Background processes (&) in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this command sequence?

Consider the following commands run in a Linux shell:

sleep 2 & echo Done

What will be printed immediately after running these commands?

Linux CLI

sleep 2 & echo Done

A Done after 2 seconds

B Done

C No output until sleep finishes

D sleep: command not found

💡 Hint

The ampersand (&) runs the first command in the background, so the shell does not wait for it.

The 'sleep 2 &' runs the sleep command in the background, so the shell immediately runs 'echo Done' and prints 'Done' right away.

💻 Command Output

intermediate

What happens when you run multiple background jobs?

You run these commands in a Linux shell:

sleep 3 & sleep 1 & sleep 2 & wait

What will happen after running these commands?

Linux CLI

sleep 3 & sleep 1 & sleep 2 & wait

A The shell waits until all three sleep commands finish before returning to prompt

B The shell returns immediately without waiting

C Only the first sleep command runs in background, others run in foreground

D The shell waits only for the first sleep command

💡 Hint

The 'wait' command waits for all background jobs to finish.

All three sleep commands run in background because of '&'. The 'wait' command pauses the shell until all background jobs complete.

🔧 Debug

advanced

Why does this background command fail to run properly?

You try to run this command in a shell:

echo Hello & & echo World

What error will the shell produce?

Linux CLI

echo Hello & & echo World

A bash: syntax error near unexpected token `&'

B Hello\nWorld

C No output, command hangs

D Hello

💡 Hint

Check how many ampersands are used and their placement.

The shell sees two ampersands in a row without a command between them, causing a syntax error.

🚀 Application

advanced

How to run a script in background and save its output to a file?

You want to run a script named backup.sh in the background and save both its output and errors to backup.log. Which command does this correctly?

A ./backup.sh >& backup.log &

B ./backup.sh & > backup.log 2>&1

C ./backup.sh &> backup.log 2>&1

D ./backup.sh > backup.log 2>&1 &

💡 Hint

Redirect output before putting command in background.

The correct syntax redirects stdout and stderr to the file, then runs the command in background with '&'.

🧠 Conceptual

expert

What is the effect of running a command with nohup and & together?

You run this command:

nohup longtask.sh &

What happens to the process if you close the terminal?

A The process stops immediately when terminal closes

B The process pauses until terminal is reopened

C The process keeps running even after terminal closes

D The process restarts automatically after terminal closes

💡 Hint

nohup prevents hangup signals from stopping the process.

nohup makes the process ignore hangup signals, so combined with '&' it runs in background and keeps running after terminal closes.

Practice

1. What does adding & at the end of a command in Linux do?

easy

A. Runs the command with higher priority

B. Stops the command from running

C. Runs the command in the background, freeing the terminal immediately

D. Saves the command output to a file

Solution

1. Step 1: Understand the role of & in Linux commands

   Adding & at the end of a command tells the shell to run it in the background.

2. Step 2: Effect on terminal usability

   Running a command in the background frees the terminal immediately for other commands.

3. Final Answer:

   Runs the command in the background, freeing the terminal immediately -> Option C

4. Quick Check:

   Background process = command + & [OK]

Hint: Remember: & means run in background [OK]

Common Mistakes:

• Thinking & stops the command
• Confusing & with output redirection
• Assuming & increases priority

2. Which of the following is the correct syntax to run sleep 10 in the background?

easy

A. sleep & 10

B. & sleep 10

C. sleep 10 &&

D. sleep 10 &

Solution

1. Step 1: Identify correct placement of &

   The & must be at the end of the command to run it in the background.

2. Step 2: Check each option

   sleep 10 & places & correctly after sleep 10. Others place it incorrectly or use wrong syntax.

3. Final Answer:

   sleep 10 & -> Option D

4. Quick Check:

   Command + & at end = background run [OK]

Hint: Put & at the end of the command [OK]

Common Mistakes:

• Placing & before the command
• Using && instead of &
• Splitting & from command arguments

3. What will be the output after running these commands?

sleep 5 &
jobs

medium

A. Immediately exits the terminal

B. Lists the sleep job running in the background

C. Displays an error about sleep command

D. Shows no jobs running

Solution

1. Step 1: Run sleep 5 &

   This starts the sleep command in the background for 5 seconds.

2. Step 2: Run jobs

   This command lists all background jobs currently running, including the sleep job.

3. Final Answer:

   Lists the sleep job running in the background -> Option B

4. Quick Check:

   jobs shows background tasks [OK]

Hint: Use jobs to see background tasks [OK]

Common Mistakes:

• Expecting jobs to show nothing
• Confusing jobs with ps command
• Thinking sleep runs in foreground without &

4. You ran sleep 10 & but the job does not appear when you run jobs. What is the likely problem?

medium

A. You ran the command in a different shell session

B. The sleep command does not support background execution

C. You forgot to add & at the end

D. The jobs command is not installed

Solution

1. Step 1: Understand job control scope

   Background jobs are tracked per shell session. Running jobs in a different session won't show jobs from another.

2. Step 2: Check other options

   Sleep supports background execution, & was used, and jobs is a built-in command, so those are unlikely issues.

3. Final Answer:

   You ran the command in a different shell session -> Option A

4. Quick Check:

   jobs show current shell's background tasks only [OK]

Hint: jobs only shows current shell's background tasks [OK]

Common Mistakes:

• Assuming jobs shows all system background tasks
• Thinking sleep can't run in background
• Believing jobs is an external command

5. You want to run a script backup.sh in the background and bring it to the foreground later. Which sequence of commands is correct?

hard

A. ./backup.sh & fg %1

B. ./backup.sh && fg

C. ./backup.sh & jobs; fg %2

D. ./backup.sh & bg %1

Solution

1. Step 1: Run the script in background

   Use ./backup.sh & to start the script in the background.

2. Step 2: Bring the background job to foreground

   Use fg %1 to bring job number 1 (the backup script) to the foreground.

3. Step 3: Analyze options

   ./backup.sh & fg %1 correctly runs the script in background and then brings job 1 to foreground. ./backup.sh && fg uses logical && instead of &, so runs in foreground. ./backup.sh & jobs; fg %2 uses wrong job number %2. ./backup.sh & bg %1 uses bg which resumes background jobs, not foreground.

4. Final Answer:

   ./backup.sh & fg %1 -> Option A

5. Quick Check:

   Run with & then fg %job_number to foreground [OK]

Hint: Use fg %job_number to bring background job forward [OK]

Common Mistakes:

• Using && instead of & for background
• Using bg instead of fg to foreground
• Using wrong job number like %2