jobs command in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of jobs command with background processes

You run these commands in a Linux shell:

sleep 30 &
sleep 40 &
jobs

What is the expected output of the jobs command?

A

[1]-  Running                 sleep 30 &
[2]+  Running                 sleep 40 &

B No output, jobs command shows nothing

C

[1]+  Done                    sleep 30 &
[2]-  Done                    sleep 40 &

D

[1]+  Running                 sleep 30 &
[2]-  Running                 sleep 40 &

💡 Hint

Jobs are numbered in order started; the last job is marked with a plus (+).

The jobs command lists background jobs with their job numbers. The most recent job is marked with a plus (+) and the previous with a minus (-). Both sleep commands run in background, so status is Running.

💻 Command Output

intermediate

Effect of fg command on jobs

You start a background job:

sleep 60 &

Then run jobs and see:

[1]+  Running                 sleep 60 &

What happens when you run fg %1?

A Syntax error: fg requires a job name, not a number

B The sleep 60 job is terminated immediately

C Nothing happens, job stays in background

D The sleep 60 job moves to foreground and blocks the shell until it finishes

💡 Hint

The fg command brings a background job to foreground.

The fg command resumes the specified job in the foreground, so the shell waits until the job completes.

🔧 Debug

advanced

Why does jobs show no output?

You run this sequence:

sleep 20 &
jobs

But jobs shows no output. What is the most likely reason?

A The jobs command requires root privileges

B The sleep command finished before jobs was run

C The shell session is not interactive, so jobs command shows nothing

D The sleep command was run in foreground, so no jobs exist

💡 Hint

Jobs only work in interactive shells.

The jobs command only reports jobs in interactive shells. In scripts or non-interactive shells, it shows nothing.

🧠 Conceptual

advanced

Understanding job control signals

You have a job running in foreground. You press Ctrl+Z. What happens to the job and how does jobs reflect this?

A The job continues running in background, jobs shows 'Running'

B The job is suspended (stopped), and jobs shows it as 'Stopped'

C The job is terminated, and jobs shows no entry

D The job is paused but jobs shows 'Done'

💡 Hint

Ctrl+Z sends SIGTSTP to suspend the job.

Ctrl+Z suspends the foreground job, stopping it temporarily. The jobs command then lists it as 'Stopped'.

🚀 Application

expert

Counting background jobs with jobs command

You run multiple background jobs:

sleep 10 &
sleep 20 &
sleep 30 &

Write a one-liner command using jobs and other shell tools to count how many background jobs are currently running.

A jobs | grep Running | wc -l

B jobs -p | wc -l

C jobs | wc -l

D ps aux | grep sleep | wc -l

💡 Hint

Filter jobs output for Running status, then count lines.

The command 'jobs | grep Running | wc -l' counts only the lines with 'Running', corresponding to currently running background jobs. Other options may count stopped jobs or unrelated processes.

Practice

1. What does the jobs command do in a Linux shell?

easy

A. Lists background and stopped tasks in the current shell

B. Shows all running processes on the system

C. Terminates all background jobs

D. Starts a new background job

Solution

1. Step 1: Understand the purpose of jobs

   The jobs command shows tasks started in the current shell that are either stopped or running in the background.

2. Step 2: Compare with other options

   Options A, B, and C describe different commands or actions unrelated to jobs.

3. Final Answer:

   Lists background and stopped tasks in the current shell -> Option A

4. Quick Check:

   jobs lists current shell jobs [OK]

Hint: Remember: jobs shows your shell's paused or background tasks [OK]

Common Mistakes:

• Confusing jobs with ps command
• Thinking jobs starts or kills tasks
• Assuming jobs shows system-wide processes

2. Which of the following is the correct syntax to list all jobs with their process IDs using the jobs command?

easy

A. jobs -a

B. jobs --list-pid

C. jobs -p

D. jobs -l

Solution

1. Step 1: Recall jobs options

   The -l option with jobs lists jobs with their process IDs.

2. Step 2: Verify other options

   -p lists only PIDs without job info, --list-pid is invalid, and -a shows all jobs including those not started by the shell.

3. Final Answer:

   jobs -l -> Option D

4. Quick Check:

   jobs -l shows jobs with PIDs [OK]

Hint: Use -l to see job numbers plus process IDs [OK]

Common Mistakes:

• Using invalid options like --list-pid
• Confusing -p with -l
• Assuming -a shows PIDs

3. Given the following shell session:

$ sleep 100 &
[1] 12345
$ sleep 200 &
[2] 12346
$ jobs
[1]+  Running                 sleep 100 &
[2]-  Running                 sleep 200 &

What will be the output of jobs -l?

medium

A. [1]+ 12345 Stopped sleep 100 &
[2]- 12346 Stopped sleep 200 &

B. [1]+ Running sleep 100 &
[2]- Running sleep 200 &

C. [1]+ 12345 Running sleep 100 &
[2]- 12346 Running sleep 200 &

D. No jobs to display

Solution

1. Step 1: Understand jobs output

   The jobs command shows job number, status, and command. The -l option adds the process ID after the job number.

2. Step 2: Match output format

   Since both jobs are running with PIDs 12345 and 12346, jobs -l will show job number, PID, status, and command.

3. Final Answer:

   [1]+ 12345 Running sleep 100 & [2]- 12346 Running sleep 200 & -> Option C

4. Quick Check:

   jobs -l adds PIDs to normal jobs output [OK]

Hint: jobs -l shows PIDs next to job numbers [OK]

Common Mistakes:

• Ignoring PIDs in output
• Confusing Running with Stopped status
• Expecting no jobs listed

4. You ran sleep 300 & and then jobs shows the job as stopped. Which command will resume the job in the background?

medium

A. kill %1

B. bg %1

C. fg %1

D. stop %1

Solution

1. Step 1: Understand job states and commands

   A stopped job is paused. The bg command resumes it in the background, while fg resumes it in the foreground.

2. Step 2: Choose correct command for background resume

   Since the question asks to resume in background, bg %1 is correct. kill terminates, and stop is invalid here.

3. Final Answer:

   bg %1 -> Option B

4. Quick Check:

   Resume stopped job in background = bg %job_number [OK]

Hint: Use bg %job_number to resume stopped jobs in background [OK]

Common Mistakes:

• Using fg instead of bg for background resume
• Trying to kill instead of resume
• Using invalid stop command

5. You have multiple background jobs running. You want to bring job number 3 to the foreground and then stop it temporarily. Which sequence of commands should you use?

hard

A. fg %3, then press Ctrl+Z

B. bg %3, then press Ctrl+C

C. kill %3, then fg %3

D. jobs %3, then stop %3

Solution

1. Step 1: Bring job 3 to foreground

   Use fg %3 to bring job number 3 to the foreground so you can interact with it.

2. Step 2: Stop the job temporarily

   Pressing Ctrl+Z sends a stop signal, pausing the job and returning control to the shell.

3. Final Answer:

   fg %3, then press Ctrl+Z -> Option A

4. Quick Check:

   Foreground job then Ctrl+Z to stop it [OK]

Hint: Use fg %job then Ctrl+Z to pause foreground job [OK]

Common Mistakes:

• Using bg instead of fg to bring job forward
• Trying to kill instead of stop
• Using invalid stop command