Top Command in Linux: Monitor CPU, Memory, and Processes in Real Time

Table of Contents

• What is the top Command?
• How to Launch and Exit top
• Understanding the Summary Area (The Header)
• Decoding the Process List (The Columns)
• Essential Interactive Keyboard Shortcuts
• Top Command Line Options
• 20 Frequently Asked Questions (FAQ)

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What is the top Command in linux?

Every aspiring system administrator, developer, or power user should master the top command in linux . The command top in linux acts as a first responder in determining if your system is sluggish and/or which processes are consuming the maximum amount of memory (RAM) on your system.

This article will provide a step-by-step approach to using the Linux top command, enabling you to move from a novice user of the command line to a system monitoring expert.

How to Launch and Exit top

To start the utility, simply open your terminal and type:

top

To exit the interface and return to your terminal prompt, simply press the q key on your keyboard.

Understanding the Summary Area

A comprehensive view of the actual/current health of your environment is provided by the first five rows of the top window.

1. System Uptime + Load Average
   • System Time: The current time on the clock.
   • Up Time: The time elapsed since the last system reboot.
   • Load Average: This three-part figure depicts the average system load for the last one, five or 15 minutes, respectively.

   Please Note: If the load average exceeds the total number of CPU cores, you are probably putting too much load onto your system and queuing.

2. Tasks

   This describes the total number of processes and their respective states, which include Running, Sleeping, Stopped or Zombie.

3. CPU Status

   The new view of your processor’s time breakdown includes us (user-level applications) and sy (kernel/system-level tasks).

Decoding the Process List

Below the header is the list of individual processes. Here are the most important columns:

Column	Description
PID	Process ID: A unique number assigned to every task.
USER	The owner of the process.
PR / NI	Priority and “Nice” value (Lower numbers = higher priority).
RES	Resident Memory: Actual physical RAM used.
%CPU	Percentage of CPU time used since the last update.

Essential Interactive Keyboard Shortcuts

While top is running, you can use these keys to manipulate the view:

• h: Display the help screen.
• k: Kill a process (you will be prompted for the PID).
• M: Sort the list by Memory usage.
• P: Sort the list by CPU usage (the default view).
• 1: Toggle between a single CPU average and showing individual CPU cores.
• z: Toggle color mode to make the interface more readable.

Top Command Line Options

You can change how top starts by using flags:

• top -d [seconds]: Change the refresh interval (e.g., top -d 1 for 1 second).
• top -u [username]: Monitor only processes belonging to a specific user.
• top -p [PID]: Monitor only a specific process ID.

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20 Frequently Asked Questions (FAQ)

1. How do I save my top in linux configuration?

Press W (Shift + w) while running to save your current view to ~/.toprc.

2. Why is my CPU usage over 100%?

In top, 100% represents one full CPU core. On a 4-core system, you can see up to 400% usage.

3. What is a “Zombie” process?

A process that has finished execution but still has an entry in the process table. It doesn’t consume RAM but indicates a bug in the parent program.

4. How do I see the full command path?

Press c while top is running.

5. How do I sort by memory using top in linux?

Press Shift + M.

6. Can I use top in linux to kill a process without sudo?

Only if you own the process. For system processes, you must run sudo top.

7. What is the difference between VIRT and RES?

VIRT is total memory the process can access; RES is the actual physical RAM it is currently using.

8. How do I change the update speed?

Press s and enter the number of seconds.

9. How do I show only active processes?

Press i to toggle the “idle” filter; this hides processes that aren’t doing anything.

10. What does “load average” mean?

It’s the average number of processes waiting for CPU time or disk I/O over 1, 5, and 15 minutes.

11. How do I highlight the sorted column?

Press x to turn on highlighting for the column currently used for sorting.

12. Is there a better version of top?

Many prefer htop for its color and mouse support, but top is installed on every Linux system by default.

13. How do I see threads using top in linux?

Press H to toggle thread monitoring.

14. How do I filter by a specific process name using top command in linux?

Press o and type a filter like COMMAND=python.

15. What is “NI” (Nice value)?

A range from -20 to 19. A higher number makes the process “nicer,” letting others use the CPU first.

16. How do I toggle the summary area using top in linux?

Use l, t, and m to toggle the load, task, and memory headers.

17. Can I use my mouse in top?

Standard top does not support the mouse. You should use htop if you need that feature.

18. How do I run top in a script?

Use top -b -n 1 > output.txt to capture a single snapshot in plain text.

19. What does “st” (Steal Time) mean?

In virtual machines, it’s the time the physical CPU was taken by the hypervisor to serve other VMs.

20. How do I display memory in MB or GB using top in linux?

Press E for the summary area and e for the process list to cycle through units.

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