24.5. The ps Command

The ps command varies from system to system. (The ps on one Red Hat Linux system reads a PS_PERSONALITY environment variable with 21 possible settings!) This article describes several different versions. Yours is probably different in some ways, so check your ps manual page for details.

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

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 (Section 24.6). -- TOR] It is often useful to pipe this output to head (Section 12.12), which will display the most active processes:

% ps -aux | head -5
USER       PID %CPU %MEM   SZ  RSS TTY STAT  TIME COMMAND
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 24-1.

The first line of this output shows that user martin is running a FORTRAN compilation (f77). This process has PID (Section 24.3) 12923 and is currently either running or runnable. User chavez's process (PID 16725), executing the program g94, is also running or runnable, 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.

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

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:

Another way to view the process information is with the top command. Unlike ps, top is an interactive screen program that updates its information every few seconds. It's a good way to get a quick pulse of your system. Not only is process information displayed, but memory statistics and the system uptime are also shown. You can find the full range of available interactive commands by typing h once top has started. You can sort processes in a variety of ways including CPU and memory usage, as well as by user. You can even kill processes from within top.

--AF, from Essential System Administration (O'Reilly, 2002), and JJ

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24.4. Subshells		24.6. The Controlling Terminal

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