until loop in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does an until loop do in Bash scripting?

An until loop runs the commands inside it repeatedly until the given condition becomes true. It keeps running while the condition is false.

beginner

How is an until loop different from a while loop?

A while loop runs while the condition is true. An until loop runs while the condition is false, stopping when it becomes true.

beginner

Write the basic syntax of an until loop in Bash.

until [ condition ] do commands done

beginner

What will this script print?

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

It will print: 1 2 3 The loop runs until count is greater than 3, so it prints 1, 2, and 3.

intermediate

Why might you choose an until loop instead of a while loop?

You use until when you want to repeat commands until a condition becomes true. It can make your code easier to read if you think in terms of "keep going until this happens."

What condition causes an until loop to stop running?

A When the condition becomes true

B When the condition becomes false

C After one iteration

D Never stops

An until loop runs while the condition is false and stops when it becomes true.

Which of these is the correct syntax to start an until loop?

A while [ condition ]; do

B until [ condition ]; do

C for i in [ condition ]; do

D if [ condition ]; then

The until loop starts with until [ condition ]; do.

What will this script print?

count=5
until [ $count -lt 3 ]
do
  echo $count
  ((count--))
done

A Nothing

B 3 4 5

C 1 2 3

D 5 4 3

The loop runs until count is less than 3, so it prints 5, 4, and 3.

Which loop runs while the condition is true?

A until loop

B none

C while loop

D both until and while

A while loop runs while the condition is true.

What happens if the condition in an until loop is true at the start?

A The loop does not run at all

B The loop runs forever

C The loop runs once

D The loop runs twice

If the condition is true at the start, the until loop does not run.

Explain how an until loop works in Bash and give a simple example.

Think about repeating something until a condition becomes true.

Compare and contrast until and while loops in Bash scripting.

Focus on when each loop continues and stops.

Practice

1. What does an until loop do in bash scripting?

easy

A. It repeats commands until the condition becomes true.

B. It repeats commands while the condition is true.

C. It runs commands only once regardless of condition.

D. It runs commands only if the condition is false.

Solution

1. Step 1: Understand the until loop behavior

   An until loop runs repeatedly until its condition becomes true, meaning it stops when the condition is true.

2. Step 2: Compare options with definition

   It repeats commands until the condition becomes true. correctly states the loop repeats until the condition is true. It repeats commands while the condition is true. describes a while loop. Options C and D are incorrect behaviors.

3. Final Answer:

   It repeats commands until the condition becomes true. -> Option A

4. Quick Check:

   until loop = repeat until true [OK]

Hint: Remember: until stops when condition is true [OK]

Common Mistakes:

• Confusing until with while loop
• Thinking until runs only once
• Believing until runs while condition is true

2. Which of the following is the correct syntax for an until loop in bash?

easy

A. until [ condition ]; do commands; done

B. until ( condition ) { commands }

C. until { condition } do commands done

D. until condition do commands done

Solution

1. Step 1: Recall bash until loop syntax

   The correct syntax uses square brackets for condition, followed by do and done keywords: until [ condition ]; do commands; done

2. Step 2: Check each option

   until [ condition ]; do commands; done matches correct syntax. Options B, C, and D use incorrect brackets or missing semicolons.

3. Final Answer:

   until [ condition ]; do commands; done -> Option A

4. Quick Check:

   Syntax = until [ condition ]; do ... done [OK]

Hint: Use square brackets and do/done keywords [OK]

Common Mistakes:

• Using parentheses instead of square brackets
• Omitting do or done keywords
• Missing semicolon before do

3. What will be the output of this script?

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

medium

A. 1 2

B. 1 2 3 4

C. 1 2 3

D. No output, infinite loop

Solution

1. Step 1: Analyze loop condition and increments

   The loop runs until count is greater than 3. Starting at 1, it prints count and increments it each time.

2. Step 2: Trace iterations

   count=1 (prints 1), count=2 (prints 2), count=3 (prints 3), count=4 stops loop because 4 > 3.

3. Final Answer:

   1 2 3 -> Option C

4. Quick Check:

   Prints 1 to 3 before stopping [OK]

Hint: Loop stops when count > 3, so prints 1 to 3 [OK]

Common Mistakes:

• Thinking it prints 4
• Confusing until with while loop condition
• Assuming infinite loop

4. Identify the error in this until loop script:

count=1
until [ $count -lt 5 ]
do
  echo $count
  ((count++))
done

medium

A. Missing semicolon after condition.

B. The loop will never run because condition is initially true.

C. The variable count is not initialized.

D. The condition should use -gt instead of -lt.

Solution

1. Step 1: Understand the until loop condition

   until runs while the condition is false and stops when true. Here, condition is count -lt 5, which is true at start (count=1).

2. Step 2: Analyze loop execution

   Since condition is true initially, loop does not run even once.

3. Final Answer:

   The loop will never run because condition is initially true. -> Option B

4. Quick Check:

   until stops immediately if condition true [OK]

Hint: until runs only while condition is false [OK]

Common Mistakes:

• Confusing -lt with -gt in condition
• Assuming loop runs at least once
• Missing that until stops when condition true

5. You want to write a bash script that waits until a file named ready.txt exists before continuing. Which until loop correctly implements this?

hard

A. until [ ! -f ready.txt ]; do sleep 1; done

B. until [ ! -f ready.txt ]; do echo 'Waiting'; done

C. until [ -d ready.txt ]; do sleep 1; done

D. until [ -f ready.txt ]; do sleep 1; done

Solution

1. Step 1: Understand the goal and condition

   The script should wait until the file exists. The loop runs while the condition is false, so condition should be "file exists" [ -f ready.txt ].

2. Step 2: Check each option's condition

   until [ -f ready.txt ]; do sleep 1; done loops while [ -f ] is false (file missing), sleeping until file exists. B, C (-d for directory), and D use wrong conditions.

3. Final Answer:

   until [ -f ready.txt ]; do sleep 1; done -> Option D

4. Quick Check:

   Loop while file missing, stops when exists [OK]

Hint: Condition [ -f file ] stops when file exists [OK]

Common Mistakes:

• Using -d instead of -f for file check
• Using ! -f negation incorrectly
• Not including sleep to avoid busy wait