LPI Linux Certification in a Nutshell (19 page)

ps

ps [
options
]

This command generates a one-time snapshot of the
current processes on standard output.

Simply entering the
ps
command with no
options will yield a brief list of processes owned by you and
attached to your terminal:

$
ps

Use the
-a, -u
, and
-x
options to include processes owned by others
and not attached to terminals as well as to display them in the
“user” mode. The command is valid with or without the dash:

$
ps -aux
$
ps aux

In this case, the dash is optional. However, certain
ps
options require the dash. (See the manpage
for details.)

If you are interested in finding process information on a
particular command, use the
-C
option. This
command displays all web server processes:

$
ps u -C httpd

You’ll note that the
-C
option
requires
the dash, but the
u
option won’t work with it if a dash is
included. This confusion exists because the
ps
command as implemented on Linux understands options in three
differing forms:

All of these option types may be freely intermixed. Instead of
the
-C
option, you may wish to use
ps
with other options that you usually use and
pipe the output to
grep
, searching for process
names, PIDs, or anything else you know about the process:

$
ps aux | grep httpd

In this case, the result would be the same list of
httpd
servers, as well as the
grep
command itself.

pstree

pstree [
options
] [
pid
|
user
]

This command displays a hierarchical list of processes
in a tree format.
pstree
is very handy for
understanding how
parent/child process relationships are set up.

If the
PID is specified, the displayed tree is rooted at that
process. Otherwise, it is rooted at the
init
process, which has PID 1. If
user
(a valid username) is specified,
trees for all processes owned by
user
are
shown. The tree is represented using characters that appear as
lines, such as
|
for vertical
lines and
+
for intersections
(VT100 line-drawing characters, displayed as solid lines by most
terminals, are optional). The output looks similar to this:

httpd-+-httpd
|-httpd
|-httpd
|-httpd
'-httpd

By default, visually identical branches of the tree are merged
to reduce output. Merged lines are preceded by a count indicating
the actual number of similar processes. The preceding example is
normally displayed on a single line:

httpd---5*[httpd]

This behavior can be turned off with the
-c
option.

Display a process tree including PIDs:

#
pstree -p
init(1)-+-atd(468)
|-bdflush(5)
|-crond(454)
|-httpd(440)-+-httpd(450)
|            |-httpd(451)
|            |-httpd(452)
|            |-httpd(453)
|            |-httpd(455)
|            |-httpd(456)
|            |-httpd(457)
|            '-httpd(458)
|-keventd(2)
|-kjournald(7)
|-klogd(335)
|-ksoftirqd_CPU0(3)
|-kswapd(4)
|-kupdated(6)
|-login(475)---bash(478)---pstree(518)
|-sendmail(420)
|-sshd(385)
|-syslogd(330)
'-xinetd(402)

top

top [
options
]

The
top
command offers output
similar to
ps
, but in a continuously updated
display. This is useful for situations in which you need to watch
the status of one or more processes or to see how they are using
your system.

In addition, a header of useful uptime, load, CPU status, and
memory information is displayed. By default, the process status
output is generated with the most CPU-intensive processes at the top
of the listing (and is named for the “top” processes). To format the
screen,
top
must understand how to control the
terminal display. The type of terminal (or terminal window) in use
is stored in the environment variable
TERM
. If this variable is not set or
contains an unknown terminal type,
top
may not
execute.

Dashes are optional in
top
options:

Once
top
is running interactively, it can
be given a number of commands via the keyboard to change its
behavior. These commands are single-key commands, some of which
cause
top
to prompt for input:

Simply executing
top
without options
gives a full status display updated every five seconds:

$
top

Use the
q
command to quit.

To run
top
with a faster refresh rate,
use the interval option, specified here with a one-second
refresh:

$
top -d 1

You may wish to use
top
to log its output
to a file. Use the
-b
(batch) option for this
purpose. In this batch example, the
-i
option
eliminates idle processes, the
-n
option, with
its argument, indicates five iterations, and the
-d
option indicates a one-second interval.
Results will be redirected to
file1
. This
command will take five seconds to execute and does not use the
optional dashes:

$
top –bi -n 5 -d 1 > file1

The single-key interactive commands can be used when
top
is running interactively. For example, if
you type the
h
command,
top
displays a help screen. If you enter the
n
command,
top
prompts you
for the number of lines you wish to display.

Using
top
to change the
nice
(priority modifier) value for a process is
discussed in
Objective 6: Modify Process Execution Priorities
.