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

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this command?

You run the command ps -e | grep ssh. What does this command do?

Linux CLI

ps -e | grep ssh

A Starts the ssh service if it is not running.

B Lists only ssh processes without showing any other processes.

C Shows the ssh service status without listing processes.

D Lists all running processes and filters to show only those with 'ssh' in their name or details.

💡 Hint

Think about what 'ps -e' and 'grep ssh' do separately and then combined.

The command 'ps -e' lists all running processes. Piping it to 'grep ssh' filters the list to only show lines containing 'ssh'.

💻 Command Output

intermediate

What does this command output?

What is the output of ps -u root?

Linux CLI

ps -u root

A Lists all processes owned by the user 'root'.

B Lists all processes except those owned by 'root'.

C Shows system-wide CPU usage statistics.

D Displays the current user's processes only.

💡 Hint

The '-u' option filters processes by user.

The command lists all processes that belong to the user 'root'.

📝 Syntax

advanced

Which command correctly lists all processes with full command line?

You want to see all running processes with their full command lines. Which command is correct?

A ps auxww

B ps -e -o cmd

C ps -ef

D ps -u all

💡 Hint

Look for options that show all users and full command lines.

The 'ps auxww' command lists all processes with detailed info and full command lines. 'ps -ef' shows all processes but may truncate commands. 'ps -e -o cmd' shows commands but not full details. 'ps -u all' is invalid.

🔧 Debug

advanced

Why does this command fail?

You run ps -u without specifying a user and get an error. Why?

Linux CLI

ps -u

A Because '-u' is not a valid option for ps.

B Because '-u' requires a username argument and none was given.

C Because ps needs root privileges to run '-u'.

D Because the system has no users defined.

💡 Hint

Check the man page for the '-u' option.

The '-u' option requires a username to filter processes. Without it, ps shows an error.

🚀 Application

expert

How many processes are running under the current user?

Write a command that counts how many processes are running under your current user and outputs only the number.

A ps -u $(whoami) | wc -l

B ps -U $(whoami) | grep -c .

C ps -u $(whoami) | tail -n +2 | wc -l

D ps aux | grep $(whoami) | wc -l

💡 Hint

Remember the header line in ps output and how to exclude it.

Option C correctly lists processes for current user, skips the header line with 'tail -n +2', then counts lines. Option C counts header too. Option C uses '-U' which is similar but may differ by system. Option C counts all lines containing username, including grep itself.

Practice

1. What does the ps command do in Linux?

easy

A. It shows the disk usage of files and folders.

B. It lists the currently running processes on the system.

C. It displays the network connections.

D. It updates the system software.

Solution

1. Step 1: Understand the purpose of ps

   The ps command is used to show processes running on the system.

2. Step 2: Compare with other options

   Disk usage is handled by du, network connections by netstat or ss, and software updates by apt or yum.

3. Final Answer:

   It lists the currently running processes on the system. -> Option B

4. Quick Check:

   ps shows running processes [OK]

Hint: Remember: ps = process status, shows running programs [OK]

Common Mistakes:

• Confusing ps with disk or network commands
• Thinking ps updates software
• Assuming ps shows file sizes

2. Which of the following is the correct syntax to list all processes with full details using ps?

easy

A. ps -l

B. ps -xv

C. ps auxf

D. ps -ef

Solution

1. Step 1: Identify the common option for full details

   The -ef option with ps lists all processes with full details.

2. Step 2: Check other options

   ps -xv is an invalid combination, ps auxf displays a tree view rather than a plain list, and ps -l shows limited info for current processes.

3. Final Answer:

   ps -ef -> Option D

4. Quick Check:

   ps -ef shows all processes fully [OK]

Hint: Use ps -ef for full process list [OK]

Common Mistakes:

• Using ps -xv which is invalid
• Confusing ps auxf as a single option
• Using ps -l which shows limited info

3. What will be the output of the command ps -e | grep sshd if the sshd process is running?

medium

A. It will show the sshd process details in the list.

B. It will show an error because grep cannot be used with ps.

C. It will show all processes except sshd.

D. It will show nothing because sshd is filtered out.

Solution

1. Step 1: Understand ps -e and pipe to grep

   ps -e lists all processes; piping to grep sshd filters lines containing 'sshd'.

2. Step 2: Interpret the output if sshd is running

   If sshd is running, its process line will appear in the output.

3. Final Answer:

   It will show the sshd process details in the list. -> Option A

4. Quick Check:

   ps -e | grep sshd shows sshd if running [OK]

Hint: Pipe ps output to grep to find specific processes [OK]

Common Mistakes:

• Thinking grep causes error with ps
• Assuming grep excludes matching lines
• Believing sshd won't appear if running

4. You run ps -ef | grep but get an error. What is the likely cause?

medium

A. You forgot to provide a search pattern after grep.

B. The ps command does not support pipes.

C. The -ef option is invalid for ps.

D. grep cannot be used with ps output.

Solution

1. Step 1: Analyze the command structure

   ps -ef outputs all processes; piping to grep requires a pattern to search.

2. Step 2: Identify the error cause

   Without a pattern, grep throws an error because it expects a search term.

3. Final Answer:

   You forgot to provide a search pattern after grep. -> Option A

4. Quick Check:

   grep needs a pattern after it [OK]

Hint: Always give grep a pattern to avoid errors [OK]

Common Mistakes:

• Thinking ps can't pipe output
• Believing -ef is invalid
• Assuming grep works without a pattern

5. You want to list all processes owned by user 'alice' with detailed info. Which command should you use?

hard

A. ps aux | grep alice

B. ps -ef | grep alice

C. ps -u alice -f

D. ps -U alice

Solution

1. Step 1: Understand options for user filtering

   ps -u alice lists processes for user alice; -f adds full details.

2. Step 2: Compare other options

   Using ps -ef | grep alice or ps aux | grep alice relies on grep which may match username in other fields, not just owner. ps -U alice lists processes by real user ID but lacks full details.

3. Final Answer:

   ps -u alice -f -> Option C

4. Quick Check:

   ps -u alice -f lists alice's processes fully [OK]

Hint: Use ps -u username -f for detailed user processes [OK]

Common Mistakes:

• Using grep which may show unrelated lines
• Using -U without details
• Confusing -u and -U options