The top command offers a quick overview of system information. It reports data about current processes which is refreshed (by default) every 3 seconds.
Using and understanding a program like top is an essential system administration skill. In this article, I will explain the basic commands and teach you how to navigate top with confidence.
top
Top is straight-forward. This simple command will bring you into the top application. When you want to exit, type the letter q to quit. There are also other key commands that we will discuss later.
You can easily check CPU load, the number of current tasks, as well as viewing information about memory usage. You'll notice that while running there are essentially 2 sections with information.
At the top of our terminal, we get our overview data including memory usage and cpu load. Below that, we have a table with specific processes with plenty of details.
There is a lot of detailed information displayed in a condensed format. Let's break it into smaller pieces so that we have a more complete understanding. The first four lines contain the overview data I mentioned before. Let's take them line by line and explain what's going on.
top - 03:37:31 up 24 min, 1 user, load average: 0.00, 0.01, 0.07
The first piece of information is a timestamp, which is pretty self-explanatory. Next to it you will see the system up time. In my case, it reads 24 mins. This refers to how long the computer has been running without interuption. Next we see three values for the load average. These figures are readings over 1 minute, 5 minutes, and 15 minutes, in that respective order.
For our purposes load values can be understood as an estimate of the "stress" on your system's processing power. For example, a load of 1.0 would mean that 100% of your processing power is currently at work.
Tasks: 205 total, 1 running, 204 sleeping, 0 stopped, 0 zombie
System processes are also known as tasks. An operating system relies on a number of processes running in the background to make other activities possible, such as using a web browser.
Processes can be run in many different ways and are prioritized using a variety of algorithims. This serves to optimize how and when the computer performs the task. This is intended as a high-level overview so we will keep it simple here. We want our readers to know that there is more to computing than the states described here.
That said, let's look at what each of these means:
| State | Description |
|---|---|
| Running | Active / in Queue to be Processed |
| Sleeping | Waiting for a Process to Complete |
| Stopped | Interrupted by Job Control Signal (CTRL+C, for example) |
| Zombie | Made up of "Orphaned" Child Tasks / No Longer Running |
The first three intuitively make sense, but Zombie tasks may require a little more explanation. What do I mean by orphaned child tasks?
Zombie processes are usually created by buggy code that fails to end a subprocess. These incomplete bits of data hang around after they should have perished, like the "undead". They do not tax the CPU, but are usually an indication of a problematic piece of software. To get rid of them, you must find the parent process and then send a signal to kill the child process.
%Cpu(s):100.0 us, 0.0 sy, 0.0 ni, 0.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
Each of the values here is a percentage of time spent on the respective tasks. The table below explains what is meant by each abbreviation. This gives a fairly detailed breakdown of how processing power is being used.
| Abbreviation | Description |
|---|---|
| us | Processing done in User Mode |
| sy | Processing done in System/Kernel Mode |
| ni | Time Spent on "Nice" Processes |
| id | Time Spent Idle |
| wa | Waiting for a I/O Process to Complete |
| hi | Time Spent Due to Hardware Interrupts |
| si | Time Spent Due to Software Interrupts |
| st | Time "Stolen" by Virtual Machine Resources |
MiB Mem : 7974.7 total, 5974.0 free, 1046.9 used, 953.9 buff/cache
MiB Swap: 4095.5 total, 4095.5 free, 0.0 used. 6660.1 avail Mem
These lines mimic the output of the free command.
This is really the heart of the top command. If you came from windows, you're probably familar with using CTRL+ALT+DEL to access Task Manager. This is sort of a more "stripped" down version of that. You see the table of all the currently running tasks.
Here is a sample of some tasks displayed in top.
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1703 christo+ 20 0 2338396 342200 114744 S 7.9 4.2 0:25.05 gnome-shell
1285 root 20 0 237252 72360 46012 S 4.3 0.9 0:04.00 Xorg
2158 christo+ 20 0 496976 42952 32192 S 4.0 0.5 0:01.74 gnome-terminal-
2703 christo+ 20 0 21528 3880 3204 R 0.7 0.0 0:00.40 top
Here is a table to help make sense of the column header abbreviations.
| Abbreviation | Description |
|---|---|
| PID | Process ID (Unique Positive Integer) |
| USER | User Name |
| PR | Priority (Kernel) |
| NI | Nice Value |
| VIRT | Virtual Memory |
| RES | Resident Size (Physical Memory) |
| SHR | Shared Memory |
| S | Process Status (Running, Stopped, etc.) |
| %CPU | CPU Load |
| %MEM | Percentage of RES / Total RAM |
| TIME + | Total Time Spent on Process |
| COMMAND | Command That Began Process |
These are the default fields on my distribution. Your system may be slightly different. These are customizable by typing F while top is running. This will display a complete list of available options
Now we can take what we've learned and put it into practice. Top is more than just a display of system information, it is interactive and we can use the software's built-in commands to meet many different needs.
We can easily add color to our output to make it easier to read.
Here is a screen capture where I cycle through the available colors. These can be changed independently for the various fields. The instructions for making those edits are below.
Toggle Color On/Off:
[while top is running] z
You can type Z while top is running to get a full list of options.
Edit Color Options (Z):
available toggles:
B =disable bold globally (Off),
z =color/mono (On)
b =tasks "bold"/reverse (On)
1) Select a target as an upper case letter, current target is T :
S = Summary Data, M = Messages/Prompts,
H = Column Heads, T = Task Information
2) Select a color as a number, current color is 1 :
0 = black, 1 = red, 2 = green, 3 = yellow,
4 = blue, 5 = magenta, 6 = cyan, 7 = white
3) Then use these keys when finished:
'q' to abort changes to window '1:Def'
'a' or 'w' to commit & change another, <Enter> to commit and end
It is not mentioned in these instructions, but in order to save your color settings, you need to hit [Enter] to commit and then save your settings by typing W. This saves the current top settings to your configuration file. Typing W will display the location upon save confirmation.
It's a good idea to backup your default settings by making a copy of the file before making any changes. Find the appropriate location and use the following command to create a copy:
cp ~./toprc ~./toprc_original
To restore, run the same command but reverse the file names Be aware, this will override any changes you have made.
Once you have your display set up in a way that you like. It's time to get some work done. Let's take a look at the full list of options for top (accessible by typing h) and look at some examples.
Z,B,E,e Global: 'Z' colors; 'B' bold; 'E'/'e' summary/task memory scale
We have covered the color changes, but you can also modify the memory scale from bytes all the way up to petabytes. E modifies the figures from the summary section, while a e adjusts the scale in the task table.
l,t,m Toggle Summary: 'l' load avg; 't' task/cpu stats; 'm' memory info
Each respective key will adjust the display options. The default is a text based output, but there are also simulated bars or you can turn items off completely.
0,1,2,3,I Toggle: '0' zeros; '1/2/3' cpus or numa node views; 'I' Irix mode
I only have one CPU on my virtual machine, but you can see how it would work if there were multiples. You can separate into individual nodes with their own statitstics.
f,F,X Fields: 'f'/'F' add/remove/order/sort; 'X' increase fixed-width
We can add or remove any fields we want displayed, we can also adjust the fixed-width of the text.
L,&,<,> . Locate: 'L'/'&' find/again; Move sort column: '<'/'>' left/right
R,H,V,J . Toggle: 'R' Sort; 'H' Threads; 'V' Forest view; 'J' Num justify
These commands give you options for organizing task information. Sort will display all of the tasks by PID, rather than the default sort by activity function. Threads will change your detailed and summary view to thread-oriented instead of task-oriented. Forest view arranges processes in a 'tree' structure to demonstrate parent-child relationships. Numeric justify changes the default left-aligned position of numeric fields.
c,i,S,j . Toggle: 'c' Cmd name/line; 'i' Idle; 'S' Time; 'j' Str justify
When you toggle c, you will see the full path of commands. Idle removes all non-active tasks from the display. S uses cumulative time. String justify changes the default left-aligned position of string fields.
x,y . Toggle highlights: 'x' sort field; 'y' running tasks
Using x to toggle sort field and y to sort running tasks.
u,U,o,O . Filter by: 'u'/'U' effective/any user; 'o'/'O' other criteria
We can easily filter out specific users or field/values. This is great for a system with many users or a lot of open tasks. Filters require a comparison operator between the field and the value. As you can see here I typed COMMAND=top to filter all commands except for top. Using shift with either command toggles case-sensitivity.
n,#,^O . Set: 'n'/'#' max tasks displayed; Show: Ctrl+'O' other filter(s)
Using either n or # will give you the option to enter a numeric value. This will show only the number of processes desired.
k,r Manipulate tasks: 'k' kill; 'r' renice
Using these commands, we can change the nice value or kill the process. I filtered out the specific PID to make it easier to see. I changed the nice value to 20. Then I sent a kill command. Using kill by default sends a SIGTERM signal. This allows it to finish executing code before terminating.
d or s Set update interval
Here, I have changed the default update time from every 3.0 seconds, to every 1 second. You can adjust any time that suits your needs. It's possible to use d or s to perform the same operation.
Did you enjoy our guide to top? I hope all of these tips taught you something new about working with top. If you like this guide, please share it on social media. If you have any comments or questions, leave them below. If you have any suggestions for topics you'd like to see covered, feel free to leave those as well. Thanks for reading.