if-then-else in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the basic syntax of an if-then-else statement in Bash?

The basic syntax is:

if [ condition ]; then
  commands
else
  other_commands
fi

This checks the condition; if true, runs commands, else runs other_commands.

beginner

How do you check if a file exists using if-then-else in Bash?

Use the test command with -e:

if [ -e filename ]; then
  echo "File exists"
else
  echo "File does not exist"
fi

beginner

What does the 'fi' keyword do in a Bash if-then-else statement?

'fi' marks the end of the if statement. It is 'if' spelled backward and tells Bash where the if block finishes.

beginner

Can you use multiple commands inside the then or else blocks? How?

Yes, you can put many commands inside then or else blocks by writing them on separate lines:

if [ condition ]; then
  command1
  command2
else
  command3
  command4
fi

beginner

What happens if the condition in an if statement is false and there is no else block?

If the condition is false and no else block exists, Bash simply skips the then block and continues running the script after fi.

Which keyword ends an if-then-else block in Bash?

A done

B end

C fi

D stop

The keyword 'fi' is used to close an if statement in Bash.

What symbol is used to test a condition in an if statement?

A {}

B []

C ()

D <>

Square brackets [] are used to test conditions in Bash if statements.

How do you write an else block in Bash?

A else { commands }

B else: commands

C else commands

D else followed by commands on next lines

In Bash, 'else' is followed by commands on the next lines before 'fi'.

What will this script print if the file 'test.txt' does not exist?
if [ -e test.txt ]; then echo "Found"; else echo "Not found"; fi

A Not found

B Nothing

C Error

D Found

If the file does not exist, the else block runs and prints 'Not found'.

Can you put multiple commands inside the then block?

A Yes, separated by semicolons or new lines

B No, only one command is allowed

C Only if you use curly braces

D Only if you use parentheses

Multiple commands can be placed inside then block separated by new lines or semicolons.

Explain how an if-then-else statement works in Bash with an example.

Think about checking a condition and running different commands based on true or false.

Describe how to check if a file exists using if-then-else in Bash and what happens if the file does not exist.

Use the test command inside the if condition.

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