Shell Scripting Decision Making

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In shell scripting, you can implement decision-making using conditional statements. The most common conditional statements in shell scripting are if, elif (short for "else if"), and else. These statements allow you to execute different code blocks based on whether certain conditions are met.

Uses of these conditional statements in shell scripts:

1. The if Statement:

   • The if statement is used to test a condition and execute a block of code if the condition is true.
   • The basic syntax is as follows:

     if [ $x -eq 10 ]; then
    echo "x is equal to 10"
fi

   • You can use various operators and commands within the [ ] to check conditions. For example:

     if [ $x -eq 10 ]; then
    echo "x is equal to 10"
fi

2. The elif Statement:

   • The elif statement allows you to test multiple conditions in a sequential manner.
   • If the previous if or elif conditions are false, the code block associated with the first true condition is executed.
   • The syntax is as follows:

     if [ condition1 ]; then
    # Code to execute if condition1 is true
elif [ condition2 ]; then
    # Code to execute if condition2 is true
else
    # Code to execute if none of the conditions are true
fi

3. The else Statement:

   • The else statement is used to provide a default code block to execute when none of the preceding conditions are true.
   • It is often used in conjunction with if or elif to handle cases where none of the conditions match.
   • The else block is optional.
   • Example:

     if [ $x -eq 10 ]; then
    echo "x is equal to 10"
else
    echo "x is not equal to 10"
fi

4. Logical Operators:

   • You can use logical operators like -eq (equal), -ne (not equal), -lt (less than), -le (less than or equal), -gt (greater than), -ge (greater than or equal), && (logical AND), and || (logical OR) to form complex conditions.
   • Example:

     if [ $x -eq 10 ] && [ $y -lt 20 ]; then
    echo "x is 10 and y is less than 20"
fi

5. Nested if Statements:

   • You can nest if statements within other if statements to create more complex decision structures.
   • Example:

     if [ $x -eq 10 ]; then
    if [ $y -lt 20 ]; then
        echo "Both conditions are true"
    fi
fi

Here's a complete example of a shell script that uses if, elif, and else statements to make decisions based on user input:

#!/bin/bash

echo "Enter a number: "
read num

if [ $num -gt 0 ]; then
    echo "The number is positive."
elif [ $num -lt 0 ]; then
    echo "The number is negative."
else
    echo "The number is zero."
fi

This script takes user input and determines whether the entered number is positive, negative, or zero based on the conditions specified.

Example

The if...else statement as follows −

Output:

 

The if...elif...fi statement is the one level advance form of control statement that allows Shell to make correct decision out of several conditions.

Example

The  if..elif..else..fi statement as follows −

Output:

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