How to Use the Top Command in Linux

Introduction

The top command in Linux displays real-time system information, including running processes, active users, and resource utilization such as CPU and memory. You can use top to monitor, troubleshoot, and identify performance issues in your system.

This article explains how to use the top command in Linux.

The top Command Syntax

The following is the basic syntax of the Linux top command:

top [options]

In the above command, options modify the command's behavior to customize the output.

Most Common Linux top Command Options

The following are the most common Linux top command options:

Check out how to use the tar command in Linux for quick file archiving and unzipping.

Understand the Linux top Command Interface

The top command interface continuously displays a list of processes and information about CPU usage, memory usage, load averages, and other system statistics by default. The list shows the most resource-intensive processes at the top.

Run the top command without any options to access the default interface to view the system processes, CPU, and memory usage.

$ top

Output:

Press Ctrl or Q to exit the interface.

Sections of a top Command Interface

The top command interface has the following sections:

• System Information: Displays general information about the system, including uptime, number of active users, load averages, and number of running processes.

  For example:

  top - 15:24:15 up 2 days,  4:36,  2 users,  load average: 0.08, 0.09, 0.10

• Task Summary: Displays a summary of the total active tasks including the number of processes and their status.

  For example:

  Tasks: 142 total,   1 running, 141 sleeping,   0 stopped,   0 zombie

• System Process List: Displays detailed information about each process, including the PID, user, CPU usage, memory usage, and command information.

  For example:

  PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+  COMMAND
  1234 root      20   0  123M  45M  15M S   0.3  2.5   0:01.23 process_name

  Within the above section, each header represents a specific value as described below:

  • PID: The unique process ID of each task.
  • PR: The system process priority, a lower number represents a higher priority.
  • VIRT: Total virtual memory a process uses.
  • USER: The system user that owns the running process.
  • %CPU: The percentage of CPU a process has consumed.
  • SHR: The amount of shared memory a process uses in KBs (Kilobytes).
  • NI: The niceness value that prioritizes processes for CPU use. A negative value represents a higher priority, while a positive value represents a lower priority.
  • %MEM: Displays the memory a process uses in percentage.
  • RES: Displays the amount of resident memory used by the process in KBs (Kilobytes).
  • TIME+: Displays the total CPU time a process has used since it started.
  • COMMAND: Shows the command that started a process.

• CPU Usage: Displays the CPU usage statistics including the time spent on system processes and the average Input/Output (I/O) processing time.

  For example:

  %Cpu(s):  2.3 us,  1.5 sy,  0.0 ni, 95.6 id,  0.4 wa,  0.0 hi,  0.2 si,  0.0 st

  In the above output:

  • us: Time spent on user processes.
  • sy: Time spent on system (kernel) processes.
  • id: Idle time.
  • wa: Time spent waiting for I/O operations to complete.

• Memory Usage: Displays the system's memory usage statistics, including the used and available memory.

  For example:

  MiB Mem :   8000.0 total,   2048.0 free,   4500.0 used,   1452.0 buff/cache
  MiB Swap:   2048.0 total,   2048.0 free,      0.0 used.   3060.0 avail Mem

  In the above output:

  • total: Total available memory.
  • used: Used memory.
  • free: Free memory available for processes.
  • buff/cache: Memory used for buffers and cache.

Run top Command in Linux with Examples

This section uses practical examples to show you how to use the Linux top command. Follow the steps below to run the samples.

1. Pass the -c flag to display the process command name instead of arguments making the processes list more readable.

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   $ top -c
   

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2. Set the delay interval between updates in seconds. For example, 5 seconds.

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   $ top -d 5
   

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   Output:

   

3. Monitor specific process IDs. For example, 1234 and 5678.

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   $ top -p 1234,5678
   

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4. Run top with a specific number of iterations before exiting. For instance, 10 iterations.

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   $ top -n 10
   

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5. Run the top command in batch mode and forward the interface's output to a file such as top_output.txt.

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   $ top -b -n 1 > top_output.txt
   

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   Output:

   

6. Display specific user's processes. Replace username with the target user.

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   $ top -u username
   

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   Output:

   

7. Ignore and hide idle and zombie processes from the top interface.

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   $ top -i
   

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8. Sort processes by memory usage in descending order.

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   $ top -o %MEM
   

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   Output:

   

9. Display individual threads.

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   $ top -H
   

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Use the top Command Interactively

Customize the top command interface with the following options to view and manage processes:

Use Advanced Linux top Command Options

The Linux top command supports more advanced options to customize the output, as illustrated below:

1. Run top in secure mode to hide or mask certain information. For instance, hide the commands for each process.

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   $ top -s
   

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   Output:

   

2. Press V within the top interface to view processes in a tree structure.

   

3. Press Z on your keyboard to modify the display color.

   

   Follow the steps below for additional customizations and color selections.

   • Press Z to access the color settings page.

     

   • Press any of the following keys to modify the target display field's color:

     • S: Summary data area.
     • M: Messages and prompts area.
     • H: Heading columns.
     • T: Information about tasks in the process list.

   • Select a target color to apply on the display field:

     • 0 = black
     • 1 = red
     • 2 = green
     • 3 = yellow
     • 4 = blue
     • 5 = magenta
     • 6 = cyan
     • 7 = white

     Press Enter to confirm your selection and apply changes to your top interface.

     

4. Use the following keys to customize the top command interface:

   • f: Displays the top configuration screen to toggle columns on or off
   • Up + Down Navigate through the configurations list.
   • Space + d: Select or deselect columns.

   

5. Run the top command and press P to monitor and sort processes by CPU usage.

   

6. Run the top command and press M to find memory hogs and sort processes by memory usage.

   

7. Run the top command, press K, and enter a signal to kill a process using the PID.

   

Conclusion

You have used the top command in Linux to monitor system processes. The top command is a powerful tool for monitoring system performance and managing active processes in real time. Use the tool to manage, troubleshoot, and identify performance bottlenecks in your server. For more information, run the man top command.