This document provides an introduction to shell programming, detailing the nature of a shell as an interface to access operating system services and how shell scripts are created for automation. It covers essential concepts such as shell variables, their definitions, and some simple shell programs demonstrating basic operations. Additionally, the document explains wildcards used in shell scripting, including types like star, question mark, and square bracket wildcards for efficient searches.

1. Submitted by: Gaurav Bisht
B.Tech (CSE) 6th Sem

2. Contents:
 What is Shell?
 Brief introduction to Shell Programming
 What are Shell Variables?
 Some Simple Shell Programs
 Wildcards

3. What is Shell?
 A shell is a computer program which works as the interface to
access the services provided by an OS.
 A shell is a command-line interpreter.
 It performs operations such as file manipulation, program
execution and printing text.

4. Introduction to Shell Programming:
 Shell programming is a computer program designed to be run by UNIX/ LINUX shell.
 We write shell scripts to avoid repetitive work and automation.
 A shell script comprises of Shell Keywords, Shell Variables, Shell Commands, functions and
Control Flow Statements.
 Shell script is saved with ’.sh’ file extension for example ’myscript.sh’.

5. Shell Variables:
 A variable is a character string to which we assign a value.
 The value assigned could be a number, text, filename, device, or any other type of data.
 The shell enables you to create, assign, and delete variables.
 Variables have naming convention such as the name of a variable can contain only letters (a to z or A
to Z), numbers ( 0 to 9) or the underscore character ( _).
‘_VAR’ ‘TOKEN_A’ ‘VAR_1’ ‘VAR_2’ These are valid variable names.
‘2_VAR’ ‘-VARIABLE’ ‘VAR1-VAR2’ ‘VAR_A!’ These are invalid variable names.

6. Shell Variables (Cont.):
 Defining Variables:
SYNTAX – For example –
variable_name = variable_value; NAME = “Gaurav Bisht”
ROLLNO = 7
 Accessing Variables:
INPUT – OUTPUT –
NAME = “Gaurav Bisht” Gaurav Bisht
echo $NAME

7. Shell Variables (Cont.):
 Variables in shell do not use special characters like ‘!’, ‘#’, ‘?’ or ‘$’ since there exists predefined
special variables in shell scripting.
S. No. Special
Variable
Description Input Output
1. $0 Display file name of current script echo “File Name: $0” File Name: ./test.sh
2. $# Display number of arguments
supported to a script
echo “Total no. of
parameters: $#”
Total no. of parameters: 2
3. $? The exit status of the last command
executed
echo $? 0
4. $$ The process number or the Process ID
(PID) of the current shell
echo $$ 29949
5. $! The process number of the last
background command
echo $! 1342

8. Some Simple Shell Programs:
 Program 1 – Adding 2 numbers.
echo "Enter first number:"
read x
echo "Enter second number:"
read y
sum=x+y
echo "The result of addition =$sum"

9. Some Simple Shell Programs (Cont.):
 Program 2 – Concatenation of 2 strings.
str1=“Hello”
str2=“World”
str3=“$str1 $str2”
echo $str3

10. Some Simple Shell Programs (Cont.):
 Program 3 – Comparison of 2 strings.
read -p "Enter first string: " VAR1
read -p "Enter second string: " VAR2
if [[ "$VAR1" == "$VAR2" ]]
then
echo "Strings are equal."
else
echo "Strings are not equal."
fi

11. Some Simple Shell Programs (Cont.):
 Program 4 – Maximum of 3 numbers.
echo " Enter A value: "
read a
echo " Enter B value: "
read b
echo " Enter C value: "
read c
if [ $a -gt $b ] && [ $a -gt $c ]
then
echo "A value is greater"
elif [ $b -gt $a ] && [ $b -gt $c ]
then
echo "B value is greatest "
else
echo "C value is greatest "
fi

12. Wildcards:
 A wildcard is a character that can be used as a substitute for any of a class of characters in a
search, thereby greatly increasing the flexibility and efficiency of searches.
 There are 3 types of wildcards:
1. Star Wildcard (*)
2. Question mark Wildcard (?)
3. Square Bracket Wildcard ([])

13. Wildcards (Cont.):
 Star Wildcard –
- It is most frequently used and most useful wildcard in shell programming.
- The star wildcard has the broadest meaning of any of the wildcards, as it can represent zero
characters, all single characters or any string.
For example –
- file * => It will display all the files present in the directory along with their type.
- ls *.txt => It will list all the files with extension ‘.txt’.

14. Wildcards (Cont.):
 Question mark Wildcard –
- The question mark wildcard is used to represent exactly one character, which can be any
single character.
- Two question marks in succession would represent any two characters .
- Three question marks in succession would represent any string consisting of three
characters.
For example –
- file ??? => It will display data in the present directory whose name is exactly 3 characters in length
irrespective if the extension.
- file ? ?? ??? => It will display data on all objects whose names are one, two or three characters in
length

15. Wildcards (Cont.):
 Square Bracket Wildcard –
- The square bracket wildcard is used to represent any of the characters enclosed in the
square brackets.
For example –
- file *[xyz]* => It will display information about all objects in the current directory that have an x, y or z
in them.
- ls *.[xyz]* => It will list all files that had an extension that begins with x, y or z.