Conditional statements are a fundamental part of programming based on logic. They allow programs to make decisions based on certain conditions being true or false.

Variations

The general variations of conditional statements are:

• if - executes a block of code if a condition is true

• else - executes a different block of code if the condition is false

• else if (or elif) - checks another condition if the first if the condition is false

• switch/case - selects one of many code blocks to execute based on different conditions

• ternary operator - a short-hand if-else expression

Conditions are based on logical expressions that evaluate true or false. This could be comparisons, logical operations, or any expression that resolves to a boolean value.

Importance

The importance of conditional statements in structured programming is that they allow programs to:

• Make decisions based on data and conditions

• Have different execution paths based on those decisions

• Handle exceptions and edge cases

• Simplify complex logic into readable chunks

Without conditional statements, programs would be linear with no ability to branch or make choices. This would severely limit what programs could accomplish.

In summary, conditional statements are an essential part of almost all programming languages, based on logical expressions that evaluate true or false. They allow programs to have different execution paths based on conditions, which is key for structured programming.

if Statement

The if statement allows us to execute commands conditionally:

if condition 
then 
    command1 
    command2
fi

condition can be:

• A test [ .. ]

• An arithmetic expression $((..))

• A string comparison == or !=

Example:

if [ $AGE -gt 18 ] 
then
    echo "You are an adult"
fi

else Clause

We can add an else clause to execute an alternative:

if condition; then
     command1 
else 
     command2
fi

Example:

if [ $AGE -gt 18 ]; then
    echo "You can vote"
else 
    echo "You cannot vote"
fi

elif Clause

We can have multiple conditions using elif (else if):

if condition1; then 
     command1
elif condition2; then
     command2 
elif condition3; 
     command3
else
     command4
fi

Example:

if [ $AGE -lt 16 ]; then
    echo "You cannot drive"
elif [ $AGE -ge 16 ] && [ $AGE -lt 18 ]; then
    echo "You can drive with restrictions" 
elif [ $AGE -ge 18 ]; then
    echo "You can drive"
else
    echo "Invalid age!"
fi

So in summary, if, else if (elif) and else clauses allow us to execute code conditionally in Bash scripts. Sometimes, an "if statement" is called a "selection statement", because enables you to select the branch you want to execute depending on the conditional value.

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Script Example

Here is a Bash script example to display the day of the week using if statements:

#!/bin/bash

echo "Enter the number of the day (1-7): "
read day

if [ $day -eq 1 ]
then 
    echo "Monday"
elif [ $day -eq 2 ]  
then
    echo "Tuesday"
elif [ $day -eq 3 ]
then 
    echo "Wednesday"    
elif [ $day -eq 4 ]
then
    echo "Thursday"
elif [ $day -eq 5 ]
then
    echo "Friday"
elif [ $day -eq 6 ]  
then 
    echo "Saturday"
elif [ $day -eq 7 ]    
then   
    echo "Sunday"
else
    echo "Invalid day"
fi

Breaking it down:

• We use #!/bin/bash to indicate this is a Bash script

• We prompt the user to enter a number from 1 to 7 representing the day

• We read the input into the $day variable

• We then have a series of if/elif statements checking the value of $day

• For each day number, we print the corresponding day of the week

• We have an else clause to handle invalid input

When run, the output will be:

Enter the number of the day (1-7): 
3
Wednesday

This shows a basic Bash script using if/elif statements to implement logic based on user input. We can expand this to handle exceptions, ask for more input, etc.

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Disclaim: This article was created with AI (Rix)