if-then-else in Bash Scripting

Introduction

Use if-then-else to make decisions in your script. It helps your script choose what to do based on conditions.

Check if a file exists before using it.

Decide what message to show based on user input.

Run different commands depending on system status.

Verify if a number is positive or negative.

Choose actions based on the time of day.

Syntax

Bash Scripting

if [ condition ]
then
  commands
else
  other_commands
fi

Use spaces around brackets: [ condition ] is required.

End the if block with fi (which is if spelled backward).

Examples

Checks if the variable name is Alice, then prints a greeting or a question.

Bash Scripting

if [ "$name" = "Alice" ]
then
  echo "Hello, Alice!"
else
  echo "Who are you?"
fi

Checks if file.txt exists and prints a message accordingly.

Bash Scripting

if [ -f "file.txt" ]
then
  echo "File exists."
else
  echo "File not found."
fi

Sample Program

This script asks the user for a number and tells if it is positive or not.

Bash Scripting

#!/bin/bash

read -p "Enter a number: " num

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

Important Notes

Always put spaces after [ and before ] in conditions.

Use -gt for 'greater than' when comparing numbers.

Quotes around variables prevent errors if they are empty.

Summary

if-then-else lets your script choose between two paths.

Conditions go inside [ ] with spaces around them.

End the block with fi to close the if statement.

Practice

1. What does the if-then-else structure do in a bash script?

easy

A. It comments out lines in the script.

B. It repeats a set of commands multiple times.

C. It defines a function to reuse code.

D. It lets the script choose between two paths based on a condition.

Solution

1. Step 1: Understand the purpose of if-then-else

   The if-then-else structure checks a condition and runs code based on whether the condition is true or false.

2. Step 2: Compare with other options

   Repeating commands is done with loops, functions reuse code, and comments do not execute code.

3. Final Answer:

   It lets the script choose between two paths based on a condition. -> Option D

4. Quick Check:

   if-then-else = choose path [OK]

Hint: If you see a condition, it's likely if-then-else [OK]

Common Mistakes:

• Confusing if-then-else with loops
• Thinking if-then-else defines functions
• Mixing comments with control flow

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

easy

A. if [ condition ]; then

B. if { condition } then

C. if(condition) then

D. if condition then

Solution

1. Step 1: Recall bash if syntax

   Bash requires spaces around brackets and a semicolon before then: if [ condition ]; then.

2. Step 2: Check other options

   if(condition) then misses spaces and semicolon, B uses braces incorrectly, D misses brackets and semicolon.

3. Final Answer:

   if [ condition ]; then -> Option A

4. Quick Check:

   Spaces and semicolon needed [OK]

Hint: Remember spaces around [ ] and semicolon before then [OK]

Common Mistakes:

• No spaces around brackets
• Missing semicolon before then
• Using parentheses or braces incorrectly

3. What will be the output of this script?

#!/bin/bash
x=5
if [ $x -gt 3 ]; then
  echo "Greater"
else
  echo "Smaller"
fi

medium

A. No output

B. Smaller

C. Greater

D. Syntax Error

Solution

1. Step 1: Evaluate the condition

   The condition checks if x (5) is greater than 3, which is true.

2. Step 2: Determine which branch runs

   Since condition is true, the script prints "Greater".

3. Final Answer:

   Greater -> Option C

4. Quick Check:

   5 > 3 = Greater [OK]

Hint: Check numeric comparison carefully [OK]

Common Mistakes:

• Confusing -gt with -lt
• Missing spaces causing syntax errors
• Assuming else runs when condition is true

4. Find the error in this script:

#!/bin/bash
if [ $num -eq 10 ]
  echo "Ten"
else
  echo "Not Ten"
fi

medium

A. Missing then after condition

B. Wrong comparison operator

C. Missing fi at the end

D. No error

Solution

1. Step 1: Check if statement syntax

   The if condition must be followed by then keyword to start the true block.

2. Step 2: Identify missing part

   The script misses then after the condition line.

3. Final Answer:

   Missing then after condition -> Option A

4. Quick Check:

   if needs then [OK]

Hint: Always put then after condition in if [ ] [OK]

Common Mistakes:

• Forgetting then keyword
• Using wrong comparison operators
• Missing fi to close if

5. You want a script that prints "Even" if a number is even, and "Odd" if it is odd. Which script correctly uses if-then-else to do this?

num=4
# Choose correct script

hard

A. if [ $num % 2 -eq 0 ]; then
echo "Even"
else
echo "Odd"
fi

B. if (( num % 2 == 0 )); then
echo "Even"
else
echo "Odd"
fi

C. if [ $num % 2 == 0 ]; then
echo "Even"
else
echo "Odd"
fi

D. if [ $num / 2 -eq 0 ]; then
echo "Even"
else
echo "Odd"
fi

Solution

1. Step 1: Understand arithmetic evaluation in bash

   Using double parentheses (( )) allows arithmetic expressions and comparisons directly.

2. Step 2: Check each option

   if [ $num % 2 -eq 0 ]; then echo "Even" else echo "Odd" fi uses % inside [ ] without arithmetic evaluation. if [ $num % 2 == 0 ]; then echo "Even" else echo "Odd" fi uses % inside [ ] without arithmetic evaluation. if [ $num / 2 -eq 0 ]; then echo "Even" else echo "Odd" fi uses / instead of % for modulus. if (( num % 2 == 0 )); then echo "Even" else echo "Odd" fi correctly uses (( )) with modulus and comparison.

3. Final Answer:

   if (( num % 2 == 0 )); then echo "Even" else echo "Odd" fi -> Option B

4. Quick Check:

   Use (( )) for arithmetic if [OK]

Hint: Use (( )) for math conditions in bash [OK]

Common Mistakes:

• Using [ ] with arithmetic operators incorrectly
• Confusing / with % for modulus
• Missing then keyword