fg and bg commands in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this sequence of commands?

You run the following commands in a Linux shell:

sleep 30 &
jobs
fg %1

What will the jobs command output before fg %1 is run?

Linux CLI

sleep 30 &
jobs
fg %1

A [1]+ Done sleep 30

B [1]+ Stopped sleep 30

C [1]- Running sleep 30 &

D [1]+ Running sleep 30 &

💡 Hint

Think about what happens when a command is run in the background with & and how jobs lists them.

When you run 'sleep 30 &' the job runs in the background and is marked as 'Running'. The jobs command shows it with a plus sign for the current job and the '&' indicates background.

💻 Command Output

intermediate

What happens after running bg on a stopped job?

You start a command and stop it with Ctrl+Z. Then you run bg %1. What will be the job status if you run jobs immediately after?

Linux CLI

sleep 30
# Press Ctrl+Z to stop
bg %1
jobs

A [1]+ Stopped sleep 30

B [1]+ Running sleep 30 &

C [1]+ Done sleep 30

D [1]+ Terminated sleep 30

💡 Hint

bg resumes a stopped job in the background.

The bg command resumes the stopped job in the background, so jobs will show it as Running with an ampersand.

📝 Syntax

advanced

Which command correctly brings job 2 to the foreground?

You have multiple jobs running. Which of these commands correctly brings job number 2 to the foreground?

A fg %2

B fg job2

C fg 2

D fg #2

💡 Hint

Jobs are referenced by a percent sign followed by the job number.

The correct syntax to bring job number 2 to foreground is 'fg %2'. 'fg 2' is invalid because it lacks the percent sign. 'fg job2' and 'fg #2' are invalid syntaxes.

🔧 Debug

advanced

Why does this bg command fail with 'no such job' error?

You run:
sleep 100 &
bg %1

But get the error: 'bg: no such job'. Why?

Linux CLI

sleep 100 &
bg %1

A Because the percent sign is missing in the command.

B Because the job number 1 does not exist yet.

C Because bg requires the job to be stopped, not running.

D Because the job is already running in the background, bg cannot resume it.

💡 Hint

bg only works on stopped jobs.

The bg command resumes stopped jobs in the background. If the job is already running, bg cannot resume it and gives 'no such job' error.

🚀 Application

expert

How to bring the most recent stopped job to foreground using a single command?

You have multiple jobs stopped and running. Which command brings the most recent stopped job to the foreground?

A fg %+

B fg %%

C fg %-

D fg %1

💡 Hint

The %+ symbol refers to the current job.

In job control, '%+' refers to the current job, which is usually the most recent stopped or background job. Using 'fg %+' brings it to foreground.

Practice

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

easy

A. Brings a background job to the foreground for interaction

B. Stops a running job immediately

C. Lists all running jobs

D. Terminates a background job

Solution

1. Step 1: Understand the purpose of fg

   The fg command is used to bring a job running in the background to the foreground so you can interact with it directly.

2. Step 2: Compare with other commands

   jobs lists jobs, bg resumes jobs in background, and stopping or terminating jobs use other commands.

3. Final Answer:

   Brings a background job to the foreground for interaction -> Option A

4. Quick Check:

   fg = bring job to foreground [OK]

Hint: Remember: fg = foreground, bg = background [OK]

Common Mistakes:

• Confusing fg with bg
• Thinking fg lists jobs
• Assuming fg stops jobs

2. Which of the following is the correct syntax to resume a stopped job in the background?

easy

A. fg %1

B. bg %1

C. jobs %1

D. stop %1

Solution

1. Step 1: Identify the command to resume a stopped job in background

   The bg command resumes a stopped job in the background, allowing the terminal to be free.

2. Step 2: Understand job specification

   Using %1 specifies job number 1. So bg %1 resumes job 1 in background.

3. Final Answer:

   bg %1 -> Option B

4. Quick Check:

   bg %jobnumber resumes job in background [OK]

Hint: Use bg with %jobnumber to resume in background [OK]

Common Mistakes:

• Using fg instead of bg to resume in background
• Trying to use jobs command to resume
• Using stop which is invalid

3. Given the following commands run in a terminal:

sleep 100 &
jobs
fg %1

What will happen after running fg %1?

medium

A. The sleep command will continue running in the background

B. An error will occur because %1 is invalid

C. The sleep command will be terminated

D. The sleep command will be brought to the foreground and block the terminal

Solution

1. Step 1: Analyze the commands

   sleep 100 & runs sleep in background. jobs lists it as job 1. fg %1 brings job 1 to foreground.

2. Step 2: Understand effect of fg

   Bringing sleep to foreground means it blocks terminal until it finishes or is stopped.

3. Final Answer:

   The sleep command will be brought to the foreground and block the terminal -> Option D

4. Quick Check:

   fg %1 = foreground job blocks terminal [OK]

Hint: fg brings job to foreground and blocks terminal [OK]

Common Mistakes:

• Thinking fg keeps job in background
• Assuming fg terminates job
• Believing %1 is invalid without jobs output

4. You ran a command in the background with sleep 200 & but accidentally typed fg 1 instead of fg %1. What will happen?

medium

A. The job will be brought to foreground successfully

B. The job will continue running in background silently

C. An error message will appear: 'fg: 1: no such job'

D. The shell will terminate

Solution

1. Step 1: Understand job specification syntax

   The fg command requires job IDs to be prefixed with %, e.g., fg %1.

2. Step 2: Analyze incorrect command

   Typing fg 1 without % causes shell to look for a job named '1' which does not exist, causing an error.

3. Final Answer:

   An error message will appear: 'fg: 1: no such job' -> Option C

4. Quick Check:

   Job IDs need % prefix for fg/bg [OK]

Hint: Always prefix job number with % in fg/bg commands [OK]

Common Mistakes:

• Omitting % before job number
• Assuming fg accepts plain numbers
• Ignoring error messages

5. You have two stopped jobs listed by jobs as:

[1]+  Stopped                 nano file1.txt
[2]-  Stopped                 vim file2.txt

How do you resume vim file2.txt in the background and then bring nano file1.txt to the foreground?

hard

A. bg %2 followed by fg %1

B. fg %2 followed by bg %1

C. bg %1 followed by fg %2

D. fg %1 followed by fg %2

Solution

1. Step 1: Resume vim in background

   Use bg %2 to resume job 2 (vim) in background so it runs without blocking terminal.

2. Step 2: Bring nano to foreground

   Use fg %1 to bring job 1 (nano) to foreground for interaction.

3. Final Answer:

   bg %2 followed by fg %1 -> Option A

4. Quick Check:

   bg %job runs in background, fg %job runs in foreground [OK]

Hint: Use bg for background, fg for foreground with correct job IDs [OK]

Common Mistakes:

• Mixing up job numbers
• Using fg for both jobs
• Not using % before job number