Conditional Statements and Operators in Shell Scripting

Table of Contents

1. Introduction to Conditional Statements
2. Types of Conditional Statements

• If Statement
• If-Else Statement
• If-Else If Statement
• Case Statement

1. Operators in Shell Scripting

• Comparison Operators
• Logical Operators
• String Operators

1. Practical Examples
2. Conclusion

1. Introduction to Conditional StatementsConditional statements allow a script to execute specific blocks of code based on whether certain conditions are true or false. By using these statements, you can control the behavior of your script in response to different scenarios, making your scripts more powerful and flexible.2. Types of Conditional Statements

2.1. If Statement

The simplest form of a conditional statement is the if statement. It executes a block of code if the specified condition is true.Syntax:bash
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if [ condition ]; then
# commands to execute if condition is true
fi
Example:bash
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#!/bin/bash
num=10
if [ $num -gt 5 ]; then
echo "$num is greater than 5"
fi

2.2. If-Else Statement

The if-else statement allows you to define an alternative action when the condition is false.Syntax:bash
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if [ condition ]; then
# commands if condition is true
else
# commands if condition is false
fi
Example:bash
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#!/bin/bash
num=3
if [ $num -gt 5 ]; then
echo "$num is greater than 5"
else
echo "$num is not greater than 5"
fi

2.3. If-Else If Statement

For multiple conditions, you can use if-else if statements.Syntax:bash
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if [ condition1 ]; then
# commands if condition1 is true
elif [ condition2 ]; then
# commands if condition2 is true
else
# commands if all conditions are false
fi
Example:bash
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#!/bin/bash
num=5
if [ $num -gt 5 ]; then
echo "$num is greater than 5"
elif [ $num -eq 5 ]; then
echo "$num is equal to 5"
else
echo "$num is less than 5"
fi

2.4. Case Statement

The case statement is a multi-way branch statement that allows you to execute code based on the value of a variable.Syntax:bash
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case variable in
pattern1)
# commands for pattern1
;;
pattern2)
# commands for pattern2
;;
*)
# default commands
;;
esac
Example:bash
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#!/bin/bash
read -p "Enter a number (1-3): " num
case $num in
1)
echo "You selected one"
;;
2)
echo "You selected two"
;;
3)
echo "You selected three"
;;
*)
echo "Invalid selection"
;;
esac
3. Operators in Shell ScriptingOperators are used to perform operations on variables and values. Below are some of the essential operators used in conditional statements.

3.1. Comparison Operators

These operators compare numeric values.OperatorDescription-eqEqual to-neNot equal to-gtGreater than-ltLess than-geGreater than or equal to-leLess than or equal toExample:bash
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#!/bin/bash
a=5
b=10
if [ $a -lt $b ]; then
echo "$a is less than $b"
fi

3.2. Logical Operators

Logical operators are used to combine multiple conditions.OperatorDescription-aLogical AND-oLogical OR!Logical NOTExample:bash
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#!/bin/bash
num=7
if [ $num -gt 5 -a $num -lt 10 ]; then
echo "$num is between 5 and 10"
fi

3.3. String Operators

String operators are used to compare strings.OperatorDescription=Equal to!=Not equal to-zString is null (length is zero)-nString is not null (length is non-zero)Example:bash
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#!/bin/bash
string1="Hello"
string2="World"
if [ "$string1" != "$string2" ]; then
echo "$string1 and $string2 are not equal"
fi
4. Practical Examples

Example 1: Number Guessing Game

bash
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#!/bin/bash
secret_number=42
read -p "Guess the secret number: " guess

if [ $guess -eq $secret_number ]; then
echo "Congratulations! You guessed it right."
else
echo "Try again!"
fi

Example 2: File Existence Check

bash
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#!/bin/bash
filename="example.txt"

if [ -e "$filename" ]; then
echo "$filename exists."
else
echo "$filename does not exist."
fi
5. ConclusionConditional statements and operators are vital for controlling the flow of execution in shell scripting. By understanding how to use if statements, case statements, and various operators, you can create powerful scripts that automate tasks, handle user input, and respond to different conditions effectively.Mastering these concepts will greatly enhance your Bash scripting capabilities, enabling you to write more complex and useful scripts for various applications in system administration, data processing, and more.