top (1) is an interactive process lister for Linux and Unix systems. It is one of the most useful programs available on the standard Linux command line. Unlike the more standard ps tool, which prints out a customisable listing of the processes running on the system, top provides a real-time updating list of processes, along with a significant amount of useful information.

A typical top output on a lightly-loaded, Linux 2.6-based system is:

top - 14:53:25 up  3:33,  1 user,  load average: 1.16, 1.17, 1.20
Tasks:  83 total,   2 running,  81 sleeping,   0 stopped,   0 zombie
Cpu(s):  8.3% us,  4.3% sy, 83.7% ni,  0.0% id,  0.0% wa,  3.7% hi,  0.0% si
Mem:    514236k total,   509300k used,     4936k free,    12068k buffers
Swap:  1834288k total,       60k used,  1834228k free,   246228k cached

  607 rpgeek    39  19 70712  10m 2112 R 86.8  2.1 189:01.27 FahCore_65.exe
  373 root      15   0 83996  45m  41m S  2.7  9.1   4:53.80 XFree86
 2176 rpgeek    15   0 33924  17m  30m S  2.7  3.4   0:00.58 kdeinit
  377 rpgeek    15   0  3536 2024 2572 S  1.0  0.4   0:37.77 famd
 2178 rpgeek    16   0  2088  988 1876 R  0.7  0.2   0:00.15 top
  480 rpgeek    16   0 27244  14m  24m S  0.3  2.9   0:09.83 kwin
 2034 rpgeek    15   0 97876  68m  43m S  0.3 13.7   0:06.86 juk
    1 root      16   0  1520  444 1368 S  0.0  0.1   0:05.63 init
    2 root      34  19     0    0    0 S  0.0  0.0   0:00.00 ksoftirqd/0
    3 root       5 -10     0    0    0 S  0.0  0.0   0:00.46 events/0
    4 root       5 -10     0    0    0 S  0.0  0.0   0:00.08 kblockd/0
This shows a wealth of instantaneous information on the running system. It is divided between the whole-system information at the top and the per-process information at the bottom. In addition to some basic statistics about the system load, it displays the current CPU, RAM, and swap usage in the top half. The important part, however, is the sortable list of processes in the bottom half of the output.

It is possible to sort the process list by any of the pieces of information shown. Sorting by PID or process name is relatively useless, adding nothing to the non-interactive ps command. It can be useful to sort processes by user, priority, or execution time, but these things change rarely and can also be done with ps. The most useful ways to sort the list are by memory usage (with a capital M keystroke) and processor utilisation (with a capital P keystroke). When doing work on a Unix system it is often useful to know what programs are using the available CPU time and memory, and top provides a list, updating in realtime, of the top users of one of these resources (this is why it's called 'top')

Top also provides an interface for management of processes, specificially sending them signals (equivalent to the 'kill' command) and changing their priority (equivalent to the 'renice' command). These features are little used compared to top's display capabilities. Top includes a terse but useful online help system, accessible by pressing '?', explaining top's complex interface for changing the display and performing process management tasks.

Due to its frequent display updates, top uses a relatively large amount of CPU time, reading the entire process table around every second. Since the process table is read while top is an active process, top itself is biased towards in the processor usage statistics, often briefly jumping to the top of the list on a relatively idle system. Top is often reniced to a lower priority than standard so that this does not disrupt more important tasks.

The interface and display style of top are familiar enough to Linux hackers that several more specialised monitor programs provide a similar interface. Examples are slabtop for probing Linux kernel memory and xrestop for monitoring the usage of X Window System server resources. Graphical programs that provide similar services to top are KSysGuard and GKrellM, both of which have larger scope and friendlier interfaces, at the price of size and complexity.

This writeup is copyright 2004-2005 D.G. Roberge and is released under the Creative Commons Attribution-NonCommercial-ShareAlike licence. Details can be found at .