11.5 top

Finally, there's a command you can use to display updating information about the processes running on the system. This command is called top(1), and is started like so:

% top

This will display a full screen of information about the processes running on the system, as well as some overall information about the system. This includes load average, number of processes, the CPU status, free memory information, and details about processes including PID, user, priority, CPU and memory usage information, running time, and program name.

 6:47pm  up 1 day,  18:01,  1 user,  load average: 0.02, 0.07, 0.02
61 processes: 59 sleeping, 2 running, 0 zombie, 0 stopped
CPU states:  2.8% user,  3.1% system,  0.0% nice, 93.9% idle
Mem:   257992K av,  249672K used,    8320K free,   51628K shrd,  78248K buff
Swap:   32764K av,     136K used,   32628K free,                 82600K cached

PID USER    PRI  NI  SIZE  RSS  SHARE  STAT  LIB %CPU %MEM  TIME COMMAND
 112 root     12   0 19376  18M   2468  R       0  3.7  7.5 55:53 X
4947 david    15   0  2136 2136   1748  S       0  2.3  0.8  0:00 screenshot
3398 david     7   0 20544  20M   3000  S       0  1.5  7.9  0:14 gimp
4946 root     12   0  1040 1040    836  R       0  1.5  0.4  0:00 top
 121 david     4   0   796  796    644  S       0  1.1  0.3 25:37 wmSMPmon
 115 david     3   0  2180 2180   1452  S       0  0.3  0.8  1:35 wmaker
4948 david    16   0   776  776    648  S       0  0.3  0.3  0:00 xwd
   1 root      1   0   176  176    148  S       0  0.1  0.0  0:13 init
 189 david     1   0  6256 6156   4352  S       0  0.1  2.4  3:16 licq
4734 david     0   0  1164 1164    916  S       0  0.1  0.4  0:00 rxvt
   2 root      0   0     0    0      0  SW      0  0.0  0.0  0:08 kflushd
   3 root      0   0     0    0      0  SW      0  0.0  0.0  0:06 kupdate
   4 root      0   0     0    0      0  SW      0  0.0  0.0  0:00 kpiod
   5 root      0   0     0    0      0  SW      0  0.0  0.0  0:04 kswapd
  31 root      0   0   340  340    248  S       0  0.0  0.1  0:00 kerneld
  51 root      0   0    48   48     32  S       0  0.0  0.0  0:00 dhcpcd
  53 bin       0   0   316  316    236  S       0  0.0  0.1  0:00 rpc.portmap
  57 root      0   0   588  588    488  S       0  0.0  0.2  0:01 syslogd

It's called top because the most CPU intensive programs will be listed at the top. An interesting note is that top will be listed first on most inactive (and some active) systems because of its CPU utilization. However, top is quite useful for determining what program is misbehaving and needs to be killed off.

But suppose you only want a list of your own processes, or the processes of some other user. The processes you want to see might not be among the most CPU intensive programs currently running. The -u option allows you to specify a username or UID and monitor only those processes owned by that UID.

% top -u alan
 PID USER      PR  NI  VIRT RES   SHR S %CPU %MEM   TIME+   COMMAND
3622 alan      13   0 11012  10m 6956 S  1.0  2.1   0:03.66 gnome-terminal
3739 alan      13   0  1012 1012  804 R  0.3  0.2   0:00.06 top
3518 alan       9   0  1312 1312 1032 S  0.0  0.3   0:00.09 bash
3529 alan       9   0   984  984  848 S  0.0  0.2   0:00.00 startx
3544 alan       9   0   640  640  568 S  0.0  0.1   0:00.00 xinit
3548 alan       9   0  8324 8320 6044 S  0.0  1.6   0:00.30 gnome-session
3551 alan       9   0  7084 7084 1968 S  0.0  1.4   0:00.50 gconfd-2
3553 alan       9   0  2232 2232  380 S  0.0  0.4   0:00.05 esd
3555 alan       9   0  2552 2552 1948 S  0.0  0.5   0:00.10 bonobo-activati
3557 alan       9   0  2740 2740 2224 S  0.0  0.5   0:00.05 gnome-smproxy
3559 alan       9   0  6496 6492 5004 S  0.0  1.3   0:00.31 gnome-settings-
3565 alan       9   0  1740 1740 1440 S  0.0  0.3   0:00.28 xscreensaver
3568 alan       9   0  7052 7052 4960 S  0.0  1.4   0:02.28 metacity
3572 alan       9   0 11412  11m 7992 S  0.0  2.2   0:01.58 gnome-panel
3574 alan       9   0 12148  11m 8780 S  0.0  2.4   0:00.64 nautilus
3575 alan       9   0 12148  11m 8780 S  0.0  2.4   0:00.00 nautilus
3576 alan       9   0 12148  11m 8780 S  0.0  2.4   0:00.00 nautilus

As you can see, I'm currently running X, top, a gnome-terminal (in which I'm writing this) and many other X-related processes which take up the most CPU time for me. This is a good way to monitor how hard your users are working your system.

top also supports monitoring processes by their PID, ignoring idle and zombied processes, and many other options. The best place to get a handle on these options is the man page for top.