while loop in Bash Scripting - Step-by-Step Execution

Concept Flow - while loop

Initialize variable

↓

Check condition

|Yes↓

Execute loop body

↓

Update variable

Back to Check↓

Exit loop

↩Back to Check

The while loop starts by checking a condition. If true, it runs the loop body, updates variables, then checks again. It stops when the condition is false.

Execution Sample

Bash Scripting

count=1
while [ $count -le 3 ]
do
  echo "Count is $count"
  ((count++))
done

This script prints the count from 1 to 3 using a while loop.

Execution Table

Step	count value	Condition [ $count -le 3 ]	Action	Output
1	1	True	Print 'Count is 1', increment count to 2	Count is 1
2	2	True	Print 'Count is 2', increment count to 3	Count is 2
3	3	True	Print 'Count is 3', increment count to 4	Count is 3
4	4	False	Exit loop

💡 count reaches 4, condition 4 <= 3 is False, loop stops

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
count	1	2	3	4	4

Key Moments - 2 Insights

Why does the loop stop when count is 4 even though the loop body increments count after printing?

The condition is checked before the loop body runs each time. When count is 4, the condition [ $count -le 3 ] is false, so the loop exits before printing or incrementing again, as shown in step 4 of the execution_table.

What happens if we forget to increment count inside the loop?

If count is never incremented, the condition stays true forever, causing an infinite loop. The execution_table shows count changing each step; without that, the loop never reaches the exit condition.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of count at step 3?

A 4

B 2

C 3

D 1

💡 Hint

Check the 'count value' column in row 3 of the execution_table.

At which step does the condition become false and the loop stops?

A Step 4

B Step 3

C Step 2

D Step 1

💡 Hint

Look at the 'Condition' column in the execution_table and find where it says False.

If we start count at 5 instead of 1, what happens to the loop?

A It runs 3 times

B It does not run at all

C It runs once

D It runs infinitely

💡 Hint

Refer to the variable_tracker and execution_table logic: condition is checked before loop body.

Concept Snapshot

while loop syntax in bash:
while [ condition ]
do
  commands
  update variables
 done

The loop runs as long as the condition is true.
Condition is checked before each iteration.
Remember to update variables to avoid infinite loops.

Full Transcript

This visual execution shows how a bash while loop works. We start with count=1. The loop checks if count is less or equal to 3. If yes, it prints the count and increments it. This repeats until count becomes 4, when the condition fails and the loop stops. The variable_tracker shows count changing from 1 to 4 step by step. Key points: the condition is checked before running the loop body, and forgetting to update count causes an infinite loop. The quiz questions help check understanding of count values and loop stopping conditions.