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<td width="80%" align="center" valign="bottom">Chapter 11 Process Control</td>
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<h1 class="SECT1"><a id="PROCESS-CONTROL-PS" name="PROCESS-CONTROL-PS">11.3 <tt
class="COMMAND">ps</tt></a></h1>
<p>So now you know how to switch back and forth between several processes that you've
started from the command line. And you also know that there are lots of processes running
all the time. So how do you list all of these programs? Well, you make use of the <tt
class="COMMAND">ps</tt>(1) command. This command has a lot of options, so we'll only
cover the most important ones here. For a complete listing, see the man page for ps. Man
pages are covered in-depth in <a href="help.html#HELP-SYSTEM-MAN">Section 2.1.1</a>.</p>
<p>Simply typing <tt class="COMMAND">ps</tt> will get you a listing of the programs
running on your terminal. This incudes the foreground processes (which include whatever
shell you are using, and of course, <tt class="COMMAND">ps</tt> itself). Also listed are
backgrounded processes you may have running. Many times, that will be a very short
listing:</p>
<div class="FIGURE"><a id="FIG-PROCESS-CONTROL-PS-SHORT"
name="FIG-PROCESS-CONTROL-PS-SHORT"></a>
<p><b>Figure 11-1. Basic <tt class="COMMAND">ps</tt> output</b></p>
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<pre class="SCREEN">
<samp class="PROMPT">%</samp> <kbd class="USERINPUT">ps</kbd>
PID TTY TIME CMD
7923 ttyp0 00:00:00 bash
8059 ttyp0 00:00:00 ps
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<p>Even though this is not a lot of processes, the information is very typical. You'll
get the same columns using regular ps no matter how many processes are running. So what
does it all mean?</p>
<p>Well, the <var class="LITERAL">PID</var> is the <span class="emphasis"><i
class="EMPHASIS">process ID</i></span>. All running processes are given a unique
identifier which ranges between 1 and 32767. Each process is assigned the next free PID.
When a process quits (or is killed, as you will see in the next section), it gives up its
PID. When the max PID is reached, the next free one will wrap back around to the lowest
free one.</p>
<p>The <var class="LITERAL">TTY</var> column indicates which terminal the process is
running on. Doing a plain <tt class="COMMAND">ps</tt> will only list all the programs
running on the current terminal, so all the processes give the same information in the
TTY column. As you can see, both processes listed are running on <tt
class="FILENAME">ttyp0</tt>. This indicates that they are either running remotely or from
an X terminal of some variety.</p>
<p>The <var class="LITERAL">TIME</var> column indicated how much CPU time the process has
been running. This is different from the actual amount of time that a process runs.
Remember that Linux is a multitasking operating system. There are many processes running
all the time, and these processes each get a small portion of the processor's time. So,
the TIME column should show much less time for each process than it actually takes to
run. If you see more than several minutes in the TIME column, it could mean that
something is wrong.</p>
<p>Finally, the <var class="LITERAL">CMD</var> column shows what the program actually is.
It only lists the base name of the program, not any command line options or similar
information. To get that information, you'll need to use one of the many options to <tt
class="COMMAND">ps</tt>. We'll discuss that shortly.</p>
<p>You can get a complete listing of the processes running on your system using the right
combination of options. This will probably result in a long listing of processes
(fifty-five on my laptop as I write this sentence), so I'll abbreviate the output:</p>
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<samp class="PROMPT">%</samp> <kbd class="USERINPUT">ps -ax</kbd>
PID TTY STAT TIME COMMAND
1 ? S 0:03 init [3]
2 ? SW 0:13 [kflushd]
3 ? SW 0:14 [kupdate]
4 ? SW 0:00 [kpiod]
5 ? SW 0:17 [kswapd]
11 ? S 0:00 /sbin/kerneld
30 ? SW 0:01 [cardmgr]
50 ? S 0:00 /sbin/rpc.portmap
54 ? S 0:00 /usr/sbin/syslogd
57 ? S 0:00 /usr/sbin/klogd -c 3
59 ? S 0:00 /usr/sbin/inetd
61 ? S 0:04 /usr/local/sbin/sshd
63 ? S 0:00 /usr/sbin/rpc.mountd
65 ? S 0:00 /usr/sbin/rpc.nfsd
67 ? S 0:00 /usr/sbin/crond -l10
69 ? S 0:00 /usr/sbin/atd -b 15 -l 1
77 ? S 0:00 /usr/sbin/apmd
79 ? S 0:01 gpm -m /dev/mouse -t ps2
94 ? S 0:00 /usr/sbin/automount /auto file /etc/auto.misc
106 tty1 S 0:08 -bash
108 tty3 SW 0:00 [agetty]
109 tty4 SW 0:00 [agetty]
110 tty5 SW 0:00 [agetty]
111 tty6 SW 0:00 [agetty]
[output cut]
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<p>Most of these processes are started at boot time on most systems. I've made a few
modifications to my system, so your mileage will most likely vary. However, you will see
most of these processes on your system too. As you can see, these options display command
line options to the running processes. Recently, a kernel vulnerability in <tt
class="COMMAND">ptrace</tt> facilitated a fix which no longer shows command line options
for many running processes. These are now listed in brackets like PIDs 108 through 110.
It also brings up a few more columns and some other interesting output.</p>
<p>First, you'll notice that most of these processes are listed as running on tty
“?”. Those are not attached to any particular terminal. This is most common
with daemons, which are processes which run without attaching to any particular terminal.
Common daemons are sendmail, BIND, apache, and NFS. They typically listen for some
request from a client, and return information to it upon request.</p>
<p>Second, there is a new column: <var class="LITERAL">STAT</var>. It shows the status of
the process. <var class="LITERAL">S</var> stands for sleeping: the process is waiting for
something to happen. <var class="LITERAL">Z</var> stands for a zombied process. A zombied
processes is one whose parent has died, leaving the child processes behind. This is not a
good thing. <var class="LITERAL">D</var> stands for a process that has entered an
uninterruptible sleep. Often, these processes refuse to die even when passed a SIGKILL.
You can read more about SIGKILL later in the next section on <tt
class="COMMAND">kill</tt> . W stands for paging. A dead process is marked with an <var
class="LITERAL">X</var>. A process marked <var class="LITERAL">T</var> is traced, or
stopped. <var class="LITERAL">R</var> means that the process is runable.</p>
<p>If you want to see even more information about the running processes, try this
out:</p>
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<samp class="PROMPT">%</samp> <kbd class="USERINPUT">ps -aux</kbd>
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1 0.0 0.0 344 80 ? S Mar02 0:03 init [3]
root 2 0.0 0.0 0 0 ? SW Mar02 0:13 [kflushd]
root 3 0.0 0.0 0 0 ? SW Mar02 0:14 [kupdate]
root 4 0.0 0.0 0 0 ? SW Mar02 0:00 [kpiod]
root 5 0.0 0.0 0 0 ? SW Mar02 0:17 [kswapd]
root 11 0.0 0.0 1044 44 ? S Mar02 0:00 /sbin/kerneld
root 30 0.0 0.0 1160 0 ? SW Mar02 0:01 [cardmgr]
bin 50 0.0 0.0 1076 120 ? S Mar02 0:00 /sbin/rpc.port
root 54 0.0 0.1 1360 192 ? S Mar02 0:00 /usr/sbin/sysl
root 57 0.0 0.1 1276 152 ? S Mar02 0:00 /usr/sbin/klog
root 59 0.0 0.0 1332 60 ? S Mar02 0:00 /usr/sbin/inet
root 61 0.0 0.2 1540 312 ? S Mar02 0:04 /usr/local/sbi
root 63 0.0 0.0 1796 72 ? S Mar02 0:00 /usr/sbin/rpc.
root 65 0.0 0.0 1812 68 ? S Mar02 0:00 /usr/sbin/rpc.
root 67 0.0 0.2 1172 260 ? S Mar02 0:00 /usr/sbin/cron
root 77 0.0 0.2 1048 316 ? S Mar02 0:00 /usr/sbin/apmd
root 79 0.0 0.1 1100 152 ? S Mar02 0:01 gpm
root 94 0.0 0.2 1396 280 ? S Mar02 0:00 /usr/sbin/auto
chris 106 0.0 0.5 1820 680 tty1 S Mar02 0:08 -bash
root 108 0.0 0.0 1048 0 tty3 SW Mar02 0:00 [agetty]
root 109 0.0 0.0 1048 0 tty4 SW Mar02 0:00 [agetty]
root 110 0.0 0.0 1048 0 tty5 SW Mar02 0:00 [agetty]
root 111 0.0 0.0 1048 0 tty6 SW Mar02 0:00 [agetty]
[output cut]
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<p>That's a whole lot of information. Basically, it adds information including what user
started the process, how much of the system resources the process is using (the %CPU,
%MEM, VSZ, and RSS columns), and on what date the process was started. Obviously, that's
a lot of information that could come in handy for a system administrator. It also brings
up another point: the information now goes off the edge of the screen so that you cannot
see it all. The <var class="OPTION">-w</var> option will force <tt
class="COMMAND">ps</tt> to wrap long lines.</p>
<p>It's not terribly pretty, but it does the job. You've now got the complete listings
for each process. There's even more information that you can display about each process.
Check out the very in-depth man page for <tt class="COMMAND">ps</tt>. However, the
options shown above are the most popular ones and will be the ones you need to use the
most often.</p>
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