Basics of the Linux Top Command

Recently, I had a tip submitted that referred to using the Linux top command to “check overall CPU utilization and individual CPU utilization”. Thanks, Sworna Dunant. It’s a good tip. However, top can be used for so much more that I decided to expand on it a little…

Before we get started, please note that the top command has it’s own versions and has slightly different options with each version. To check your top command version use the following:

$ top -v
top: procps version 3.3.9

Also, the options explained below are those that I have found useful and are by no means a comprehensive list of what can be done using top. More detail can be found in the man pages or here.

Display Running Processes

The screen below displays a table of the currently running processes with some very useful information:

  • PID – Process ID
  • User – The UserID running the process
  • %CPU – CPU used by the process
  • %MEM – Memory used by the process
  • Command – The command the process is running

If you look in the header there is even more info:

  • Uptime
  • # of Users on the system
  • Load Average for the last 5 min, 10 min & 15 min
  • More explained in detail here
$ top
top - 06:42:15 up 228 days, 11:32,  1 user,  load average: 0.35, 0.52, 0.60
Tasks: 195 total,   1 running, 194 sleeping,   0 stopped,   0 zombie
Cpu(s): 16.8%us,  2.3%sy,  0.0%ni, 78.5%id,  0.0%wa,  0.7%hi,  1.7%si,  0.0%st
Mem:   8175332k total,  5788888k used,  2386444k free,   510972k buffers
Swap: 16779852k total,       96k used, 16779756k free,  2179896k cached

11159 user      15   0  217m  81m  19m S 19.0  1.0  18:38.26 runbatch
28053 user      15   0  168m  41m  18m S 12.6  0.5   1:28.43 runbatch
16222 user      15   0  127m  27m  15m S  2.0  0.3   3:34.47 jdenet_k
16262 user      15   0 1398m  98m  24m S  1.7  1.2   4:30.66 jdenet_k
16339 user      15   0 60232 3848 3144 S  1.7  0.0   2:40.67 jdenet_n
16242 user      15   0  803m 146m  30m S  1.3  1.8   8:03.14 jdenet_k

Sort by Memory, CPU Usage, Process ID or Runtime

Use the following key strokes to sort the Top data:

  • Shift+M – sort by descending Memory usage
  • Shift+P – sort by descending CPU usage
  • Shift+N – sort by descending Process ID
  • Shift+T – sort by descending Runtime

Change The Refresh Interval

By default the top command refreshes the data every 3 seconds. That delay can be changed by pressing the [d] key. Top will ask you to enter your desired value, which can be less than 1.

top - 00:25:49 up 229 days,  5:15,  1 user,  load average: 1.09, 1.27, 1.16
Tasks: 194 total,   2 running, 192 sleeping,   0 stopped,   0 zombie
Cpu(s): 38.4%us,  1.3%sy,  0.0%ni, 56.7%id,  0.0%wa,  0.8%hi,  2.8%si,  0.0%st
Mem:   8175332k total,  6829240k used,  1346092k free,   562332k buffers
Swap: 16779852k total,       96k used, 16779756k free,  2809648k cached
Change delay from 3.0 to: .5
16243 jde900    15   0  654m 311m  36m S  0.0  3.9  15:34.52 jdenet_k
16242 jde900    15   0  804m 146m  30m S  0.3  1.8  15:44.62 jdenet_k

Display The Full Command Path and Arguments

Press the [c] key to display the full command path and arguments of the processes.

top - 00:31:50 up 229 days,  5:21,  1 user,  load average: 1.39, 1.37, 1.22
Tasks: 196 total,   3 running, 193 sleeping,   0 stopped,   0 zombie
Cpu(s): 49.3%us,  4.5%sy,  0.0%ni, 41.3%id,  0.2%wa,  0.8%hi,  3.9%si,  0.0%st
Mem:   8175332k total,  6883860k used,  1291472k free,   562528k buffers
Swap: 16779852k total,       96k used, 16779756k free,  2812428k cached

21132 jde900    15   0  182m  60m  18m R 64.2  0.8  21:40.48 runbatch USER PASSWORD
19876 jde900    15   0  277m  45m  19m S 16.3  0.6   0:22.50 jdenet_k 6015
25324 jde900    15   0  170m  44m  19m S 11.6  0.6   0:20.20 runbatch USER PASSWORD
25330 jde900    15   0  166m  39m  18m R  8.3  0.5   0:15.26 runbatch USER PASSWORD
17067 jde900    15   0 60320 3948 3208 S  6.3  0.0   4:38.34 jdenet_n 6015
11594 jde900    15   0  185m  45m  19m S  5.3  0.6   1:57.17 runbatch USER PASSWORD
16242 jde900    15   0  804m 146m  30m S  0.3  1.8  15:45.21 jdenet_k 6015

Display CPU Core Information Separately

Press the [1] key to display the individual CPU core information in the header.

Tasks: 195 total,   1 running, 194 sleeping,   0 stopped,   0 zombie
Cpu0  :  3.0%us,  0.3%sy,  0.0%ni, 96.6%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Cpu1  : 37.0%us,  1.3%sy,  0.0%ni, 56.9%id,  0.0%wa,  1.3%hi,  3.4%si,  0.0%st
Mem:   8175332k total,  6856660k used,  1318672k free,   562768k buffers
Swap: 16779852k total,       96k used, 16779756k free,  2815036k cached

21132 jde900    15   0  182m  60m  18m S 30.9  0.8  25:14.79 runbatch USER PASSWORD
25330 jde900    15   0  167m  40m  18m S 13.0  0.5   0:48.80 runbatch USER PASSWORD
16262 jde900    15   0 1398m  94m  24m S  0.3  1.2   8:57.37 jdenet_k 6015


There are many options to consider when using the top command to quickly get system performance information from your Linux servers. Those above are just the ones that I have found to be useful in supporting Oracle JD Edwards EnterpriseOne. A more in depth article can be found here.

If you have any tips for the top command or any others that you use, please let me know in the comments.


5 Ways to Empty a File on Linux

Like most operations on any Operating System, there are several ways to do basic file manipulation on Linux. Below are 5 different ways to empty a file.

Simple Redirection

Redirection is usually used to output command results to a file.  However, if we redirect “nothing” to a file, we can overwrite it with a blank file. Redirection can be done using a “>” greater-than symbol or a “|” pipe symbol.

# > file.txt
# :> file.txt

REDIRECT THE echo command

Use the echo command to redirect null values to a file.

# echo -n > file.txt
  • n do not output the trailing newline.

UsE THE truncate command

Use the truncate command to shrink or expand the size of a file to the size specified by the s option value. Use zero to make the file empty.

# truncate -s 0 file.txt
# truncate file.txt --size 0

REDIRECT THE /dev/null device

The null device is usually used for disposing of unwanted output streams of a process or as a empty file for input streams or a redirection operation.

# cat /dev/null > file.txt
# cp /dev/null file.txt
cp: overwrite 'file.txt'? y


Vim is a text editor built to make creating and changing any kind of text very efficient. It is included as “vi” in most Linux/Unix systems.

# ex -sc ':%d|x' file.txt
# ex -sc ':1,$d|x' file.txt
  • ex : Enter into Ex mode
  • s : Silent; do not display prompts
  • c : Run the given ex command upon startup
  • : : Invoke an ex command
  • % : Choose all the lines
  • d : Delete selected lines
  • x : save and close
  • 2g.txt : input filename

How To Set Your Linux Command Shell

Linux Shell

One of the best things about the Linux command line is the ability to recall the last used command by hitting the up arrow ’. Yes, I realize that Windows command line has done that for a long time, but where do you think they got the idea for that?

I’m fairly new to this whole Linux thing and I had an issue where I used SSH to get to a Linux server but when I ran a command and then hit the up arrow ’, I got back a couple unreadable characters instead of the previous command. I found out that I was using the wrong command shell for what I was wanting to do.

There are a few shells that seem to be fairly common:

  • bash – Bourne again shell
  • ksh – Korn shell
  • csh – C Shell
  • dash – Debian almquist shell

I’m not sure which shell I was using that day, but I figured out that I needed the bash shell. Below are a few commands that will help you when working with your command shell:

  • To find all of the available shells in your system, type the following command:
    cat /etc/shells
  • To find out your current shell, type any of the following commands:
    echo $SHELL
    ps $$
    ps -p $$
  • To change your shell to the bash shell, type the following command:
    chsh -s /bin/bash

Anything else you want to add to the discussion about shells?


How To Find Basic Linux Hardware Info

You can use the below to find some basic information about the hardware your Linux system is installed on:

How much RAM is installed and how much of it is in use (megabytes)?

$ free -m
 total used free shared buffers cached
Mem: 3949 3189 760 0 599 1594
-/+ buffers/cache: 995 2954
Swap: 16386 109 16277

Processor type:

$ cat /proc/cpuinfo

Check the size of the hard drive and what hard drives are available in the system.
This command will also list USB drives and sticks. You need root permissions to execute the fdisk command:

$ sudo fdisk -l | grep GB
Disk /dev/sda: 42.9 GB, 42949672960 bytes
Disk /dev/sdb: 107.3 GB, 107374182400 bytes

UPDATE 01-04-2018: Another handy command for finding disk space is “df”:

$ df
Filesystem                  1K-blocks    Used Available Use% Mounted on
/dev/mapper/rootvg-usrlv      5232640 1856376   3376264  36% /usr
/dev/mapper/rootvg-homelv     8378368  189912   8188456   3% /home
/dev/sda1                      520868  122896    397972  24% /boot

Got any other tips for me?

How To Find Your Linux Version

I’ve been doing a few things in Linux lately and needed a couple things like: What version of Linux am I using?

$ lsb_release -a
LSB Version: :core-3.1-amd64:core-3.1-ia32:core-3.1-noarch:graphics-3.1-amd64:graphics-3.1-ia32:graphics-3.1-noarch
Distributor ID: EnterpriseEnterpriseServer
Description: Enterprise Linux Enterprise Linux Server release 5.5 (Carthage)
Release: 5.5
Codename: Carthage

This Linux stuff is a little new for me, so hope that helps someone else out there.