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linux-unix130.linux.and.unix.ebooks130 linux and unix ebookslinuxLearning Linux - Collection of 12 E

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<A NAME="Heading32<FONT COLOR="#000077"><B>NOTE:</B> </FONT>If you are logged

	in as <TT>root</TT>, you will see a list of all processes on the system. This is

	because the <TT>root</TT> username, being the superuser, owns everything that happens

	on the Linux system. If you are an &quot;ordinary&quot; user, but have also logged

	in on another terminal (including another virtual terminal you have selected by pressing

	Alt-Fn as discussed in Chapter 6, &quot;Getting Started&quot;), you will see the

	processes you are running on the other terminal (or terminals) as well.

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<P>One useful option with <TT>ps</TT> is <TT>u</TT>. Although it stands for &quot;user,&quot;

as in &quot;List the username as well,&quot; it actually adds quite a few more columns

of information in addition to just the username:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">darkstar:~$ ps -u

USER   PID %CPU %MEM SIZE  RSS TTY STAT START   TIME COMMAND

fido    41  0.1  6.8  364  472 v01 S    23:19   0:01 -bash

fido   138  0.0  3.3   72  228 v01 R    23:34   0:00 ps -u

</FONT></PRE>

<P>In addition to the username in the <TT>USER</TT> column, other interesting new

items include <TT>%CPU</TT>, which shows you what percentage of your computer's processing

power is being used by the process, and <TT>%MEM</TT>, which shows you what percentage

of your computer's memory is being used by the process.</P>

<P>If you want to see all processes running on the system, and not just the processes

started by your own username, you can use the <TT>a</TT> command option. (The <TT>root</TT>

login sees everyone's processes automatically and does not have to use <TT>a</TT>,

so <TT>root</TT> can get the following output by simply typing <TT>ps</TT>.)<FONT

COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">darkstar:~$ ps -a

 PID  TTY STAT  TIME COMMAND

  62  v03 S     0:00 /sbin/agetty 38400 tty3

  63  v04 S     0:00 /sbin/agetty 38400 tty4

  64  v05 S     0:00 /sbin/agetty 38400 tty5

  65  v06 S     0:00 /sbin/agetty 38400 tty6

 330  v02 S     0:00 -bash

 217  v01 S     0:00 -bash

 217  v01 S     0:00 ps -a

</FONT></PRE>

<P>As you can see, quite a few &quot;other&quot; processes are happening on the system!

In fact, most of the processes we see here will be running whether or not anyone

is actually logged into the Linux system. All the processes listed as running on

<TT>tty psf</TT> are actually system processes, and are started every time you boot

up the Linux system. Processes of the form <TT>/sbin/agetty 38400 tty6</TT> are login

processes running on a particular terminal waiting for your login.</P>

<P>It can be useful to combine the <TT>a</TT> and <TT>u</TT> options (if you're not

<TT>root</TT>).<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">darkstar:~$ ps -au

USER       PID %CPU %MEM SIZE  RSS TTY STAT START   TIME COMMAND

root        72  0.0  3.6  390  532 v01 S    17:55   0:01 -bash

root        74  0.0  1.5   41  224 v03 S    17:55   0:00 /sbin/agetty 38400 tty3

root        75  0.0  1.5   41  224 v04 S    17:55   0:00 /sbin/agetty 38400 tty4

root        76  0.0  1.5   41  224 v05 S    17:55   0:00 /sbin/agetty 38400 tty5

root        77  0.0  1.5   41  224 v06 S    17:55   0:00 /sbin/agetty 38400 tty6

root        78  0.0  1.5   56  228 s00 S    17:55   0:00 gpm -t mman

root        98  0.0  1.5   41  224 v02 S    18:02   0:00 /sbin/agetty 38400 tty2

root       108 18.8  3.6  384  528 pp0 S    18:27   0:01 -bash

</FONT></PRE>

<P>A more technical <TT>l</TT> option can sometimes be useful:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">darkstar:~$ ps -l

 F    UID   PID  PPID PRI NI SIZE  RSS WCHAN      STAT TTY   TIME COMMAND

  0   501    41     1  15  0  364  472 114d9c     S    v01   0:00 -bash

  0   501   121    41  29  0   64  208 0          R    v01   0:00 ps -l

</FONT></PRE>

<P>The interesting information is in the <TT>PPID</TT> column. PPID stands for &quot;Parent

Process ID&quot;--in other words, the process that started the particular process.

Notice that the <TT>ps -l</TT> command was started by <TT>-bash</TT>, the <TT>login</TT>

shell. In other words, <TT>ps -l</TT> was started from the command line. Notice also

that the PPID for the login shell is PID <TT>1</TT>. If you check the output from

<TT>ps -au</TT> above, you will see that the process with PID of <TT>1</TT> is <TT>init</TT>.

The <TT>init</TT> process is the one that spawns, or starts, all other processes.

If <TT>init</TT> dies, the system crashes!



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<A NAME="Heading33<FONT COLOR="#000077"><B>NOTE: </B></FONT>The Linux <TT>ps</TT>

	command has some quirks when it comes to options. First of all, the dash before the

	options is not necessary. In the earlier example, <TT>ps l</TT> would work the same

	as <TT>ps -l</TT>. Because most Linux commands do require the use of dashes with

	their command options, and other versions of UNIX might require dashes when using

	<TT>ps</TT>, it's best to use the dash anyway. Second, the order in which you enter

	the options does matter, especially if you try to combine the <TT>l</TT> and <TT>u</TT>

	options! Try typing <TT>ps -lu</TT>, and then <TT>ps -ul</TT>. This behavior is not

	covered in the <TT>ps</TT> man page. The moral is twofold: First, use the minimum

	possible number of command options. Second, the man pages are, alas, not always correct

	and complete.

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<H4 ALIGN="CENTER"><A NAME="Heading34<FONT COLOR="#000077">The Process Termination

Command: kill</FONT></H4>

<P>The <TT>kill</TT> command is used to terminate processes that can't be stopped

by other means.



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<A NAME="Heading35<FONT COLOR="#000077"><B>NOTE:</B> </FONT>Before going through

	the following procedure, if it's a program you're stuck in, make sure you can't stop

	or exit it by typing Ctrl-C or some other key combination.

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	<DD><B>1. </B>Switch to another virtual console and log in as <TT>root</TT>.<BR>

	<B><BR>

	2. </B>Run <TT>ps -u</TT> and identify the offending process. You will use its PID

	in the next step.<BR>

	<B><BR>

	3.</B> Use the <TT>kill</TT> program by typing <TT>kill &lt;</TT>PID<TT>&gt;</TT>,

	where PID is the Process ID you want to kill. Make sure that you have correctly identified

	the offending process! As root, you can kill any user process, including the wrong

	one if you misread or mistype the PID.<BR>

	<B><BR>

	4.</B> Verify that the process has been killed by using <TT>ps -u</TT> again. You

	can type <TT>ps -u &lt;</TT>PID<TT>&gt;</TT>, which shows you the status of only

	the specified PID. If there's a null result and you're just given the Linux prompt

	again, the PID is dead, so go to step 8. However, it's best to look at the complete

	<TT>ps -u</TT> list if it's not too long. Sometimes the offending process reappears

	with a new PID! If that is the case, go to step 6.<BR>

	<B><BR>

	5. </B>If the process is still alive and has the same PID, use <TT>kill</TT>'s <TT>9</TT>

	option. Type <TT>kill -9 &lt;</TT>PID<TT>&gt;</TT>. Check it as in step 4. If this

	does not kill the process, go to step 7. If the process is <BR>

	now dead, go to step 8.<BR>

	<B><BR>

	6.</B> If the offending process has reappeared with a new PID, that means that it's

	being created automatically by some other process. The only thing to do now is to

	kill the parent process, which is the true offender! You might also have to kill

	the parent process when <TT>kill -9</TT> does not work.<BR>

	<B><BR>

	7.</B> Use <TT>ps -l</TT> to identify the troublesome process's PPID. This is the

	PID of the parent process. You should check the parent's identity more closely by

	typing <TT>ps -u &lt;</TT>Parent PID<TT>&gt;</TT> before going ahead and killing

	it as described in step 3, using the PID of the parent in the <TT>kill</TT> command.

	You should follow through with step 4 and, if necessary, step 5, making sure the

	parent process has been killed.<BR>

	<B><BR>

	8.</B> The process is killed. Remember to log off. You should not leave <TT>root</TT>

	logged in on virtual consoles, because you will forget that the <TT>root</TT> logins

	are there!

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<A NAME="Heading36<FONT COLOR="#000077"><B>NOTE:</B> </FONT>Sometimes processes

	are simply unkillable! In this case, you're best off shutting down the Linux system

	and rebooting.

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<P>Linux keeps ordinary users (as opposed to root) from killing other users' processes

(maliciously or otherwise). For instance, if you are an ordinary user and you try

to kill the <TT>init</TT> process, which always has PID=1, you will see<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">darkstar:~$ kill 1

kill:  (1) - Not owner

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<P>Actually, not even <TT>root</TT> can kill the <TT>init</TT> process, although

there is no error message. The <TT>init</TT> process is one of those &quot;unkillable&quot;

processes discussed earlier, because it's such a key process. That's all for the

best!

<H3 ALIGN="CENTER"><A NAME="Heading37<FONT COLOR="#000077">Becoming Someone

Else: The su Command</FONT></H3>

<P>Usually, when you want to temporarily become a different user, you will simply

switch to an- other virtual terminal, log in as the other user, log out when you're

done, and return to your &quot;home&quot; virtual terminal. However, there are times

when this is impractical or inconvenient. Perhaps all your virtual terminals are

already busy, or perhaps you're in a situation (such as logged on via a telephone

and modem) in which you don't have virtual terminals available.</P>

<P>In these cases, you can use the <TT>su</TT> command. &quot;su&quot; stands for

&quot;super user.&quot; If you type <TT>su</TT> by itself, you will be prompted for

the <TT>root</TT> password. If you successfully enter the <TT>root</TT> password,

you will see the <TT>root</TT> <TT>#</TT> prompt, and you will have all of <TT>root</TT>'s

privileges.</P>

<P>You can also become any other user by typing <TT>su </TT>&lt;username&gt;. If

you are <TT>root</TT> when you type <TT>su </TT>&lt;username&gt;, you are not asked

for that user's password since in principle you could change the user's password

or examine all the user's files from the <TT>root</TT> login anyway. If you are an

&quot;ordinary&quot; user trying to change to another ordinary user, you will be

asked to enter the password of the user you are trying to become.



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<A NAME="Heading38<FONT COLOR="#000077"><B>NOTE:</B> </FONT>Although <TT>su</TT>

	grants you all the privileges you would get if you logged on as that user, be aware

	that you won't inherit that login's exact environment or run that login's startup

	files (if any). This means that <TT>su</TT> is not really suited to doing extended

	work, and it's quite unsuitable for troubleshooting problems with that login.

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<H3 ALIGN="CENTER"><A NAME="Heading39<FONT COLOR="#000077">The grep Command</FONT></H3>

<P>&quot;What on earth does <TT>grep</TT> mean?&quot; you ask.</P>

<P>This is a fair question. <TT>grep</TT> must be the quintessential UNIX acronym,

because it's impossible to understand even when it's spelled out in full!</P>

<P><TT>grep</TT> stands for Global Regular Expression Parser. You will understand

the use of this command right away, but when &quot;Global Regular Expression Parser&quot;

becomes a comfortable phrase in itself, you should probably consider taking a vacation.</P>

<P>What <TT>grep</TT> does, essentially, is find and display lines that contain a

pattern that you specify. There are two basic ways to use <TT>grep</TT>.</P>

<P>The first use of <TT>grep</TT> is to filter the output of other commands. The

general syntax is <TT>&lt;</TT>command<TT>&gt; | grep &lt;</TT>pattern<TT>&gt;</TT>.

For instance, if we wanted to see every actively running process on the system, we

would type <TT>ps -a | grep R</TT>. In this application, <TT>grep</TT> passes on

only those lines that contain the pattern (in this case, the single letter) R. Note

that if someone were running a program called <TT>Resting</TT>, it would show up

even if its status were <TT>S</TT> for sleeping, because <TT>grep</TT> would match

the R in <TT>Resting</TT>. An easy way around this problem is to type <TT>grep &quot;

R &quot;</TT>, which explicitly tells <TT>grep</TT> to search for an R with a space

on each side. You must use quotes whenever you search for a pattern that contains

one or more blank spaces.</P>

<P>The second use of <TT>grep</TT> is to search for lines that contain a specified

pattern in a specified file. The syntax here is <TT>grep &lt;</TT>pattern<TT>&gt;

&lt;</TT>filename<TT>&gt;</TT>. Be careful. It's easy to specify the filename first

and the pattern second by mistake! Again, you should be as specific as you can with

the pattern to be matched, in order to avoid &quot;false&quot; matches.

<H3 ALIGN="CENTER"><A NAME="Heading40<FONT COLOR="#000077">Summary</FONT></H3>

<P>By this point you should have tried enough different Linux commands to start getting

familiar (if not yet entirely comfortable) with typical Linux usage conventions.</P>

<P>It is important that you be able to use the man pages provided online by Linux.

A very good exercise at this point is to pull up man pages for all the commands we

have looked at in the past two chapters: <TT>login</TT>, <TT>passwd</TT>, <TT>who</TT>,

<TT>adduser</TT>, and so on. If some of the commands listed under &quot;See also:&quot;

look interesting, by all means take a look at their man pages too!



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<A NAME="Heading41<FONT COLOR="#000077"><B>NOTE:</B> </FONT>Some man pages,

	such as the one for <TT>bash</TT>, are extremely long. Do not plan to read them all

	in one sitting!

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<P>In Chapter 8, we head out from &quot;home&quot; and poke around in the Linux file

system. As system administrators, we should know what our hard drives contain! For

instance, there are special &quot;administrator-only&quot; directories crammed with

goodies.</P>



<P>Several more &quot;essential&quot; commands will be introduced. By the end of

the next chapter, you will have seen and tried most of the important &quot;user&quot;

Linux commands and will have had a taste of some of the &quot;administrator&quot;

commands.









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