top Command in Linux

Introduction

In this tutorial, we will delve into the Linux `top` command, an indispensable tool for any systemadmin seeking to monitor system performance and resource allocation. Our journey begins with grasping the core functionality of `top`, including its real-time display of running processes, CPU load, memory consumption, and other vital system resources. We will then explore a range of options and customizations that allow you to tailor the output of `top` to precisely fit your monitoring requirements, ultimately empowering you to analyze system performance efficiently and pinpoint potential bottlenecks.

Understand the top Command

This section focuses on the `top` command, a vital asset for system administrators who need to track system performance and resource usage within a Linux environment. The `top` command gives you a dynamic, real-time view of processes, CPU, memory, and other essential system metrics.

Let's begin by executing the `top` command:

$ top

Example output:

top - 14:22:37 up 1 day, 12:34,  0 users,  load average: 0.00, 0.01, 0.05
Tasks: 101 total,   1 running,  99 sleeping,   1 stopped,   0 zombie
%Cpu(s):  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
MiB Mem :   1992.0 total,   1537.0 free,    235.0 used,    220.0 buff/cache
MiB Swap:   2047.9 total,   2047.9 free,      0.0 used.   1555.3 avail Mem

   PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
     1 root      20   0    8788   5688   3828 S   0.0   0.3   0:02.14 systemd
     2 root      20   0       0      0      0 S   0.0   0.0   0:00.01 kthreadd
     3 root      20   0       0      0      0 I   0.0   0.0   0:00.00 rcu_gp
     4 root      20   0       0      0      0 I   0.0   0.0   0:00.00 rcu_par_gp
     5 root      20   0       0      0      0 I   0.0   0.0   0:00.00 slub_flushwq

The `top` command presents a wealth of system information, including:

  • Uptime: Indicates how long the system has been running since the last reboot.
  • Tasks: Displays a breakdown of the total number of processes categorized as running, sleeping, stopped, or zombie.
  • CPU utilization: Shows the distribution of CPU time across various modes, such as user, system, nice, idle, wait, hardware interrupts, and software interrupts.
  • Memory usage: Provides a snapshot of total, free, used, and buffered/cached memory.
  • Swap usage: Shows the total, free, and used swap space on the system.
  • Process list: A dynamically updated list of the most resource-demanding processes, typically sorted by CPU or memory consumption.

You can interact with the `top` command and customize its output using various key presses, such as:

  • h: Displays a comprehensive help menu outlining available commands.
  • 1: Toggles the display between a single, aggregate CPU utilization view and a per-CPU utilization breakdown.
  • f: Allows you to manage which columns are displayed in the process list.
  • o: Enables customization of the sorting order for the process list.
  • u: Filters the displayed processes to show only those belonging to a specific user.

To exit the `top` command and return to the shell, press q.

Explore top Command Options and Customizations

This section will guide you through the various command-line options and interactive customizations available within the `top` command, empowering you to tailor the output to meet your specific monitoring needs.

Let's begin by displaying the help menu to see the available options:

$ top -h

This will provide a list of all command-line options and a brief description of their functionality. Some frequently used options include:

  • -d: Allows you to specify the delay interval (in seconds) between updates of the display. The default is 3 seconds.
  • -n: Sets the number of iterations that `top` will run before automatically exiting. The default is to run indefinitely.
  • -p: Restricts the monitoring to specific process IDs (PIDs).
  • -u: Displays only the processes owned by a particular user.
  • -o: Allows you to sort the process list based on a specific field or column.

For example, to display the top 5 processes, sorted according to their memory usage, you would use the following command:

$ top -o %MEM -n 5

Example output:

top - 14:30:37 up 1 day, 12:42,  0 users,  load average: 0.00, 0.00, 0.00
Tasks: 101 total,   1 running,  99 sleeping,   1 stopped,   0 zombie
%Cpu(s):  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
MiB Mem :   1992.0 total,   1537.0 free,    235.0 used,    220.0 buff/cache
MiB Swap:   2047.9 total,   2047.9 free,      0.0 used.   1555.3 avail Mem

   PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
     1 root      20   0    8788   5688   3828 S   0.0   0.3   0:02.14 systemd
     2 root      20   0       0      0      0 S   0.0   0.0   0:00.01 kthreadd
     3 root      20   0       0      0      0 I   0.0   0.0   0:00.00 rcu_gp
     4 root      20   0       0      0      0 I   0.0   0.0   0:00.00 rcu_par_gp
     5 root      20   0       0      0      0 I   0.0   0.0   0:00.00 slub_flushwq

You can also interactively customize the displayed columns by pressing the f key while `top` is running. This allows you to add, remove, or rearrange columns to better suit your analysis.

To exit the `top` command, press q.

Analyze System Performance Using top

In this final section, we'll leverage the `top` command to analyze system performance, identify potential bottlenecks, and diagnose resource-related issues.

Let's begin by monitoring overall system health using the basic `top` command:

$ top

Pay close attention to these key metrics:

  • CPU utilization: Track the percentages of CPU time spent in various states (user, system, idle, etc.). High CPU utilization often signals a CPU-bound process or application.
  • Memory usage: Monitor total, free, used, and buffered/cached memory. High memory usage may indicate a memory leak or memory-intensive application.
  • Swap usage: Observe the amount of used swap space. Excessive swap usage is typically an indicator of insufficient physical memory.
  • Process list: Examine the processes consuming the most resources (CPU or memory). Identify any processes exhibiting abnormally high resource consumption.

To delve deeper into specific processes, you can use these interactive commands within `top`:

  • k: Allows you to kill a process by specifying its PID. Use with caution, especially when dealing with processes owned by root.
  • r: Allows you to renice a process, adjusting its priority to either increase or decrease its access to system resources.
  • f: Manage the columns displayed to show relevant metrics.
  • o: Customize the sort order based on different metrics.

For example, to display the top 5 processes consuming the most CPU:

$ top -o %CPU -n 5

Example output:

top - 14:36:37 up 1 day, 12:48,  0 users,  load average: 0.00, 0.00, 0.00
Tasks: 101 total,   1 running,  99 sleeping,   1 stopped,   0 zombie
%Cpu(s):  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
MiB Mem :   1992.0 total,   1537.0 free,    235.0 used,    220.0 buff/cache
MiB Swap:   2047.9 total,   2047.9 free,      0.0 used.   1555.3 avail Mem

   PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
     1 root      20   0    8788   5688   3828 S   0.0   0.3   0:02.14 systemd
     2 root      20   0       0      0      0 S   0.0   0.0   0:00.01 kthreadd
     3 root      20   0       0      0      0 I   0.0   0.0   0:00.00 rcu_gp
     4 root      20   0       0      0      0 I   0.0   0.0   0:00.00 rcu_par_gp
     5 root      20   0       0      0      0 I   0.0   0.0   0:00.00 slub_flushwq

This allows you to quickly identify CPU-intensive processes that may be impacting system performance.

To exit the `top` command, press q.

Summary

In this lab, we explored the `top` command, a vital tool for monitoring system performance and resource utilization on Linux systems. We learned how to interpret the key system metrics displayed by `top`, including uptime, task statistics, CPU and memory usage, and the process list. Furthermore, we discovered how to customize the `top` command's output using various interactive commands, allowing us to toggle between per-CPU and aggregate CPU views, manage the displayed columns, and customize the sorting order of the process list. This empowers system administrators to effectively monitor and troubleshoot performance issues on their Linux servers.

400+ Linux Commands