The Linux system provides a command called “top” which provides dynamic real-time view of running processes, memory status, cpu usage and other system information. This command is very easy to use and it updates the display automatically and user just needs to keep the command open. Then user needs to press the key “q” to close the display.

Here is the output of “top” command look like.

Tasks: 179 total,   1 running, 178 sleeping,   0 stopped,   0 zombie
Cpu(s):  7.2%us,  2.6%sy,  0.0%ni, 88.5%id,  1.5%wa,  0.1%hi,  0.0%si,  0.0%st
Mem:   2007980k total,  1046836k used,   961144k free,    55852k buffers
Swap:   803208k total,        0k used,   803208k free,   539948k cached

PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
3175 neo       20   0  215m  93m  22m S  5.8  4.8   3:36.28 firefox
2501 root      20   0 81052  10m 5580 S  1.9  0.5   2:10.15 Xorg
2927 neo       20   0  161m  23m  12m S  1.9  1.2   0:07.07 knotify4
2964 neo       20   0  138m  28m  15m S  1.9  1.4   0:06.24 konsole
3527 neo       20   0  2556  996  736 R  1.9  0.0   0:00.01 top
1 root      20   0  2008  768  564 S  0.0  0.0   0:01.14 init
2 root      15  -5     0    0    0 S  0.0  0.0   0:00.00 kthreadd
3 root      RT  -5     0    0    0 S  0.0  0.0   0:00.00 migration/0
4 root      15  -5     0    0    0 S  0.0  0.0   0:00.25 ksoftirqd/0
5 root      RT  -5     0    0    0 S  0.0  0.0   0:00.00 watchdog/0
6 root      RT  -5     0    0    0 S  0.0  0.0   0:00.00 migration/1
7 root      15  -5     0    0    0 S  0.0  0.0   0:00.24 ksoftirqd/1
8 root      RT  -5     0    0    0 S  0.0  0.0   0:00.00 watchdog/1
9 root      15  -5     0    0    0 S  0.0  0.0   0:00.94 events/0
10 root      15  -5     0    0    0 S  0.0  0.0   0:00.86 events/1
11 root      15  -5     0    0    0 S  0.0  0.0   0:00.00 khelper
85 root      15  -5     0    0    0 S  0.0  0.0   0:00.00 kintegrityd/0
86 root      15  -5     0    0    0 S  0.0  0.0   0:00.00 kintegrityd/1
88 root      15  -5     0    0    0 S  0.0  0.0   0:00.26 kblockd/0
89 root      15  -5     0    0    0 S  0.0  0.0   0:00.03 kblockd/1
91 root      15  -5     0    0    0 S  0.0  0.0   0:00.00 kacpid
92 root      15  -5     0    0    0 S  0.0  0.0   0:00.00 kacpi_notify
164 root      15  -5     0    0    0 S  0.0  0.0   0:00.00 cqueue
168 root      15  -5     0    0    0 S  0.0  0.0   0:00.51 ata/0
169 root      15  -5     0    0    0 S  0.0  0.0   0:00.00 ata/1
170 root      15  -5     0    0    0 S  0.0  0.0   0:00.00 ata_aux
172 root      15  -5     0    0    0 S  0.0  0.0   0:00.00 ksuspend_usbd
177 root      15  -5     0    0    0 S  0.0  0.0   0:00.00 khubd
180 root      15  -5     0    0    0 S  0.0  0.0   0:00.00 kseriod
222 root      20   0     0    0    0 S  0.0  0.0   0:00.00 pdflush
223 root      20   0     0    0    0 S  0.0  0.0   0:00.08 pdflush

The first line of output tells you the total number of tasks or processes currently running in the system and their process states.

The second line tells you about the CPU usage in the system and the percentage of states in which CPU spent its time. The states are explained below.

Code Meaning
us user
sy system
ni nice
id idle
wa I/O wait state
hi hardware interrupt
si software interrupt
st steal

The third and fourth lines tell you about the RAM and Swap usage respectively. Then follows the list of processes ordered by CPU and memory usage.

The meaning of following line explained below:

PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
Code Meaning
PID The unique process ID
USER Name of the user who owns this process
PR Priority of the process. Read More
NI Nice value. A negative value means higher priority.
VIRT Total amount of virtual memory used by this process in KBytes
RES Non-swapped memory (Resident memory size)  in KBytes
SHR Amount of shared memory used by this process in KBytes
S Status of the process.

D = Uninterruptible sleep

R = Running

S = Sleeping

T = Traced or Stopped

Z = Zombie
%CPU CPU share of this process since last screen update
%MEM Current share of available physical memory (RES)
TIME+ Total CPU time used since the process started.
COMMAND Name of the command or process name

There is no special option provided by “top” to limit the number of processes. However using “watch”, “head” and “tail” commands, you can monitor exactly top 10 processes as shown below. You need to press “Ctrl+C” to close the display.

[neo@techpulp ~]$ watch -n 1 "top -b -n 1 | head -n 17 | tail -n 11"
Every 10.0s: top -b -n 1 | head -n 17 | tail -n 11                                                                     Thu Jan 29 23:20:19 2009

PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
3175 neo       20   0  225m 103m  22m S  7.8  5.3   6:24.63 firefox
2501 root      20   0 82948  12m 6700 S  2.0  0.6   5:25.48 Xorg
3253 neo       20   0  107m  19m 9868 S  2.0  1.0   2:40.87 npviewer.bin
3839 neo       20   0  2424  972  720 R  2.0  0.0   0:00.01 top
1 root      20   0  2008  768  564 S  0.0  0.0   0:01.15 init
2 root      15  -5     0    0    0 S  0.0  0.0   0:00.00 kthreadd
3 root      RT  -5     0    0    0 S  0.0  0.0   0:00.00 migration/0
4 root      15  -5     0    0    0 S  0.0  0.0   0:00.53 ksoftirqd/0
5 root      RT  -5     0    0    0 S  0.0  0.0   0:00.00 watchdog/0
6 root      RT  -5     0    0    0 S  0.0  0.0   0:00.00 migration/1
[neo@techpulp ~]$