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

Concept Flow - until loop

Initialize variable

↓

Check condition

↓

EXIT

↓

Update variable

↩Back to Check condition

The until loop runs the code inside it until the condition becomes true. It checks the condition first, runs the body only if the condition is false, then repeats.

Execution Sample

Bash Scripting

count=1
until [ $count -gt 3 ]
do
  echo $count
  ((count++))
done

This script prints numbers 1 to 3 using an until loop that stops when count is greater than 3.

Execution Table

Step	count	Condition [ $count -gt 3 ]	Condition Result	Action	Output
1	1	1 -gt 3	False	Print 1, increment count to 2	1
2	2	2 -gt 3	False	Print 2, increment count to 3	2
3	3	3 -gt 3	False	Print 3, increment count to 4	3
4	4	4 -gt 3	True	Exit loop

💡 count reaches 4, condition 4 -gt 3 is True, so 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 run when the condition is false, not true?

The until loop runs while the condition is false. It stops when the condition becomes true, as shown in execution_table rows 1-3 where condition is false and loop runs, then row 4 where condition is true and loop exits.

What happens if the condition is true at the start?

If the condition is true initially, the loop body does not run at all. The loop exits immediately, as the condition check happens before running the body (see concept_flow).

Visual Quiz - 3 Questions

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

A 2

B 4

C 3

D 1

💡 Hint

Check the 'count' column in execution_table row 3

At which step does the condition become true, causing the loop to exit?

A Step 4

B Step 2

C Step 3

D Step 1

💡 Hint

Look at the 'Condition Result' column in execution_table

If we change the condition to [ $count -ge 3 ], how many times will the loop run?

A 1 time

B 2 times

C 3 times

D 4 times

💡 Hint

Think about when the condition becomes true and compare with variable_tracker values

Concept Snapshot

until loop syntax:
until [ condition ]
do
  commands
  update variables
 done

Runs commands while condition is false.
Stops when condition becomes true.
Checks condition before each iteration.

Full Transcript

The until loop in bash runs the commands inside it repeatedly until the condition becomes true. It first checks the condition. If the condition is false, it runs the loop body. Then it updates variables and checks the condition again. This repeats until the condition is true, then the loop stops. For example, with count starting at 1, the loop prints count and increments it until count is greater than 3. When count reaches 4, the condition becomes true and the loop exits. Beginners often confuse until with while loops. Remember, until runs while the condition is false, stopping when true. Also, if the condition is true at the start, the loop body does not run at all.