Why conditionals branch script logic in Bash Scripting - See It in Action

Why conditionals branch script logic

📖 Scenario: You are writing a simple script to check if a folder exists on your computer. Depending on whether the folder is there or not, the script will print a different message. This helps you understand how conditionals can change what your script does.

🎯 Goal: Build a bash script that uses a conditional if statement to check if a folder named my_folder exists. If it exists, print Folder exists. If it does not exist, print Folder does not exist.

📋 What You'll Learn

Create a variable called folder with the value my_folder

Create a variable called exists_message with the value Folder exists

Create a variable called not_exists_message with the value Folder does not exist

Use an if statement with the test [ -d "$folder" ] to check if the folder exists

Print $exists_message if the folder exists

Print $not_exists_message if the folder does not exist

💡 Why This Matters

🌍 Real World

Scripts often need to check if files or folders exist before doing tasks like backups or installations.

💼 Career

Understanding conditionals is essential for automation, system administration, and writing reliable scripts.

1

Set up folder and message variables

Create a variable called folder and set it to my_folder. Then create two variables: exists_message with the value Folder exists and not_exists_message with the value Folder does not exist.

Bash Scripting

# Create the variables folder, exists_message, and not_exists_message
# Your code here

Hint

Use the syntax variable_name="value" to create variables in bash.

2

Write the if condition to check folder existence

Write an if statement that uses [ -d "$folder" ] to check if the folder exists. Inside the if, add a comment # Folder exists. After the if, add an else with a comment # Folder does not exist.

Bash Scripting

folder="my_folder"
exists_message="Folder exists"
not_exists_message="Folder does not exist"

# Write if condition to check if folder exists
# Your code here

Hint

Use if [ condition ]; then ... else ... fi to write conditionals in bash.

3

Add commands to print messages inside the if and else blocks

Inside the if block, add a command to print $exists_message. Inside the else block, add a command to print $not_exists_message.

Bash Scripting

folder="my_folder"
exists_message="Folder exists"
not_exists_message="Folder does not exist"

if [ -d "$folder" ]; then
    # Print exists_message
    # Your code here
else
    # Print not_exists_message
    # Your code here
fi

Hint

Use echo "$variable_name" to print the value of a variable in bash.

4

Run the script and see the output

Run the script and observe the output. The script should print Folder exists if the folder my_folder exists, or Folder does not exist if it does not.

Bash Scripting

folder="my_folder"
exists_message="Folder exists"
not_exists_message="Folder does not exist"

if [ -d "$folder" ]; then
    echo "$exists_message"
else
    echo "$not_exists_message"
fi

# Run the script to see the output

Hint

Use bash script_name.sh to run your script in the terminal. The output depends on whether the folder my_folder exists.

Practice

1. What is the main purpose of using if statements in a bash script?

easy

A. To make decisions and run different commands based on conditions

B. To repeat commands multiple times

C. To define functions

D. To declare variables

Solution

1. Step 1: Understand the role of if in scripts

   if statements check conditions and decide which commands to run next.

2. Step 2: Compare with other options

   Repeating commands is done by loops, functions define reusable code, and variables store data, so these are not the main purpose of if.

3. Final Answer:

   To make decisions and run different commands based on conditions -> Option A

4. Quick Check:

   Conditionals branch logic = A [OK]

Hint: If it chooses what to do next, it's an if statement [OK]

Common Mistakes:

• Confusing if with loops
• Thinking if defines functions
• Mixing variable declaration with conditionals

2. Which of the following is the correct syntax to start an if statement in bash?

easy

A. if condition {}

B. if (condition) {

C. if condition then

D. if [ condition ]; then

Solution

1. Step 1: Recall bash if syntax

   Bash requires square brackets around the condition and a semicolon before then.

2. Step 2: Check other options for syntax errors

   Options A, B, and C use incorrect syntax styles not valid in bash.

3. Final Answer:

   if [ condition ]; then -> Option D

4. Quick Check:

   Bash if syntax = D [OK]

Hint: Remember: if [ condition ]; then is bash style [OK]

Common Mistakes:

• Omitting semicolon before then
• Using parentheses instead of brackets
• Missing brackets around condition

3. What will be the output of this script?

#!/bin/bash
num=5
if [ $num -gt 10 ]; then
  echo "Greater than 10"
elif [ $num -eq 5 ]; then
  echo "Equals 5"
else
  echo "Less than 5"
fi

medium

A. Equals 5

B. Greater than 10

C. Less than 5

D. No output

Solution

1. Step 1: Check the value of num

   The variable num is set to 5.

2. Step 2: Evaluate the conditions in order

   First condition checks if 5 > 10 (false), second checks if 5 == 5 (true), so it prints "Equals 5".

3. Final Answer:

   Equals 5 -> Option A

4. Quick Check:

   num=5 matches elif condition = A [OK]

Hint: Check conditions top to bottom; first true runs [OK]

Common Mistakes:

• Assuming first condition is true
• Ignoring elif branch
• Confusing numeric comparison operators

4. Identify the error in this bash conditional snippet:

if [ $value = 10 ]; then
  echo "Value is 10"
fi

medium

A. Missing semicolon before then

B. Using single equals for numeric comparison

C. Brackets should be double [[ ]]

D. No error, script runs fine

Solution

1. Step 1: Understand comparison operators in bash

   For numeric comparison inside [ ], use -eq instead of =.

2. Step 2: Check the snippet

   The snippet uses = which is for string comparison, so numeric comparison is incorrect here.

3. Final Answer:

   Using single equals for numeric comparison -> Option B

4. Quick Check:

   Numeric compare needs -eq not = [OK]

Hint: Use -eq for numbers, = for strings in [ ] [OK]

Common Mistakes:

• Confusing string and numeric operators
• Thinking = works for numbers
• Ignoring bash operator rules

5. You want a script to print "Positive", "Zero", or "Negative" based on a variable num. Which conditional structure correctly branches all cases?

hard

A. if [ $num -ge 0 ]; then echo "Positive"; else echo "Negative"; fi

B. if [ $num = 0 ]; then echo "Zero"; else echo "Non-zero"; fi

C. if [ $num -gt 0 ]; then echo "Positive"; elif [ $num -eq 0 ]; then echo "Zero"; else echo "Negative"; fi

D. if [ $num -lt 0 ]; then echo "Negative"; elif [ $num -gt 0 ]; then echo "Positive"; fi

Solution

1. Step 1: Identify all required cases

   We need to handle positive (>0), zero (=0), and negative (<0) values.

2. Step 2: Check each option for coverage

   if [ $num -gt 0 ]; then echo "Positive"; elif [ $num -eq 0 ]; then echo "Zero"; else echo "Negative"; fi covers all three cases with if, elif, else branches. Others miss zero or one case.

3. Final Answer:

   if [ $num -gt 0 ]; then echo "Positive"; elif [ $num -eq 0 ]; then echo "Zero"; else echo "Negative"; fi -> Option C

4. Quick Check:

   All cases covered with if-elif-else = B [OK]

Hint: Use if-elif-else to cover all conditions [OK]

Common Mistakes:

• Missing zero case
• Using wrong comparison operators
• Not using else for last case