C-style for loop in Bash Scripting - Step-by-Step Execution

Concept Flow - C-style for loop

Initialize variable

↓

Check condition

↓

Execute body

↓

Update variable

↩Back to Check condition

The loop starts by setting a variable, then checks a condition. If true, it runs the loop body, updates the variable, and repeats. If false, it stops.

Execution Sample

Bash Scripting

for ((i=0; i<3; i++)); do
  echo $i
done

This loop prints numbers 0, 1, and 2 by increasing i from 0 to 2.

Execution Table

Iteration	i (before check)	Condition (i<3)	Body executed	Output	i (after update)
1	0	True	Yes	0	1
2	1	True	Yes	1	2
3	2	True	Yes	2	3
4	3	False	No

💡 i reaches 3, condition 3<3 is False, loop stops

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
i	0	1	2	3	3

Key Moments - 3 Insights

Why does the loop stop when i equals 3?

Because the condition i<3 becomes false at iteration 4 (see execution_table row 4), so the loop does not run the body again.

When is the variable i updated in the loop?

i is updated after the loop body executes in each iteration (see execution_table column 'i (after update)').

What happens if the initial value of i is already 3?

The condition i<3 is false immediately, so the loop body never runs (like iteration 4 in the table).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of i before the condition check in iteration 3?

A 2

B 3

C 1

D 0

💡 Hint

Check the 'i (before check)' column in row 3 of the execution_table.

At which iteration does the loop condition become false?

A Iteration 2

B Iteration 3

C Iteration 4

D Iteration 1

💡 Hint

Look at the 'Condition (i<3)' column and find where it is False.

If we change the loop to for ((i=0; i<=3; i++)), how many times will the loop body execute?

A 3 times

B 4 times

C 5 times

D 2 times

💡 Hint

Think about the condition i<=3 and compare with the variable_tracker values.

Concept Snapshot

C-style for loop syntax in bash:
for (( init; condition; update )); do
  commands
 done

- init runs once at start
- condition checked before each iteration
- loop runs while condition is true
- update runs after each iteration
- useful for counting loops

Full Transcript

A C-style for loop in bash scripting starts by initializing a variable, then checks a condition before each loop run. If the condition is true, the loop body runs, then the variable updates. This repeats until the condition is false. For example, 'for ((i=0; i<3; i++))' prints 0, 1, and 2. The loop stops when i reaches 3 because the condition i<3 is no longer true. The variable i changes after each loop body execution. If i starts at 3, the loop never runs because the condition is false from the start.

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