing. In CUPS, print files are always sent to the scheduler via IPP which has the same effect.

-d destination Prints files to the destination printer.

-h hostname[:port] Chooses an alternate server.
-i job-id Specifies an existing job to modify.
-m Sends an email when the job is completed.
-n copies Sets the number of copies to print from 1 to 100.

-o "name=value [name=value ...]" Sets one or more job options.

-q priority Sets the job priority from 1 (lowest) to 100 (highest). The default priority is 50.
-s Do not report the resulting job IDs.
-t "name" Sets the job name.

Specifies when the job should be printed. A value of immediate will print the file immediately, a value of hold will hold the job
-H hh:mm-H hold-H immediate-H restart-H
indefinitely, and a UTC time value (HH:MM) will hold the job until the specified UTC (not local) time. Use a value of resume with the -i
resume
option to resume a held job. Use a value of restart with the -i option to restart a completed job.

Specifies which pages to print in the document. The list can contain a list of numbers and ranges (#-#) separated by commas (e.g. 1,3-
-P page-list 5,16). The page numbers refer to the output pages and not the document's original pages - options like "number-up" can affect the
numbering of the pages.
od
• od command in Linux is used to convert the content of input in different
formats with the octal format as the default format. This command is especially
useful when debugging Linux scripts for unwanted changes or characters. If
multiple files are specified, the od command concatenates them in the listed
order to form the input. It can display output in a variety of other formats,
including hexadecimal, decimal, and ASCII. It is useful for visualizing data that is
not in a human-readable format, like a program’s executable code.
• Syntax: od [OPTION]... [FILE]...
• -b Option: It displays the contents of input in octal format.
• -c Option: It displays the contents of input in character format.
• -An Option: It displays the contents of input in character format but with no
offset
• -A Option: It displays input contents in different formats by concatenating
some special character with -A.
tar
• The Linux ‘tar’ stands for tape archive, which is used to create Archive and extract the Archive files. tar command in Linux is
one of the important commands which provides archiving functionality in Linux. We can use the Linux tar command to
create compressed or uncompressed Archive files and also maintain and modify them.
• Syntax:
• tar [options] [archive-file] [file or directory to be archived]
• Options:
• -c: Creates Archive
• -x: Extract the archive
• -f: creates archive with the given filename
• -t: displays or lists files in the archived file
• -u: archives and adds to an existing archive file
• -v: Displays Verbose Information
• -A: Concatenates the archive files
• -z: zip, tells tar command that creates tar file using gzip
• -j: filter archive tar file using tbzip
• -W: Verify an archive file
• -r: update or add file or directory in an already existing .tar file
 gzip

• gzip command compresses files. Each single file is compressed into a single file. The
compressed file consists of a GNU zip header and deflated data.
If given a file as an argument, gzip compresses the file, adds a “.gz” suffix, and deletes
the original file. With no arguments, gzip compresses the standard input and writes
the compressed file to standard output.
File Permissions
Every file in Unix has the following attributes –

• Owner permissions − The owner's permissions determine what actions

the owner of the file can perform on the file.
• Group permissions − The group's permissions determine what actions a
user, who is a member of the group that a file belongs to, can perform
on the file.
• Other (world) permissions − The permissions for others indicate what
action all other users can perform on the file.
The Permission Indicators
• While using ls -l command, it displays various information related to file
permission as follows :

• The permissions are broken into groups of threes, and each position in the
group denotes a specific permission, in this order: read (r), write (w), execute
(x) −
• The first three characters (2-4) represent the permissions for the file's owner.
For example, -rwxr-xr-- represents that the owner has read (r), write (w) and
execute (x) permission.
• The second group of three characters (5-7) consists of the permissions for the
group to which the file belongs. For example, -rwxr-xr-- represents that the
group has read (r) and execute (x) permission, but no write permission.
• The last group of three characters (8-10) represents the permissions for
everyone else. For example, -rwxr-xr-- represents that there is read (r) only
permission.
File Access Modes
• The permissions of a file are the first line of defense in the security of
a Unix system. The basic building blocks of Unix permissions are the
read, write, and execute permissions, which have been described
below −
• Read Grants the capability to read, i.e., view the contents of the file.
• Write Grants the capability to modify, or remove the content of the
file.
• Execute Users with execute permissions can run a file as a program.
Directory Access Modes
• Directory access modes are listed and organized in the same
manner as any other file. There are a few differences that need to
be mentioned:
• Read Access to a directory means that the user can read the
contents. The user can look at the filenames inside the directory.
• Write Access means that the user can add or delete files from the
directory.
• Execute Executing a directory doesn't really make sense, so think of
this as a traverse permission. A user must have execute access to
the bin directory in order to execute the ls or the cd command.
Changing Permissions
• To change the file or the directory permissions, you use the chmod
(change mode) command. There are two ways to use chmod — the
symbolic mode and the absolute mode.
• Using chmod in: The easiest way for a beginner to modify file or
directory permissions is to use the symbolic mode. With symbolic
permissions you can add, delete, or specify the permission set you
want by using the operators in the following table. Symbolic Mode
Then each example chmod command from the preceding table is run on the testfile, followed by
ls –l, so you can see the permission changes –
Using chmod with Absolute mode
• The second way to modify permissions with the chmod command is to
use a number to specify each set of permissions for the file. Each
permission is assigned a value, as the following table shows, and the
total of each set of permissions provides a number for that set
Changing Owners and Groups
While creating an account on Unix, it assigns a owner ID and a group ID to
each user. All the permissions mentioned above are also assigned based
on the Owner and the Groups.
Two commands are available to change the owner and the group of files −
chown − The chown command stands for "change owner" and is used to
change the owner of a file.
chgrp − The chgrp command stands for "change group" and is used to
change the group of a file
Write the commands for :
• Rahul wants to keep documentation of the work he does on the
command prompt.
• Delete the password of the system.
• Seema wants to know the version of the kernel of the system she is
working.
• Ram wants to know the users currently logged in
• Change the input terminal
• Count the number of lines in the file
• Print the file and send the message when the job is done.
• Compress the file