Understanding top Command

This post helps understanding top command on Linux. top is a tool that outputs a list of the currently running processes of the system and the output updates in every 3 seconds by default. It also shows CPU load, currently used and free RAM as well as a lot of other useful information.

This tool is installed by default in most Linux distributions. I have provided an example output below:

top - 12:14:43 up 92 days,  6:28,  1 user,  load average: 0.00, 0.10, 0.03
Tasks:  22 total,   1 running,  21 sleeping,   0 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
Mem:   1048576k total,    48376k used,  1000200k free,        0k buffers
Swap:        0k total,        0k used,        0k free,    22128k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND                                                                            
    1 root      20   0 19232  784  536 S  0.0  0.1   0:00.04 init                                                                               
    2 root      20   0     0    0    0 S  0.0  0.0   0:00.00 [kthreadd/324]                                                                     
    3 root      20   0     0    0    0 S  0.0  0.0   0:00.00 [khelper/324]                                                                      
  130 root      16  -4 10644  604  248 S  0.0  0.1   0:00.00 /sbin/udevd -d                                                                     
  554 root      20   0 17372 1284  268 S  0.0  0.1   0:00.03 /sbin/udevd

Interactive commands

Before proceeding to explain every field in the output of top command, please read about the interactive commands used in top.

Key	Name	function
u	User	Filter a user’s processes
k	kill	End process
r	renice	Set process Nice value
d	delay	Set update interval in seconds
n	number	Maximum number of displayed processes
N	Number	Sort by PID
P	Percent	Sort by CPU usage
M	Memory	Sort by RAM usage
q	quit	Quitting top
H	help	Show help

Also, pressing the space bar refreshes the screen output.

Understanding top command: Summary Area

1st Row — top

This line shows the time since the system was online. It also shows the load averages of last 1, 5 and 15 minutes.

top - 12:14:43 up 92 days,  6:28,  1 user,  load average: 0.00, 0.10, 0.03

Understand each sections in 1st row in order:

Current time – 12:14:43
Uptime – 92 days, 6:28
Currently logged in Users – 1 user
Average load – load average: 0.00, 0.10, 0.03 (The 3 values refer to the last minute, five minutes and 15 minutes)

2nd Row – task

Displays the number of processes active at the time of the last update. It also shows how many processes are running, sleeping, stopped or undead

Tasks:  22 total,   1 running,  21 sleeping,   0 stopped,   0 zombie

Understand each sections in 2nd row in order:

Total Processes – 22 total
Running Processes – 1 running
Sleeping Processes – 21 sleeping
Stopped Processes – 0 stopped
Zombie Processes – 0 zombie

Zombie Processes: Zombie process also known as defunct process is a process that finished the execution but it still has an entry in the process table to report to its parent process.

3rd Row – Cpu(s)

This section displays the CPU utilization in percent.

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

Understand each sections in 3rd row in order:

Percentage of CPU for user processes – 0.0%us
Percentage of CPU for system processes – 0.0%sy
Percentage of CPU processes that have no default priority (nice) – 0.0%ni
Percentage of CPU not used – 100.0%id
Percentage of CPU processes waiting for I/O operations – 0.0%wa
Percentage of CPU serving hardware interrupts – 0.0% hi – Hardware IRQ
Percentage of CPU serving software interrupts – 0.0% si – Software Interrupts
Percentage of Steel Time – 0.0%st – Steal Time

Steal time is the percentage of time a virtual CPU waits for a real CPU while the hypervisor is servicing another virtual processor.

4th and 5th Row – Memory usage

This section shows the statistics on the utilization of the main memory and also displays the swap utilization.

Mem:   1048576k total,    48376k used,  1000200k free,        0k buffers
Swap:        0k total,        0k used,        0k free,    22128k cached

The above rows shows the usage of RAM and swap in the order Total memory, Memory in use, Free memory and buffers cached.

All Other rows: Processes list

PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
  1 root      20   0 19232  784  536 S  0.0  0.1   0:00.04 init
  2 root      20   0     0    0    0 S  0.0  0.0   0:00.00 [kthreadd/324]
  3 root      20   0     0    0    0 S  0.0  0.0   0:00.00 [khelper/324]
130 root      16  -4 10644  604  248 S  0.0  0.1   0:00.00 /sbin/udevd -d

PID – This is the ID of a process.
USER – The owner of the process
PR – priority of the process
NI – NICE value of the process
VIRT – Virtual memory used by process
RES – Physical memory used by a process
SHR – Shared memory of the process
S – Status of the process. The Different status’ are given below:

S - sleeping
D - uninterruptible sleep
R - running
W - swapped out process
Z - zombies
I - Ideal process
T - stopped or traced
N - process with positive nice value
< - process with negative nice value

%CPU – Percentage of CPU used a process
%MEM – Percentage of RAM used a process
TIME+ – The total time of activity of process (0:17.75)
COMMAND – It shows the exact command or name used by a process.

That’s it!

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Understanding top Command