Home Linux ServerLinux Fundementals Instructions for analyzing and explaining the TOP command in Linux

Instructions for analyzing and explaining the TOP command in Linux

by Thạch Phạm

The top command in Linux is used to display all the processes running in the Linux environment. This tutorial will show you how to use the top command by explaining the different parameters available and what they display.

What is the Top command in Linux?

In Linux, there is a command you will most often use to monitor the status of server information, which is top. This command will help you to see an overview of the server, such as the percentage of CPU used, the overload through the load average parameter, the number of active processes and much other useful information.

How to use the top command

Basically, all you need to do to display the currently running processes is to type in the terminal the following:

top

Important information of the top command

A lot of information will be displayed when you use the top command, I will explain all this information in an easy-to-understand and simple way.

The following information is displayed when you run the top command in Linux:

Line 1

  • Current system time
  • Uptime
  • Number of users
  • Load average
  • The load average shows the system load time for the last 1 minute, 5 minutes and the last 15 minutes.

The load average for each system will be different. If your Server/VPS has 2 CPU(s), the load average should be less than 2. Similarly, if Server/VPS has 40 CPU(s), the load average should be less than 40. If this number is higher than the number of your CPU, the system is overloaded, the amount of processing work exceeds what the current CPU can handle.

Line 2

  • Total number of tasks available on the server
  • Number of running tasks
  • Number of tasks in the “sleeping” state
  • Number of stopped tasks
  • Number of zombie tasks (progress does not exist or is broken)

Line 3

  • CPU usage by users as a percentage
  • CPU usage by system as a percentage
  • CPU usage by low priority processes as a percentage
  • CPU usage by idle process (process indicates the processor is idle) in percentage
  • CPU usage by io wait (time spent in idle CPU waiting for disk I/O to complete) as a percentage
  • CPU usage by hardware interrupt in percentage
  • CPU usage by a software interrupt in percentage
  • CPU usage by steal time (the time the virtual CPU “waits” for the real CPU, while the hypervisor is serving another virtual processor) as a percentage

Line 4

  • Total system memory (in KiB units)
  • Free memory
  • Used memory
  • Buffer cache
Line 5
  • Total available swap (in KiB)
  • Total swap available
  • Total swap used
  • Available memory

Swap is virtual RAM used when physical memory (RAM) is full. Always enable Swap so that when the system is full of physical RAM, it will not hang.

Currently, I am turning off Swap (not recommended), to re-enable swap you use the command:

swapon -a

In some cases, your swap used parameter will increase when the RAM is full, and when the RAM is no longer full, the amount of swap will not decrease. To reduce the swap used to 0, you use the following 2 commands:

swapoff -a

swapon -a

You wait for the swapoff -a command to finish running and then run the swapon -a command, the time is long or short, depending on how much swap you have used. During this process, the swap used will increase to 100%.

I turned off swap because I finished using the command swapoff -a but forgot to run swapon -a , please don’t be like me.

Main board

  • Process ID
  • Users (I have hidden some users, but the default is not empty)
  • Priority level
  • Nice level (calls a shell script with a specific priority)
  • Virtual memory used by the process
  • The “resident” memory that a process uses (i.e. the process is always in memory and cannot be moved to another storage device)
  • Shareable memory
  • CPU used by the process as a percentage of 1 CPU.
  • Memory used by the process as a percentage
  • The time the process was run
  • Command

Keep the top command running in the background on Linux computers

You can easily keep the top command running without having to enter the command into the Terminal every time you turn on your computer.

To pause the top command and resume using the terminal, press CTRL + Z on your keyboard.

To bring the top command back to the foreground, type fg.

The main parameters for the top command

ParameterNameDescribe
-hHelpDisplay the content of the Top command instruction
-vVersionView the version of the library in use
-bBatch ModeUsing the top command with batch mode allows sending the returned content of the command to another file or to another application
-cToggle command line/program name
Progress displayed in the COMMAND column in the top command will be displayed as a command instead of a program name. Or switch back and forth while viewing the top command with the c key.
-dDelay time intervalSpecify the refresh time of the top command interface.
-e [k | m | g | t | p]task memory scalingOption to display the unit of memory in use, you can optionally display k(kibibytes), m(mebibytes), g(gibibytes), t(tebibytes), p(pebibytes), e(exbibytes).
-uMonitor user IDs modeView command top with specified user processes
-pMonitor process IDs modeView command top with the specified process ID
-iIdle process toggleDon’t show idle processes

Show current version

Enter the following to display the current version details for the top command:

top -h

The output will be as follows:

procps -ng version 3.3.10

Specify the time delay between screen refreshes

To specify the delay between screen refreshes while using top, you can enter the following command:

top -d

To refresh the screen every 5 seconds, you can enter:

top -d 5

Get a list of columns to sort by order

Get a list of columns where you can sort the top command by typing the following:

top -O

There are a lot of columns listed, so you can reduce the output as:

top -O | less

Sort columns in the top command by column name

Use the command in the previous section to find a sortable column, and then use the following syntax to sort by that column:

top -o

To sort by % CPU, type the following command:

top -o %CPU

Show only processes for a specific user

To show only processes that a specific user is running, use the following syntax:

top -u

For example, to show all the processes the user is running, you can run following command:

top -u gary

Hide Idle Task

The default view in the top command can be a bit messy, and if you only want to see active processes (i.e., those that are not idle), you can run the following command:

top -i

Add the column displayed with the top command:

While running the top command, you can press the F key to display the list of fields that can be displayed in the table:

  • Use the arrow keys to move up and down the list of fields.
  • To display a field on the screen, press the D key. To delete the field, press D on the field again. An asterisk (*) will appear next to the displayed fields.
  • You can sort the table by field by pressing the S key on the field you want to sort.
  • Press the Enter key to confirm the change and press Q to exit.

Switch between modes

While running the top command, you can press the A key to switch between the standard and alternate screens.

Change color

Press the Z key to change the color of the values in the top command.

There are three stages required to change color:

  1. Press S for summary data, M for notifications, H for column headers, or T for task information to target the area to change color.
  2. Select a color for the specified target: 0 is black, 1 is red, 2 is green, 3 is yellow, 4 is blue, 5 is magenta, 6 is cyan and 7 is white
  3. Press Enter to confirm the change

Press the B key to bold the text.

Change display in when the top command is running

While the top command is running, you can enable/disable many features by pressing the relevant keys.

The following table shows the key to press and the function it provides:

Function keysDescribe
AAlternate display (default off)
dRefresh the screen after the specified delay, in seconds (default is 1.5 seconds)
pPID monitoring (default off), display all processes
BBold enabled (default on), values are displayed in bold text
lShow load average (default on)
tDefine how tasks are displayed (default 1 + 1)
mDefines how memory usage is displayed (default 2 lines)
1Single CPU (default off) – i.e., display for multiple CPUs
JAlign numbers to the right (default on)
jAlign text to the right (default off)
RReverse Sort (default on) – From highest processes to lowest processes
SCumulative time (default off)
uUser filter (default off), show only euid
UUser filter (default off), show any uid
VForest view (on by default), displayed as branches
xHighlight column (default off)
zMulticolor or monochrome display (default on)

CThere are even more parameters for the top command. You can read more about them by typing the following in a terminal window:

man top

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