C-style for loop in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a C-style for loop in bash scripting?

A C-style for loop in bash uses the syntax: for (( initialization; condition; increment )); do commands; done. It works like the for loop in C language, controlling the loop with a counter variable.

beginner

Write the syntax of a C-style for loop in bash.

for (( i=0; i<10; i++ )); do # commands done

beginner

How does the increment part work in a C-style for loop in bash?

The increment part updates the loop variable after each iteration. For example, i++ adds 1 to i each time the loop runs.

beginner

What will this loop print? for (( i=1; i<=3; i++ )); do echo $i; done

It will print: 1 2 3 Each number on its own line, counting from 1 to 3.

intermediate

Can you use negative increments in a C-style for loop in bash?

Yes, you can. For example, for (( i=5; i>0; i-- )) will count down from 5 to 1.

What is the correct syntax to start a C-style for loop in bash?

A for i in 0 1 2 3 4

B while (( i<5 ))

C for i=0 to 5

D for (( i=0; i<5; i++ ))

The C-style for loop uses double parentheses with initialization, condition, and increment separated by semicolons.

What does the increment part in a C-style for loop do?

A Checks if the loop should continue

B Updates the loop variable after each iteration

C Initializes the loop variable

D Ends the loop

The increment part changes the loop variable, usually increasing or decreasing it, after each loop cycle.

Which of these loops counts down from 3 to 1?

A for (( i=0; i<3; i++ ))

B for (( i=1; i<=3; i++ ))

C for (( i=3; i>0; i-- ))

D for (( i=3; i<0; i++ ))

The loop starts at 3, continues while i>0, and decreases i by 1 each time.

What will this loop print? for (( i=0; i<3; i++ )); do echo $i; done

A 0 1 2

B 1 2 3

C 0 1 2 3

D 1 2

The loop prints 0, then 1, then 2, because it runs while i is less than 3.

Can you use a C-style for loop to loop over a list of strings in bash?

A No, you must use 'for var in list' syntax

B No, C-style loops only work with numbers

C Yes, but only with arrays

D Yes, directly with C-style syntax

C-style for loops are for numeric loops. To loop over strings, use 'for var in list' syntax.

Explain how a C-style for loop works in bash scripting.

Think about how you set a counter, check it, and change it each time.

Write a C-style for loop in bash that counts from 10 down to 1 and prints each number.

Use decrement operator and condition i>=1.

Practice

1. What does the following C-style for loop do in bash?
for ((i=1; i<=3; i++)); do echo $i; done

easy

A. Prints numbers 1, 2, and 3 each on a new line

B. Prints numbers 0, 1, and 2 each on a new line

C. Prints numbers 1, 2, 3, and 4 each on a new line

D. Causes a syntax error

Solution

1. Step 1: Understand the loop initialization and condition

   The loop starts with i=1 and runs while i is less than or equal to 3.

2. Step 2: Analyze the increment and output

   Each iteration prints the current value of i, then increases i by 1 until i becomes 4 and stops.

3. Final Answer:

   Prints numbers 1, 2, and 3 each on a new line -> Option A

4. Quick Check:

   Loop from 1 to 3 prints 1, 2, 3 [OK]

Hint: Check start, end, and increment values carefully [OK]

Common Mistakes:

• Starting loop at 0 instead of 1
• Confusing <= with < in condition
• Forgetting to increment i

2. Which of the following is the correct syntax for a C-style for loop in bash?

easy

A. for (i=0; i<5; i++) do echo $i; done

B. for ((i=0; i<5; i++)); do echo $i; done

C. for [i=0; i<5; i++] do echo $i; done

D. for {i=0; i<5; i++} do echo $i; done

Solution

1. Step 1: Identify correct C-style for loop syntax in bash

   Bash requires double parentheses (( )) around the loop control with semicolons separating init, condition, and increment.

2. Step 2: Check each option

   for ((i=0; i<5; i++)); do echo $i; done uses double parentheses and semicolons correctly; others use incorrect brackets or missing parentheses.

3. Final Answer:

   for ((i=0; i<5; i++)); do echo $i; done -> Option B

4. Quick Check:

   Double parentheses and semicolons = correct syntax [OK]

Hint: Look for double parentheses and semicolons inside [OK]

Common Mistakes:

• Using single parentheses instead of double
• Using square or curly brackets
• Missing semicolons between parts

3. What is the output of this bash script?

for ((i=2; i<=6; i+=2)); do echo $i; done

medium

A. 2 3 4 5 6

B. Syntax error

C. 2 4 6 8

D. 2 4 6

Solution

1. Step 1: Understand loop initialization and increment

   The loop starts at i=2, runs while i is less than or equal to 6, and increments i by 2 each time.

2. Step 2: List values printed

   Values printed are 2, then 4, then 6. After 6, i becomes 8 which breaks the condition.

3. Final Answer:

   2 4 6 -> Option D

4. Quick Check:

   Increment by 2 from 2 to 6 prints 2,4,6 [OK]

Hint: Check increment step and loop condition carefully [OK]

Common Mistakes:

• Assuming increment is 1 by default
• Including numbers beyond the condition
• Confusing <= with <

4. Identify the error in this bash C-style for loop:

for ((i=0; i<5 i++)); do echo $i; done

medium

A. Missing semicolon between condition and increment

B. Using single parentheses instead of double

C. Incorrect variable name

D. Missing do keyword

Solution

1. Step 1: Check loop syntax parts

   The loop control must have three parts separated by semicolons: initialization, condition, and increment.

2. Step 2: Locate missing semicolon

   Between 'i<5' and 'i++' there is no semicolon, causing a syntax error.

3. Final Answer:

   Missing semicolon between condition and increment -> Option A

4. Quick Check:

   Semicolons separate parts in C-style for loops [OK]

Hint: Check for three semicolon-separated parts inside (( )) [OK]

Common Mistakes:

• Omitting semicolons between parts
• Using single parentheses
• Forgetting do keyword

5. You want to print all odd numbers from 1 to 9 using a C-style for loop in bash. Which script achieves this?

hard

A. for ((i=1; i<10; i++)); do if (( i % 2 == 0 )); then echo $i; fi; done

B. for ((i=0; i<10; i++)); do if (( i % 2 == 0 )); then echo $i; fi; done

C. for ((i=1; i<=9; i+=2)); do echo $i; done

D. All of the above

Solution

1. Step 1: Analyze for ((i=1; i<=9; i+=2)); do echo $i; done

   for ((i=1; i<=9; i+=2)); do echo $i; done starts at 1, increments by 2, prints 1,3,5,7,9 directly.

2. Step 2: Analyze options B and C

   Both loop with i++ and check if (( i % 2 == 0 )), which selects even numbers: B prints 0,2,4,6,8; C prints 2,4,6,8.

3. Final Answer:

   for ((i=1; i<=9; i+=2)); do echo $i; done -> Option C

4. Quick Check:

   Increment by 2 from 1 to <=9 prints odds 1-9 [OK]

Hint: Use increments or modulo checks to filter odd numbers [OK]

Common Mistakes:

• Using wrong increment step
• Starting from 0 without filtering
• Forgetting to check odd condition