ps (list processes) in Linux CLI - Mini Project: Build & Apply

Listing and Filtering Processes with ps

📖 Scenario: You are managing a Linux system and want to see which programs are running. You will use the ps command to list processes and filter them.

🎯 Goal: Learn to use the ps command to list all running processes, filter by user, and display specific columns.

📋 What You'll Learn

Use the ps command to list processes

Filter processes by user

Display specific columns like PID and command

Use command options correctly

💡 Why This Matters

🌍 Real World

System administrators often need to check running processes to monitor system health or troubleshoot issues.

💼 Career

Knowing how to list and filter processes is a basic skill for Linux system administrators and developers working on servers.

1

List your current processes

Type the command ps to list the processes running in your current shell session.

Linux CLI

# Type the command 'ps' to list current shell processes
# Your code here

Hint

The ps command without options shows processes in your current shell.

2

List all processes for your user

Type the command ps -u $USER to list all processes owned by your user account.

Linux CLI

ps
# Now list all processes for your user with 'ps -u $USER'
# Your code here

Hint

The -u option followed by your username shows all your user processes.

3

Show only PID and command columns

Use ps -u $USER -o pid,comm to list your user processes showing only the PID and command name columns.

Linux CLI

ps
ps -u $USER
# Add the option '-o pid,comm' to show only PID and command
# Your code here

Hint

The -o option lets you choose which columns to display.

4

Print the final command output

Run the command ps -u $USER -o pid,comm and print its output to see your user processes with PID and command columns.

Linux CLI

ps
ps -u $USER
ps -u $USER -o pid,comm
# Run the last command to display output
# Your code here

Hint

The output should show a header with 'PID' and 'COMMAND' and list your processes.

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