at command for one-time jobs in Linux CLI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to schedule a command to run at 5 PM today using the at command.

Linux CLI

echo "ls -l" | at "[1]"

Drag options to blanks, or click blank then click option'

Atomorrow 5pm

B17:00

Cnow + 1 hour

D5pm

Attempts:

3 left

💡 Hint

Common Mistakes

Using '17:00' might not work as expected in all shells.

Using 'now + 1 hour' schedules the job one hour from now, not at 5 PM.

Using 'tomorrow 5pm' schedules the job for tomorrow, not today.

The at command accepts time in formats like '5pm' to schedule jobs for that time today.

2fill in blank

medium

Complete the code to schedule a script backup.sh to run at 10:30 AM tomorrow.

Linux CLI

at "[1]" -f backup.sh

Drag options to blanks, or click blank then click option'

A10:30 tomorrow

B10:30am tomorrow

Ctomorrow 10:30

Dnow + 1 day 10:30

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'tomorrow 10:30' might not be recognized correctly.

Using 'now + 1 day 10:30' is not a valid at time format.

Using '10:30 tomorrow' misses the AM/PM and may cause errors.

The at command accepts '10:30am tomorrow' to schedule a job at 10:30 AM the next day.

3fill in blank

hard

Fix the error in the command to schedule echo Hello to run at 2 AM tomorrow.

Linux CLI

echo "echo Hello" | at "[1]"

Drag options to blanks, or click blank then click option'

Atomorrow 2am

B2am tomorrow

C2:00 tomorrow

D2am

Attempts:

3 left

💡 Hint

Common Mistakes

Using '2am' alone schedules the job for 2 AM today if that time is in the future.

Using '2:00 tomorrow' is not a valid at time format.

Using '2am tomorrow' may cause errors depending on shell parsing.

The correct format is 'tomorrow 2am' to specify 2 AM the next day for the at command.

4fill in blank

hard

Fill both blanks to schedule a command to run 15 minutes from now and list all scheduled jobs.

Linux CLI

echo "date" | at "[1]" && at [2]

Drag options to blanks, or click blank then click option'

Anow + 15 minutes

B-l

C-c

Dlist

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'list' instead of '-l' to list jobs causes errors.

Using 'now + 15' without 'minutes' schedules immediately or errors.

Using '-c' lists job contents but is not the standard list command.

Use 'now + 15 minutes' to schedule the job 15 minutes later, and 'at -l' to list all scheduled jobs.

5fill in blank

hard

Fill both blanks to schedule a job to run a script cleanup.sh at 11:45 PM today, then remove the job by its ID.

Linux CLI

at "[1]" -f cleanup.sh
job_id=$(atq | head -n 1 | cut -f1)
atrm [2]

Drag options to blanks, or click blank then click option'

A11:45pm

B$job_id

C1

Dnow + 1 hour

Attempts:

3 left

💡 Hint

Common Mistakes

Using '1' to remove the job may delete the wrong job.

Using 'now + 1 hour' schedules the job at the wrong time.

Not capturing the job ID before removing causes errors.

Schedule the job at '11:45pm' today, then remove it using the job ID stored in the variable job_id.

Practice

1. What is the primary purpose of the at command in Linux?

easy

A. To list all running processes

B. To schedule a one-time job to run at a specific time

C. To monitor system resource usage continuously

D. To create a recurring job every day

Solution

1. Step 1: Understand the at command function

   The at command is designed to schedule commands or scripts to run once at a specified future time.

2. Step 2: Compare with other options

   Options A, C, and D describe other commands or functionalities unrelated to at. For example, listing processes is done by ps, monitoring resources by top, and recurring jobs by cron.

3. Final Answer:

   To schedule a one-time job to run at a specific time -> Option B

4. Quick Check:

   at schedules one-time jobs = B [OK]

Hint: Remember: at is for one-time future tasks [OK]

Common Mistakes:

• Confusing at with cron for recurring jobs
• Thinking at lists running processes
• Assuming at monitors system resources

2. Which of the following is the correct syntax to schedule a job at 3:30 PM using the at command?

easy

A. at 3-30

B. at 3.30pm

C. at 15:30pm

D. at 3:30pm

Solution

1. Step 1: Identify valid time format for at

   The at command accepts time in formats like HH:MMam or HH:MMpm. So 3:30pm is valid.

2. Step 2: Check other options for syntax errors

   B: at 3.30pm uses dot instead of colon, invalid. C: at 15:30pm mixes 24-hour format with pm, invalid. D: at 3-30 uses dash instead of colon, invalid.

3. Final Answer:

   at 3:30pm -> Option D

4. Quick Check:

   Correct at time format = A [OK]

Hint: Use colon and am/pm for time with at [OK]

Common Mistakes:

• Using dot or dash instead of colon in time
• Omitting am/pm in 12-hour format
• Confusing 24-hour and 12-hour formats

3. What will be the output of the following commands sequence?

echo "echo Hello World" | at now + 1 minute
sleep 70
atq

medium

A. No jobs listed because the job already ran

B. Shows an error because at cannot read from pipe

C. Lists the scheduled job with its job number

D. Prints 'Hello World' immediately

Solution

1. Step 1: Understand the job scheduling and timing

   The command schedules a job to run 1 minute from now that prints 'Hello World'. Then the script waits 70 seconds (more than 1 minute), so the job runs during sleep.

2. Step 2: Check the atq output after sleep

   Since the job already executed, atq will show no pending jobs.

3. Final Answer:

   No jobs listed because the job already ran -> Option A

4. Quick Check:

   Job runs after 1 min, sleep 70s, then atq empty = C [OK]

Hint: Wait past scheduled time, then atq shows no jobs [OK]

Common Mistakes:

• Expecting atq to list completed jobs
• Thinking at cannot read from pipe
• Assuming immediate output from scheduled job

4. You tried to schedule a job with at 25:00 but got an error. What is the likely cause and how to fix it?

medium

A. You must run at as root user

B. Missing command after at; add a command

C. Invalid hour; use a valid 24-hour time like 23:00

D. The at daemon is not installed

Solution

1. Step 1: Identify the error cause

   The hour 25 is invalid because valid 24-hour times range from 00 to 23.

2. Step 2: Correct the time format

   Use a valid time like 23:00 or a 12-hour format with am/pm.

3. Final Answer:

   Invalid hour; use a valid 24-hour time like 23:00 -> Option C

4. Quick Check:

   Hour must be 0-23 in at time = A [OK]

Hint: Hours must be 0-23 in 24-hour format for at [OK]

Common Mistakes:

• Using invalid hour values above 23
• Assuming at needs root for scheduling
• Confusing missing command error with time format error

5. You want to schedule a script /home/user/backup.sh to run once at 11:45 PM today and then remove it from the queue after scheduling. Which sequence of commands achieves this correctly?

hard

A. echo "/home/user/backup.sh" | at 11:45pm && atrm $(atq | awk '{print $1}')

B. at 11:45pm < /home/user/backup.sh && atrm $(atq | awk '{print $1}')

C. at 11:45pm <<< "/home/user/backup.sh" && atrm $(atq | awk '{print $1}')

D. echo "/home/user/backup.sh" | at 11:45pm && atq && atrm $(atq | awk '{print $1}')

Solution

1. Step 1: Schedule the script using echo piped to at

   This schedules the command to run at 11:45pm by piping the script path as a command.

2. Step 2: Remove the job from the queue immediately after scheduling

   Use atrm with the job number obtained from atq. echo "/home/user/backup.sh" | at 11:45pm && atrm $(atq | awk '{print $1}') correctly schedules and then removes the job.

3. Final Answer:

   echo "/home/user/backup.sh" | at 11:45pm && atrm $(atq | awk '{print $1}') -> Option A

4. Quick Check:

   Pipe command to at, then remove job = D [OK]

Hint: Pipe command to at, then remove job with atrm [OK]

Common Mistakes:

• Using input redirection with script file instead of command
• Removing job before it is scheduled
• Confusing atq output format for job ID