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Starting, Stopping, and Killing Processes
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38.5 The ps Command

The ps command produces a report summarizing execution statistics for current processes. The bare ps command lists the process ID, the terminal the command was started from, how much CPU time it has used, and the command itself. The output looks something like this (it differs from system to system):

 1803 p5 IW    0:00 -csh (csh)
 1883 p5 IW    0:04 vi outline
 1811 p6 IW    0:01 -csh (csh)
 5353 p6 TW    0:01 vi 4890

By default, ps lists only your own processes. There are many times, though, when it's desirable to have a more complete listing with a lot of data about all of the processes currently running on the system. The options required to do this differ between BSD UNIX and System V. Under BSD UNIX, the command is ps -aux, which produces a table of all processes, arranged in order of decreasing CPU usage at the moment when the ps command was executed. [The -a option gives processes belonging to all users, -u gives a more detailed listing, and -x includes processes that no longer have a controlling terminal (38.6). -TOR ] It is often useful to pipe this output to head (25.20), which will display the most active processes:

% ps -aux | head -5
martin   12923 74.2 22.5  223  376 p5  R     2:12 f77 -o foo foo.F
chavez   16725 10.9 50.8 1146 1826 p6  R N  56:04 g94 HgO.dat
ng       17026  3.5  1.2  354  240 co  I     0:19 vi benzene.txt
gull      7997  0.2  0.3  142   46 p3  S     0:04 csh

The meanings of the fields in this output (as well as others displayed by the -l option to ps) are given in Table 38.1.

The first line of this output shows that user martin is running a FORTRAN compilation (f77). This process has PID (38.3) 12923 and is currently either running or runable. User chavez's process (PID 16725), executing the program g94, is also running or runable, though at a lowered priority. From this display, it's obvious who is using most system resources at this instant: martin and chavez have about 85% of the CPU and 73% of the memory between them. However, although it does display total CPU time, ps does not average the %CPU or %MEM values over time in any way.

Table 38.1: ps Command Output Fields
USER (BSD)Username of process owner.
UID (System V)Username of process owner.
PIDProcess ID.
%CPUEstimated fraction of CPU consumed (BSD).
%MEMEstimated fraction of system memory consumed (BSD).
SZVirtual memory used in K (BSD) or pages (System V).
RSSReal memory used (in same units as SZ).
TT, TTYTerminal port associated with process.
STAT (BSD),Current process state; one (or more under BSD) of:
S (System V)
R: Running or runnable.
S: Sleeping.
I: Idle (BSD). Intermediate state (System V).
T: Stopped (12.8).
Z: Zombie process (38.16).
D (BSD): Disk wait.
P (BSD): Page wait.
X (System V): Growing: waiting for memory.
K (AIX): Available kernel process.
W (BSD): Swapped out.
N (BSD): Niced (39.9, 39.11):execution priority lowered.
> (BSD): Execution priorityartificially raised (39.11).
TIMETotal CPU time used.
COMMANDCommand line being executed (may be truncated).
STIME (System V)Time or date process started.
C (System V),Short term CPU-use factor; used by scheduler for
CP (BSD)computing execution priority (PRI below).
FFlags associated with process (see ps manual page).
PPIDParent's PID.
PRIActual execution priority (recomputed dynamically).
NIProcess nice number (39.9).
WCHANEvent process is waiting for.

[3] Some vendors add other fields, such as the processor number for multiprocessors and additional or different process states (as in the AIX K field). These codes may differ from vendor to vendor: for example, the 0 code under Stardent UNIX means a process that is actually running (and R means runable) while 0 under AIX means a nonexistent process.

A vaguely similar listing is produced by the System V ps -ef command:

$ ps -ef
  root     0     0   0 09:36:35       ?  0:00 sched
  root     1     0   0 09:36:35       ?  0:02 /etc/init 
  gull  7997     1  10 09:49:32   ttyp3  0:04 csh
martin 12923 11324   9 10:19:49   ttyp5 56:12 f77 -o foo foo.F     
chavez 16725 16652  15 17:02:43   ttyp6 10:04 g94 HgO.dat
    ng 17026 17012  14 17:23:12 console  0:19 vi benzene.txt

The columns hold the username, process ID, parent's PID (the PID of the process that created it), the current scheduler value, the time the process started, its associated terminal, its accumulated CPU time, and the command it is running. Note that the ordering is by PID, not resource usage.

AIX's version of the ps command supports both BSD and System V options. The BSD options are not preceded by a hyphen (which is a legal syntax variation), and the System V options are. Thus, under AIX, ps -au is not the same as ps au. The command is the System V version, however, even if its output is displayed with the BSD column headings. Thus, ps aux output is displayed in PID rather than %CPU order.

ps is also useful in pipes; a common use is:

% ps -aux | grep chavez

to see what user chavez has currently running. [Under System V, use ps -u chavez. -JP]

- AF from O'Reilly & Associates' Essential System Administration, Chapter 7

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