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<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></A>

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<LI><A NAME="tex2html1897"
  HREF="#SECTION001210000000000000000">9.1 Introduction</A>
<LI><A NAME="tex2html1898"
  HREF="#SECTION001220000000000000000">9.2 <TT>
<FONT COLOR="#0000ff">ps</FONT></TT> -- List Running Processes</A>
<LI><A NAME="tex2html1899"
  HREF="#SECTION001230000000000000000">9.3 Controlling Jobs</A>
<LI><A NAME="tex2html1900"
  HREF="#SECTION001240000000000000000">9.4 Creating Background Processes</A>
<LI><A NAME="tex2html1901"
  HREF="#SECTION001250000000000000000">9.5 <TT>
<FONT COLOR="#0000ff">kill</FONT></TT><I>ing</I> a Process, Sending Signals</A>
<LI><A NAME="tex2html1902"
  HREF="#SECTION001260000000000000000">9.6 List of Common Signals</A>
<LI><A NAME="tex2html1903"
  HREF="#SECTION001270000000000000000">9.7 Niceness of Processes, Scheduling Priority</A>
<LI><A NAME="tex2html1904"
  HREF="#SECTION001280000000000000000">9.8 Process CPU/Memory Consumption, <TT>
<FONT COLOR="#0000ff">top</FONT></TT></A>
<LI><A NAME="tex2html1905"
  HREF="#SECTION001290000000000000000">9.9 Environments of Processes</A>
</UL>
<!--End of Table of Child-Links-->
<HR>

<H1><A NAME="SECTION001200000000000000000">
9. Processes and Environment Variables</A>
</H1>

<P>
<A NAME="chap:processenviron"></A>From this chapter you will get an idea about what is happening under the
hood of your U<SMALL>NIX</SMALL> system, but go have some coffee first.

<P>

<H1><A NAME="SECTION001210000000000000000">
9.1 Introduction</A>
</H1>

<P>
On U<SMALL>NIX</SMALL>, when you run a program (like any of the shell commands you have been
using), the actual computer instructions are read from a file on disk from
one of the <TT>
<FONT COLOR="#0000ff">bin/</FONT></TT> directories and placed in RAM.
The program is then executed in memory and becomes a <I>process</I>. A <I>process</I>
is some command/program/shell-script that is being run (or <I>executed</I>)
in memory. When the process has finished running, it is removed from memory.
There are usually about 50 processes running simultaneously at any one time
on a system with one person logged in. The CPU hops between each
of them to give a share of its <I>execution time</I>. <FONT COLOR="#ffa500">[Time given to carry out the instructions of a particular program. Note this
is in contrast to Windows or DOS where the program itself has to allow the others
a share of the CPU: under U<SMALL>NIX</SMALL>, the process has no say in the matter.
]</FONT>Each process is given a process number called the <I>PID</I>
(process ID). Besides the memory actually occupied by
the executable, the process itself seizes additional memory for its
operations.

<P>
In the same way that a file is owned by a particular user and group, a process
also has an owner--usually the person who ran the program. Whenever a process
tries to access a file, its ownership is compared to that of the file to decide
if the access is permissible. Because all devices are files, the only way a
process can do <I>anything</I> is through a file, and hence file permission
restrictions are the only kind of restrictions ever needed on U<SMALL>NIX</SMALL>. <FONT COLOR="#ffa500">[There are some exceptions to this.]</FONT> This
is how U<SMALL>NIX</SMALL> access control and security works.

<P>
The center of this operation is called the U<SMALL>NIX</SMALL> <I>kernel</I>.
The kernel is what actually does the hardware access, execution, allocation
of process IDs, sharing of CPU time, and ownership management.

<P>

<H1><A NAME="SECTION001220000000000000000">
9.2 <TT>
<FONT COLOR="#0000ff">ps</FONT></TT> -- List Running Processes</A>
</H1>

<P>
Log in on a terminal and type the command <TT>
<FONT COLOR="#0000ff">ps</FONT></TT>. You should get some output
like:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>&nbsp;PID&nbsp;TTY&nbsp;STAT&nbsp;&nbsp;TIME&nbsp;COMMAND</code><br>
<code>5995&nbsp;&nbsp;&nbsp;2&nbsp;S&nbsp;&nbsp;&nbsp;&nbsp;0:00&nbsp;/bin/login&nbsp;--&nbsp;myname</code><br>
<code>5999&nbsp;&nbsp;&nbsp;2&nbsp;S&nbsp;&nbsp;&nbsp;&nbsp;0:00&nbsp;-bash</code><br>
<code>6030&nbsp;&nbsp;&nbsp;2&nbsp;R&nbsp;&nbsp;&nbsp;&nbsp;0:00&nbsp;ps</code><br>
</FONT></TD></TR></TABLE><P>
<TT>
<FONT COLOR="#0000ff">ps</FONT></TT> with no options shows three processes to be running. These are the only
three processes visible to you as a user, although there are
other system processes not belonging to you. The first process
was the program that logged you in by displaying the login
prompt and requesting a password. It then ran a second process
call <TT>
<FONT COLOR="#0000ff">bash</FONT></TT>, the Bourne Again shell <FONT COLOR="#ffa500">[The Bourne shell
was the original U<SMALL>NIX</SMALL> shell]</FONT> where you have been typing commands.
Finally, you ran <TT>
<FONT COLOR="#0000ff">ps</FONT></TT>, which must have found itself
when it checked which processes were running, but then exited
immediately afterward.

<P>

<H1><A NAME="SECTION001230000000000000000">
9.3 Controlling Jobs</A>
</H1>

<P>
The shell has many facilities for controlling and executing processes--this
is called job control. Create a small script called <TT>
<FONT COLOR="#0000ff">proc.sh</FONT></TT>:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>#!/bin/sh</code><br>
<code>echo&nbsp;"proc.sh:&nbsp;is&nbsp;running"</code><br>
<code>sleep&nbsp;1000</code><br>
</FONT></TD></TR></TABLE><P>

<P>
Run the script with <TT>
<FONT COLOR="#0000ff">chmod&nbsp;0755&nbsp;proc.sh</FONT></TT> and then <TT>
<FONT COLOR="#0000ff">./proc.sh</FONT></TT>.
The shell <I>blocks</I>, waiting for the process to exit. Now press ^Z.
This will cause the process to <I>stop</I> (that is, pause but not terminate).
Now do a <TT>
<FONT COLOR="#0000ff">ps</FONT></TT> again. You will see
your script listed. However, it is not presently running because it is in the
condition of being stopped. Type <TT>
<FONT COLOR="#0000ff">bg</FONT></TT> (for <I>background</I>).
The script will now be ``unstopped'' and run in the background. You can now try to run
other processes in the meantime. Type <TT>
<FONT COLOR="#0000ff">fg</FONT></TT>, and the script returns
to the <I>foreground</I>. You can then type ^C to interrupt the process.

<P>

<H1><A NAME="SECTION001240000000000000000">
9.4 Creating Background Processes</A>
</H1>

<P>
Create a program that does something a little more interesting:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>5</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>#!/bin/sh</code><br>
<code>echo&nbsp;"proc.sh:&nbsp;is&nbsp;running"</code><br>
<code>while&nbsp;true&nbsp;&#059;&nbsp;do</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;-e&nbsp;'\a'</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;sleep&nbsp;2</code><br>
<code>done</code><br>
</FONT></TD></TR></TABLE><P>
Now perform the ^Z, <TT>
<FONT COLOR="#0000ff">bg</FONT></TT>, <TT>
<FONT COLOR="#0000ff">fg</FONT></TT>, and ^C operations from before.
To put a process immediately into the background, you can use:

<P>

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>./proc.sh&nbsp;&#038;</code><br>
</FONT></TD></TR></TABLE><P>
The <B>JOB CONTROL</B> section of the <TT>
<FONT COLOR="#0000ff">bash</FONT></TT> man page (<TT>
<FONT COLOR="#0000ff">bash</FONT></TT>(1)) looks like
this<FONT COLOR="#ffa500">(footnote follows)</FONT> <FONT COLOR="#ffa500">[Thanks to Brian Fox and Chet Ramey
for this material.]</FONT>:
(the footnotes are mine)

<P>

<BLOCKQUOTE><FONT SIZE="-1"><B>JOB CONTROL</B>
<BR>
<BR><I>Job control</I> refers to the ability to selectively stop (<I>suspend</I>)
the execution of processes and continue (<I>resume</I>) their execution at a
later point. A user typically employs this facility via an interactive interface
supplied jointly by the system's terminal driver and <B>bash</B>.
<BR>
<BR>
The shell associates a <I>job</I> with each
pipeline. <FONT COLOR="#ffa500">[What does this mean? It means that each time you execute something
in the background, it gets its own unique number, called the job number.]</FONT>It keeps a table of currently
executing jobs, which may be listed with the <B>jobs</B> command. When <B>bash</B>
starts a job asynchronously (in the <I>background</I>), it prints a line that
looks like:
<BR>
<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<B>[1] 25647</B>
<BR>
<BR>
indicating that this job is job number 1 and that the process ID of the last
process in the pipeline associated with this job is 25647. All of the processes
in a single pipeline are members of the same job. <B>Bash</B> uses the <I>job</I>
abstraction as the basis for job control.
<BR>
<BR>
To facilitate the implementation of the user interface to job control, the system
maintains the notion of a <I>current terminal process group ID</I>. Members
of this process group (processes whose process group ID is equal to the current
terminal process group ID) receive keyboard-generated signals such as <B>SIGINT</B>.
These processes are said to be in the <I>foreground</I>. <I>Background</I> processes
are those whose process group ID differs from the terminal's; such processes
are immune to keyboard-generated signals. Only foreground processes are allowed
to read from or write to the terminal. Background processes which attempt to
read from (write to) the terminal are sent a <B>SIGTTIN (SIGTTOU)</B> signal
by the terminal driver, which, unless caught, suspends the process.
<BR>
<BR>
If the operating system on which <B>bash</B> is running supports job control,
<B>bash</B> allows you to use it. Typing the <I>suspend</I> character (typically
<B>^Z</B>, Control-Z) while a process is running causes that process to
be stopped and returns you to <B>bash</B>. Typing the <I>delayed suspend</I>
character (typically <B>^Y</B>, Control-Y) causes the process to be stopped
when it attempts to read input from the terminal, and control to be returned
to <B>bash</B>. You may then manipulate the state of this job, using the <B>bg</B>
command to continue it in the background, the <B>fg</B> command to continue
it in the foreground, or the <B>kill</B> command to kill it. A <B>^Z</B>
takes effect immediately, and has the additional side effect of causing pending
output and typeahead to be discarded.
<BR>
<BR>
There are a number of ways to refer to a job in the shell. The character <B>%</B>
introduces a job name. Job number <I>n</I> may be referred to as <B>%n</B>.
A job may also be referred to using a prefix of the name used to start it, or
using a substring that appears in its command  line. For example, <B>%ce</B>
refers to a stopped <B>ce</B> job. If a prefix matches more than one job,
<B>bash</B> reports an error. Using <B>%?ce</B>, on the other hand, refers
to any job containing the string <B>ce</B> in its command  line. If the substring
matches more than one job, <B>bash</B> reports an error. The symbols <B>%%</B>
and <B>%+</B> refer to the shell's notion of the <I>current job</I>, which
is the last job stopped while it was in the foreground. The <I>previous job</I>
may be referenced using <B>%-</B>. In output pertaining to jobs (e.g., the
output of the <B>jobs</B> command), the current job is always flagged with
a <B>+</B>, and the previous job with a <B>-</B>.
<BR>
<BR>
Simply naming a job can be used to bring it into the foreground: <B>%1</B>
is a synonym for <B>``fg %1''</B>, bringing job 1 from the background into
the foreground. Similarly, <B>``%1 </B><B>&amp;</B><B>''</B> resumes job 1 in the background,
equivalent to <B>``bg %1''</B>.
<BR>
<BR>
The shell learns immediately whenever a job changes state. Normally, <B>bash</B>
waits until it is about to print a prompt before reporting changes in a job's
status so as to not interrupt any other output. If the <B>-b</B> option to
the <B>set</B> builtin command is set, <B>bash</B> reports such changes
immediately. (See also the description of <B>notify</B> variable under <B>Shell
Variables</B> above.)
<BR>
<BR>
If you attempt to exit <B>bash</B> while jobs are stopped, the shell prints
a message warning you. You may then use the <B>jobs</B> command to inspect
their status. If you do this, or try to exit again immediately, you are not
warned again, and the stopped jobs are terminated.
</FONT></BLOCKQUOTE>

<P>

<H1><A NAME="SECTION001250000000000000000">
9.5 <TT>
<FONT COLOR="#0000ff">kill</FONT></TT><I>ing</I> a Process, Sending Signals</A>
</H1>

<P>
<A NAME="sec:killingproc"></A>
<P>
To terminate a process, use the <TT>
<FONT COLOR="#0000ff">kill</FONT></TT> command:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>kill&nbsp;&#060;PID&#062;</code><br>
</FONT></TD></TR></TABLE><P>
The <TT>
<FONT COLOR="#0000ff">kill</FONT></TT> command actually sends a termination <I>signal</I> to the process. The sending of
a signal simply means that the process is asked to execute one of 30 predefined
functions. In some cases, developers would not have bothered to define a
function for a particular signal number (called <I>catching</I> the signal);
in which case the kernel will substitute
the default behavior for that signal. The default behavior for a signal is usually
to ignore the signal, to stop the process, or to terminate the process. 
The default behavior for the <I>termination</I> signal is to terminate the process. 

<P>
To send a specific signal to a process, you can name the signal on the command-line or use
its numerical equivalent:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>kill&nbsp;-SIGTERM&nbsp;12345</code><br>
</FONT></TD></TR></TABLE><P>
or

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>kill&nbsp;-15&nbsp;12345</code><br>
</FONT></TD></TR></TABLE><P>
which is the signal that <TT>
<FONT COLOR="#0000ff">kill</FONT></TT> normally sends when none is specified on the command-line.

<P>
To unconditionally terminate a process:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>kill&nbsp;-SIGKILL&nbsp;12345</code><br>
</FONT></TD></TR></TABLE><P>
or

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>kill&nbsp;-9&nbsp;12345</code><br>
</FONT></TD></TR></TABLE><P>
which should only be used as a last resort. <I>Processes are prohibited from ever catching
the</I> <TT>
<FONT COLOR="#0000ff">SIGKILL</FONT></TT> <I>signal</I>.

<P>
It is cumbersome to have to constantly look up the PID of a process.
Hence the GNU utilities have a command, <TT>
<FONT COLOR="#0000ff">killall</FONT></TT>, that sends
a signal to all processes of the same name:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>killall&nbsp;-&#060;signal&#062;&nbsp;&#060;process_name&#062;</code><br>
</FONT></TD></TR></TABLE><P>
This command is useful when you are sure that there is only one of a
process running, either because no one else is logged in
on the system or because you are not logged in as superuser.
<I>Note that on other U<SMALL>NIX</SMALL> systems, the <TT>
<FONT COLOR="#0000ff">killall</FONT></TT> command
kills <I>all</I> the processes that you are allowed to kill. If you
are root, this action would crash the machine.</I>

<P>

<H1><A NAME="SECTION001260000000000000000">
9.6 List of Common Signals</A>
</H1>

<P>
The full list of signals can be gotten from <TT>
<FONT COLOR="#0000ff">signal</FONT></TT>(7), and in the
file <TT>
<FONT COLOR="#0000ff">/usr/include/asm/signal.h</FONT></TT>.

<P>
<A NAME="page:commonsignals"></A>

<DL COMPACT>
<DT><B><TT>
<FONT COLOR="#0000ff">SIGHUP</FONT></TT></B> (1)
<DD><I>Hang up</I>. If the terminal becomes disconnected from a process,
this signal is sent automatically to the process. Sending a process this signal often
causes it to reread its configuration files, so it is useful instead of restarting the
process. Always check the man page to see if a process has this behavior.
<DT><B><TT>
<FONT COLOR="#0000ff">SIGINT</FONT></TT></B> (2)
<DD><I>Interrupt</I> from keyboard. Issued if you press ^C.
<DT><B><TT>
<FONT COLOR="#0000ff">SIGQUIT</FONT></TT></B> (3)
<DD><I>Quit</I> from keyboard. Issued if you press ^D.
<DT><B><TT>
<FONT COLOR="#0000ff">SIGFPE</FONT></TT></B> (8)
<DD><I>Floating point exception</I>. Issued automatically to a program performing some kind of illegal mathematical operation.
<DT><B><TT>
<FONT COLOR="#0000ff">SIGKILL</FONT></TT></B> (9)
<DD><I>Kill</I> signal. This is one of the signals that can never be <I>caught</I>
by a process. If a process gets this signal it <I>must</I> quit immediately and will
not perform any clean-up operations (like closing files or removing temporary files).
You can send a process a <TT>
<FONT COLOR="#0000ff">SIGKILL</FONT></TT> signal if there is no other means of destroying it.
<DT><B><TT>
<FONT COLOR="#0000ff">SIGUSR1</FONT></TT></B> (10), <B><TT>
<FONT COLOR="#0000ff">SIGUSR2</FONT></TT></B> (12)
<DD><I>User signal</I>.
These signals are available to developers when they need extra functionality. For example, some processes begin
logging debug messages when you send them <TT>
<FONT COLOR="#0000ff">SIGUSR1</FONT></TT>.
<DT><B><TT>
<FONT COLOR="#0000ff">SIGSEGV</FONT></TT></B> (11)
<DD><I>Segmentation violation</I>. Issued automatically when a process
tries to access memory outside of its allowable address space, equivalent to a <B>Fatal Exception</B> or <B>General Protection Fault</B> under Windows.
Note that programs with bugs or programs in the process of being developed often get these signals.
A program receiving a <TT>
<FONT COLOR="#0000ff">SIGSEGV</FONT></TT>, however, can never cause the rest of the system to be
compromised. If the kernel itself were to receive such an error, it would cause
the system to come down, but such is extremely rare.
<DT><B><TT>
<FONT COLOR="#0000ff">SIGPIPE</FONT></TT></B> (13)
<DD><I>Pipe</I> died. A program was writing to a pipe, the other end of
which is no longer available.
<DT><B><TT>
<FONT COLOR="#0000ff">SIGTERM</FONT></TT></B> (15)
<DD><I>Terminate</I>. Cause the program to quit gracefully
<DT><B><TT>
<FONT COLOR="#0000ff">SIGCHLD</FONT></TT></B> (17)
<DD><I>Child terminate</I>. Sent to a parent process every time
one of its spawned processes dies.
</DD>
</DL>

<P>

<H1><A NAME="SECTION001270000000000000000">
9.7 Niceness of Processes, Scheduling Priority</A>
</H1>

<P>
<A NAME="sec:nicenessschedulepri"></A>
All processes are allocated execution time by the kernel. If all
processes were allocated the same amount of time, performance
would obviously get worse as the number of processes increased.
The kernel uses heuristics <FONT COLOR="#ffa500">[Sets of rules.]</FONT> to guess how
much time each process should be allocated. The kernel tries to be
fair--two users competing for CPU usage should
both get the same amount.

<P>
Most processes spend their time waiting for either a key press,
some network input, some device to send data, or some time to
elapse. They hence do not consume CPU.

<P>
On the other hand, when more than one process runs flat
out, it can be difficult for the kernel to decide if it should be given
greater <I>priority</I>
than another process. What if a process is
doing some operation more important than another process? How
does the kernel tell? The answer is the U<SMALL>NIX</SMALL> feature of
<I>scheduling priority</I> or <I>niceness</I>. Scheduling priority
ranges from <TT>
<FONT COLOR="#0000ff">+20</FONT></TT> to <TT>
<FONT COLOR="#0000ff">-20</FONT></TT>. You can set a process's
niceness with the <TT>
<FONT COLOR="#0000ff">renice</FONT></TT> command.

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>renice&nbsp;&#060;priority&#062;&nbsp;&#060;pid&#062;</code><br>
<code>renice&nbsp;&#060;priority&#062;&nbsp;-u&nbsp;&#060;user&#062;</code><br>
<code>renice&nbsp;&#060;priority&#062;&nbsp;-g&nbsp;&#060;group&#062;</code><br>
</FONT></TD></TR></TABLE><P>

<P>
A typical example is the <I>SETI</I> program. <FONT COLOR="#ffa500">[SETI stands for Search for Extraterrestrial Intelligence.
SETI is an initiative funded by various obscure sources to scan the
skies for radio signals from other civilizations. The data
that SETI gathers has to be intensively processed. SETI distributes
part of that data to anyone who wants to run a <TT>
<FONT COLOR="#0000ff">seti</FONT></TT> 
program in the background. This puts the idle time of millions of
machines to ``good'' use. There is even a SETI screen-saver that has 
become quite popular. Unfortunately for the colleague in my office,
he runs <TT>
<FONT COLOR="#0000ff">seti</FONT></TT> at <TT>
<FONT COLOR="#0000ff">-19</FONT></TT> instead of <TT>
<FONT COLOR="#0000ff">+19</FONT></TT> scheduling
priority, so nothing on his machine works right. On the other hand,
I have inside information that the millions of other civilizations
in this galaxy and others are probably not using radio signals to
communicate at all :-)]</FONT> Set its priority to <TT>
<FONT COLOR="#0000ff">+19</FONT></TT> with:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>renice&nbsp;+19&nbsp;&#060;pid&#062;</code><br>
</FONT></TD></TR></TABLE><P>
to make it disrupt your machine as little as possible.

<P>
<I>Note that nice values have the reverse meaning that
you would expect: <TT>
<FONT COLOR="#0000ff">+19</FONT></TT> means a process that eats <I>little</I>
CPU, while <TT>
<FONT COLOR="#0000ff">-19</FONT></TT> is a process that eats <I>lots</I>. Only superuser
can set processes to negative nice values.</I>

<P>
Mostly, multimedia applications and some device utilities
are the only processes that need negative renicing, and most of
these will have their own command-line options to set the nice
value. See, for example, <TT>
<FONT COLOR="#0000ff">cdrecord</FONT></TT>(1) and <TT>
<FONT COLOR="#0000ff">mikmod</FONT></TT>(1) --
a negative nice value will prevent skips in your
playback. <FONT COLOR="#ffa500">[L<SMALL>INUX</SMALL> will soon have so called <I>real time</I>
process scheduling. This is a kernel feature that reduces scheduling
<I>latency</I> (the gaps between CPU execution time of a process,
as well as the time it takes for a process to wake). There are already
some kernel patches that accomplish this goal.]</FONT>
<P>
Also useful are the <TT>
<FONT COLOR="#0000ff">-u</FONT></TT> and <TT>
<FONT COLOR="#0000ff">-g</FONT></TT> options, which
set the priority of all the processes that a user or group owns.

<P>
Further, we have the <TT>
<FONT COLOR="#0000ff">nice</FONT></TT> command, which starts
a program under a defined niceness relative to the current nice
value of the present user. For example,

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>nice&nbsp;+&#060;priority&#062;&nbsp;&#060;pid&#062;</code><br>
<code>nice&nbsp;-&#060;priority&#062;&nbsp;&#060;pid&#062;</code><br>
</FONT></TD></TR></TABLE><P>

<P>
Finally, the <TT>
<FONT COLOR="#0000ff">snice</FONT></TT> command can both
display and set the current niceness. This command doesn't seem
to work on my machine.

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>snice&nbsp;-v&nbsp;&#060;pid&#062;</code><br>
</FONT></TD></TR></TABLE><P>

<P>

<H1><A NAME="SECTION001280000000000000000">
9.8 Process CPU/Memory Consumption, <TT>
<FONT COLOR="#0000ff">top</FONT></TT></A>
</H1>

<P>
The <TT>
<FONT COLOR="#0000ff">top</FONT></TT> command sorts all processes by their
CPU and memory consumption and
displays the <I>top</I> twenty or so in a table. Use <TT>
<FONT COLOR="#0000ff">top</FONT></TT>
whenever you want to see what's hogging your system.
<TT>
<FONT COLOR="#0000ff">top -q -d 2</FONT></TT> is useful for scheduling the <TT>
<FONT COLOR="#0000ff">top</FONT></TT>
command itself to a high priority, so that it is sure to refresh its
listing without lag. <TT>
<FONT COLOR="#0000ff">top -n 1 -b &gt; top.txt</FONT></TT> lists all processes,
and <TT>
<FONT COLOR="#0000ff">top -n 1 -b -p &lt;pid&gt;</FONT></TT> prints information on one process.

<P>
<TT>
<FONT COLOR="#0000ff">top</FONT></TT> has some useful interactive responses to key presses:
<DL>
<DT><STRONG>f</STRONG></DT>
<DD>Shows a list of displayed fields that you can alter
interactively. By default the only fields shown are
<TT>
<FONT COLOR="#0000ff">USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND</FONT></TT>
which is usually what you are most interested in. (The field meanings
are given below.)

<P>
</DD>
<DT><STRONG>r</STRONG></DT>
<DD>Renices a process.

<P>
</DD>
<DT><STRONG>k</STRONG></DT>
<DD>Kills a process.
</DD>
</DL>

<P>
The <TT>
<FONT COLOR="#0000ff">top</FONT></TT> man page describes the field meanings.
Some of these are confusing and assume knowledge of the
internals of <B>C</B> programs. The main question people ask is:
<I>How much memory is a process using?</I> The answer is given by the
<TT>
<FONT COLOR="#0000ff">RSS</FONT></TT> field, which stands for <I>Resident Set Size</I>.
<TT>
<FONT COLOR="#0000ff">RSS</FONT></TT> means the amount of RAM that a process consumes alone.
The following examples show totals for <I>all</I> processes running on my system
(which had 65536 kilobytes of RAM at the time).
They represent the total of the <TT>
<FONT COLOR="#0000ff">SIZE</FONT></TT>, <TT>
<FONT COLOR="#0000ff">RSS</FONT></TT>, and <TT>
<FONT COLOR="#0000ff">SHARE</FONT></TT>
fields, respectively.

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>5</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>10</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>echo&nbsp;`echo&nbsp;'0&nbsp;'&nbsp;&#059;&nbsp;top&nbsp;-q&nbsp;-n&nbsp;1&nbsp;-b&nbsp;|&nbsp;sed&nbsp;-e&nbsp;'1,/PID&nbsp;*USER&nbsp;*PRI/D'&nbsp;|&nbsp;\</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;awk&nbsp;'{print&nbsp;"+"&nbsp;$5}'&nbsp;|&nbsp;sed&nbsp;-e&nbsp;'s/M/\\*1024/'`&nbsp;|&nbsp;bc</code><br>
<code>68016</code><br>
<code>&nbsp;</code><br>
<code>echo&nbsp;`echo&nbsp;'0&nbsp;'&nbsp;&#059;&nbsp;top&nbsp;-q&nbsp;-n&nbsp;1&nbsp;-b&nbsp;|&nbsp;sed&nbsp;-e&nbsp;'1,/PID&nbsp;*USER&nbsp;*PRI/D'&nbsp;|&nbsp;\</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;awk&nbsp;'{print&nbsp;"+"&nbsp;$6}'&nbsp;|&nbsp;sed&nbsp;-e&nbsp;'s/M/\\*1024/'`&nbsp;|&nbsp;bc</code><br>
<code>58908</code><br>
<code>&nbsp;</code><br>
<code>echo&nbsp;`echo&nbsp;'0&nbsp;'&nbsp;&#059;&nbsp;top&nbsp;-q&nbsp;-n&nbsp;1&nbsp;-b&nbsp;|&nbsp;sed&nbsp;-e&nbsp;'1,/PID&nbsp;*USER&nbsp;*PRI/D'&nbsp;|&nbsp;\</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;awk&nbsp;'{print&nbsp;"+"&nbsp;$7}'&nbsp;|&nbsp;sed&nbsp;-e&nbsp;'s/M/\\*1024/'`&nbsp;|&nbsp;bc</code><br>
<code>30184</code><br>
</FONT></TD></TR></TABLE><P>
The <TT>
<FONT COLOR="#0000ff">SIZE</FONT></TT> represents the total memory usage of a process.
<TT>
<FONT COLOR="#0000ff">RSS</FONT></TT> is the same, but excludes memory not needing actual
RAM (this would be memory swapped to the swap partition).
<TT>
<FONT COLOR="#0000ff">SHARE</FONT></TT> is the amount shared between processes.

<P>
Other fields are described by the <TT>
<FONT COLOR="#0000ff">top</FONT></TT> man page (quoted verbatim) as follows:

<BLOCKQUOTE></BLOCKQUOTE><DL>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">uptime</FONT></TT></STRONG></DT>
<DD>This  line  displays the time the system has been up,
            and the three load averages for the system.  The load
            averages  are  the average number of processes ready to
            run during the last 1, 5 and 15 minutes.   This  line
            is  just  like  the  output of uptime(1).  The uptime
            display may be toggled by the interactive l  command.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">processes</FONT></TT></STRONG></DT>
<DD>The  total number of processes running at the time of
            the last update.  This is also broken down  into  the
            number of tasks which are running, sleeping, stopped,
            or undead. The processes and states  display  may  be
            toggled by the t interactive command.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">CPU states</FONT></TT></STRONG></DT>
<DD>Shows the percentage of CPU time in user mode, system
            mode, niced tasks, and idle.  (Niced tasks  are  only
            those  whose  nice value is negative.)  Time spent in
            niced tasks will also be counted in system  and  user
            time,  so the total will be more than 100%.  The
            processes and states display may be  toggled  by  the  t
            interactive command.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">Mem</FONT></TT></STRONG></DT>
<DD>Statistics on memory usage, including total available
            memory, free memory, used memory, shared memory,  and
            memory used for buffers. The display of memory
            information may be toggled by the m interactive command.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">Swap</FONT></TT></STRONG></DT>
<DD>Statistics on swap space, including total swap space,
            available  swap space, and used swap space.  This and
            Mem are just like the output of free(1).

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">PID</FONT></TT></STRONG></DT>
<DD>The process ID of each task.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">PPID</FONT></TT></STRONG></DT>
<DD>The parent process ID of each task.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">UID</FONT></TT></STRONG></DT>
<DD>The user ID of the task's owner.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">USER</FONT></TT></STRONG></DT>
<DD>The user name of the task's owner.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">PRI</FONT></TT></STRONG></DT>
<DD>The priority of the task.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">NI</FONT></TT></STRONG></DT>
<DD>The nice value of the task. Negative nice values are higher priority.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">SIZE</FONT></TT></STRONG></DT>
<DD>The  size  of  the  task's  code plus data plus stack
            space, in kilobytes, is shown here.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">TSIZE</FONT></TT></STRONG></DT>
<DD>The code size of the task. This gives strange  values
            for kernel processes and is broken for ELF processes.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">DSIZE</FONT></TT></STRONG></DT>
<DD>Data + Stack size. This is broken for ELF  processes.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">TRS</FONT></TT></STRONG></DT>
<DD>Text resident size.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">SWAP</FONT></TT></STRONG></DT>
<DD>Size of the swapped out part of the task.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">D</FONT></TT></STRONG></DT>
<DD>Size of pages marked dirty.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">LIB</FONT></TT></STRONG></DT>
<DD>Size of use library pages. This does not work for ELF
            processes.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">RSS</FONT></TT></STRONG></DT>
<DD>The total amount of physical memory used by the task,
            in  kilobytes,  is shown here. For ELF processes used
            library pages are counted here, for  a.out  processes
            not.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">SHARE</FONT></TT></STRONG></DT>
<DD>The amount of shared memory used by the task is shown
            in this column.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">STAT</FONT></TT></STRONG></DT>
<DD>The state of the task is shown  here.  The  state  is
            either S for sleeping, D for uninterruptible sleep, R
            for running, Z for  zombies,  or  T  for  stopped  or
            traced.  These states are modified by a trailing &lt; for a
            process with negative nice value,  N  for  a  process
            with positive nice value, W for a swapped out process
            (this does not work correctly for kernel  processes).

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">WCHAN</FONT></TT></STRONG></DT>
<DD>depending    on    the    availability    of    either
            /boot/psdatabase or the kernel  link  map
            /boot/System.map  this  shows  the  address or the name of the
            kernel function the task currently is sleeping in.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">TIME</FONT></TT></STRONG></DT>
<DD>Total CPU time the task has used  since  it  started.
            If  cumulative mode is on, this also includes the CPU
            time used by the process's children which have  died.
            You  can  set cumulative mode with the S command  line
            option or toggle it with the interactive  command  S.
            The header line will then be changed to CTIME.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">%CPU</FONT></TT></STRONG></DT>
<DD>The  task's  share  of  the  CPU  time since the last
            screen update, expressed as a percentage of total CPU
            time per processor.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">%MEM</FONT></TT></STRONG></DT>
<DD>The task's share of the physical memory.

<P>
</DD>
<DT><STRONG><TT>
<FONT COLOR="#0000ff">COMMAND</FONT></TT></STRONG></DT>
<DD>The  task's  command name, which will be truncated if
            it is too long to be displayed on one line.  Tasks in
            memory will have a full command  line, but swapped-out
            tasks will only have  the  name  of  the  program  in
            parentheses (for example, "(getty)").
</DD>
</DL><BLOCKQUOTE></BLOCKQUOTE>

<P>

<H1><A NAME="SECTION001290000000000000000">
9.9 Environments of Processes</A>
</H1>

<P>
Each process that runs does so with the knowledge of several
<I>var</I><TT>
<FONT COLOR="#0000ff">=</FONT></TT><I>value</I> text pairs. All this means is
that a process can look up the value of some variable that it
may have inherited from its parent process. The complete list of
these text pairs is called the <I>environment</I> of the
process, and each <I>var</I> is called an <I>environment
variable</I>. Each process has its own environment, which is
copied from the parent process's environment.

<P>
After you have logged in and have a shell prompt, the process
you are using (the shell itself) is just like any other process
with an environment with environment variables. To get a
complete list of these variables, just type:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>set</code><br>
</FONT></TD></TR></TABLE><P>
This command is useful for finding the value of an environment variable whose name
you are unsure of:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>set&nbsp;|&nbsp;grep&nbsp;&#060;regexp&#062;</code><br>
</FONT></TD></TR></TABLE><P>
Try <TT>
<FONT COLOR="#0000ff">set &#124; grep PATH</FONT></TT> to see the <TT>
<FONT COLOR="#0000ff">PATH</FONT></TT>
environment variable discussed previously.

<P>
The purpose of an environment is just to have an alternative way
of passing parameters to a program (in addition to command-line
arguments). The difference is that an environment is inherited from
one process to the next: for example, a shell might have a certain
variable set and may run a file manager,
which may run a word processor. The word processor inherited its
environment from file manager which inherited its environment
from the shell. If you had set an environment variable <TT>
<FONT COLOR="#0000ff">PRINTER</FONT></TT>
within the shell, it would have been inherited all the way to the
word processor, thus eliminating the need to separately configure
which printer the word processor should use.

<P>
Try

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>X="Hi&nbsp;there"</code><br>
<code>echo&nbsp;$X</code><br>
</FONT></TD></TR></TABLE><P>
You have set a variable. But now run

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>bash</code><br>
</FONT></TD></TR></TABLE><P>
You have now run a new process which is a <I>child</I> of the process you
were just in.
Type

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>echo&nbsp;$X</code><br>
</FONT></TD></TR></TABLE><P>
You will see that <TT>
<FONT COLOR="#0000ff">X</FONT></TT> is not set. The reason is that
the variable was not <TT>
<FONT COLOR="#0000ff">export</FONT></TT><I>ed</I> as an environment
variable and hence was not inherited. Now type

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>exit</code><br>
</FONT></TD></TR></TABLE><P>
which breaks to the <I>parent</I> process. Then

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>export&nbsp;X</code><br>
<code>bash</code><br>
<code>echo&nbsp;$X</code><br>
</FONT></TD></TR></TABLE><P>
You will see that the new <TT>
<FONT COLOR="#0000ff">bash</FONT></TT> now knows about <TT>
<FONT COLOR="#0000ff">X</FONT></TT>.

<P>
Above we are setting an arbitrary variable for our own use.
<TT>
<FONT COLOR="#0000ff">bash</FONT></TT> (and many other programs) automatically set many of
their own environment variables. The <TT>
<FONT COLOR="#0000ff">bash</FONT></TT> man page
lists these (when it talks about <TT>
<FONT COLOR="#0000ff">unset</FONT></TT><I>ting</I> a variable, it
means using the command <TT>
<FONT COLOR="#0000ff">unset &lt;variable&gt;</FONT></TT>). You may not
understand some of these at the moment, but they are included
here as a complete reference for later.

<P>
The following is quoted verbatim from the <TT>
<FONT COLOR="#0000ff">bash</FONT></TT> man page.
You will see that some variables are of the type that provide
special information and are read but never never set, whereas other
variables configure behavioral features of the shell (or other
programs) and can be set at any time<FONT COLOR="#ffa500">(footnote follows)</FONT> <FONT COLOR="#ffa500">[Thanks to Brian Fox and Chet Ramey for this material.]</FONT>.

<P>

<BLOCKQUOTE><FONT SIZE="-1"><B>Shell Variables</B>
</FONT></BLOCKQUOTE>
<P>
<BLOCKQUOTE><FONT SIZE="-1">The following variables are set by the shell:
</FONT></BLOCKQUOTE>
<P>
<BLOCKQUOTE></BLOCKQUOTE><DL>
<DT><STRONG>PPID</STRONG></DT>
<DD>The process ID of the shell's parent.
</DD>
<DT><STRONG>PWD</STRONG></DT>
<DD>The current working directory as set by the <B>cd</B> command.
</DD>
<DT><STRONG>OLDPWD</STRONG></DT>
<DD>The previous working directory as set by the <B>cd</B> command.
</DD>
<DT><STRONG>REPLY</STRONG></DT>
<DD>Set to the line of input read by the <B>read</B> builtin command when no arguments are supplied.
</DD>
<DT><STRONG>UID</STRONG></DT>
<DD>Expands to the user ID of the current user, initialized at shell startup.
</DD>
<DT><STRONG>EUID</STRONG></DT>
<DD>Expands to the effective user ID of the current user, initialized at shell startup.
</DD>
<DT><STRONG>BASH</STRONG></DT>
<DD>Expands to the full pathname used to invoke this instance of <B>bash</B>.
</DD>
<DT><STRONG>BASH_VERSION</STRONG></DT>
<DD>Expands to the version number of this instance of <B>bash</B>.
</DD>
<DT><STRONG>SHLVL</STRONG></DT>
<DD>Incremented by one each time an instance of <B>bash</B> is started.

<P>
</DD>
<DT><STRONG>RANDOM</STRONG></DT>
<DD>Each time this parameter is referenced, a random
integer is generated. The sequence of random numbers may be
initialized by assigning a value to <B>RANDOM</B>. If
<B>RANDOM</B> is unset, it loses its special properties, even
if it is subsequently reset.

<P>
</DD>
<DT><STRONG>SECONDS</STRONG></DT>
<DD>Each time this parameter is referenced, the
number of seconds since shell invocation is returned. If a value
is assigned to <B>SECONDS</B>. the value returned upon
subsequent references is the number of seconds since the
assignment plus the value assigned. If <B>SECONDS</B> is
unset, it loses its special properties, even if it is
subsequently reset.

<P>
</DD>
<DT><STRONG>LINENO</STRONG></DT>
<DD>Each time this parameter is referenced, the shell
substitutes a decimal number representing the current sequential
line number (starting with 1) within a script or function.  When
not in a script or function, the value substituted is not
guaranteed to be meaningful. When in a function, the value is
not the number of the source line that the command appears on
(that information has been lost by the time the function is
executed), but is an approximation of the number of
<I>simple commands</I> executed in the current function.
If <B>LINENO</B> is unset, it loses its special properties,
even if it is subsequently reset.

<P>
</DD>
<DT><STRONG>HISTCMD</STRONG></DT>
<DD>The history number, or index in the history list,
of the current command. If <B>HISTCMD</B> is unset, it loses
its special properties, even if it is subsequently reset.

<P>
</DD>
<DT><STRONG>OPTARG</STRONG></DT>
<DD>The value of the last option argument processed by
the <B>getopts</B> builtin command (see <B>SHELL BUILTIN
COMMANDS</B> below).

<P>
</DD>
<DT><STRONG>OPTIND</STRONG></DT>
<DD>The index of the next argument to be processed by
the <B>getopts</B> builtin command (see <B>SHELL BUILTIN
COMMANDS</B> below).

<P>
</DD>
<DT><STRONG>HOSTTYPE</STRONG></DT>
<DD>Automatically set to a string that uniquely
describes the type of machine on which <B>bash</B> is
executing. The default is system-dependent.

<P>
</DD>
<DT><STRONG>OSTYPE</STRONG></DT>
<DD>Automatically set to a string that describes the
operating system on which <B>bash</B> is executing. The
default is system-dependent.
</DD>
</DL><BLOCKQUOTE></BLOCKQUOTE>
<P>
<BLOCKQUOTE><FONT SIZE="-1">The following variables are used by the shell.  In some cases,
<B>bash</B>
assigns a default value to a variable; these cases are noted
below.
</FONT></BLOCKQUOTE>
<P>
<BLOCKQUOTE></BLOCKQUOTE><DL>
<DT><STRONG>IFS</STRONG></DT>
<DD>The
<I>Internal Field Separator</I>
that is used
for word splitting after expansion and to
split lines into words with the
<B>read</B>
builtin command.  The default value is
``&#60;space&#62;&#60;tab&#62;&#60;newline&#62;''.
</DD>
<DT><STRONG>PATH</STRONG></DT>
<DD>The search path for commands.  It
is a colon-separated list of directories in which
the shell looks for commands (see
<B>COMMAND EXECUTION</B>
below).  The default path is system-dependent,
and is set by the administrator who installs
<B>bash</B>.
A common value is ``/usr/gnu/bin:/usr/local/bin:/usr/ucb:/bin:/usr/bin:.''.
</DD>
<DT><STRONG>HOME</STRONG></DT>
<DD>The home directory of the current user; the default argument for the
<B>cd</B> builtin command.
</DD>
<DT><STRONG>CDPATH</STRONG></DT>
<DD>The search path for the <B>cd</B> command. This is a colon-separated
list of directories in which the shell looks for destination directories
specified by the <B>cd</B> command.  A sample value is <code>``.:~:/usr''.</code>
</DD>
<DT><STRONG>ENV</STRONG></DT>
<DD>If this parameter is set when <B>bash</B> is executing a shell script,
its value is interpreted as a filename containing commands to
initialize the shell, as in
<I>.bashrc</I>.
The value of
<B>ENV</B>
is subjected to parameter expansion, command substitution, and arithmetic
expansion before being interpreted as a pathname.
<B>PATH</B>
is not used to search for the resultant pathname.
</DD>
<DT><STRONG>MAIL</STRONG></DT>
<DD>If this parameter is set to a filename and the
<B>MAILPATH</B>
variable is not set,
<B>bash</B>
informs the user of the arrival of mail in the specified file.
</DD>
<DT><STRONG>MAILCHECK</STRONG></DT>
<DD>Specifies how
often (in seconds)
<B>bash</B>
checks for mail.  The default is 60 seconds.  When it is time to check
for mail, the shell does so before prompting.
If this variable is unset, the shell disables mail checking.
</DD>
<DT><STRONG>MAILPATH</STRONG></DT>
<DD>A colon-separated list of pathnames to be checked for mail. 
The message to be printed may be specified by separating the pathname from
the message with a `?'.  $_ stands for the name of the current mailfile. 
Example:
<BR><FONT SIZE="-2"><code>MAILPATH='/usr/spool/mail/bfox?"You have mail":~/shell-mail?"$_ has mail!"'</code></FONT>
<B>Bash</B>
supplies a default value for this variable, but the location of the user
mail files that it uses is system dependent (e.g., /usr/spool/mail/<B>$USER</B>).
</DD>
<DT><STRONG>MAIL_WARNING</STRONG></DT>
<DD>If set, and a file that <B>bash</B> is checking for mail has been
accessed since the last time it was checked, the message ``The mail in
<I>mailfile</I> has been read'' is printed.
</DD>
<DT><STRONG>PS1</STRONG></DT>
<DD>The value of this parameter is expanded (see
<B>PROMPTING</B>
below) and used as the primary prompt string.  The default value is
``<B>bash&#92;$ </B>''.
</DD>
<DT><STRONG>PS2</STRONG></DT>
<DD>The value of this parameter is expanded
and used as the secondary prompt string.  The default is
``<B>&#62;&nbsp;</B>''.
</DD>
<DT><STRONG>PS3</STRONG></DT>
<DD>The value of this parameter is used as the prompt for the
<I>select</I>
command (see
<B>SHELL GRAMMAR</B>
above).
</DD>
<DT><STRONG>PS4</STRONG></DT>
<DD>The value of this parameter is expanded
and the value is printed before each command
<B>bash</B>
displays during an execution trace.  The first character of
<B>PS4</B>
is replicated multiple times, as necessary, to indicate multiple
levels of indirection.  The default is ``<B>+&nbsp;</B>''.
</DD>
<DT><STRONG>HISTSIZE</STRONG></DT>
<DD>The number of commands to remember in the command history (see
<B>HISTORY</B>
below).  The default value is 500.
</DD>
<DT><STRONG>HISTFILE</STRONG></DT>
<DD>The name of the file in which command history is saved.  (See
<B>HISTORY</B>
below.)  The default value is <I>~/.bash_history</I>.  If unset, the
command history is not saved when an interactive shell exits.
</DD>
<DT><STRONG>HISTFILESIZE</STRONG></DT>
<DD>The maximum number of lines contained in the history file.  When this
variable is assigned a value, the history file is truncated, if
necessary, to contain no more than that number of lines.  The default
value is 500.
</DD>
<DT><STRONG>OPTERR</STRONG></DT>
<DD>If set to the value 1,
<B>bash</B>
displays error messages generated by the
<B>getopts</B>
builtin command (see
<B>SHELL BUILTIN COMMANDS</B>
below).
<B>OPTERR</B>
is initialized to 1 each time the shell is invoked or a shell
script is executed.
</DD>
<DT><STRONG>PROMPT_COMMAND</STRONG></DT>
<DD>If set, the value is executed as a command prior to issuing each primary
prompt.
</DD>
<DT><STRONG>IGNOREEOF</STRONG></DT>
<DD>Controls the
action of the shell on receipt of an
<B>EOF</B>
character as the sole input.  If set, the value is the number of
consecutive
<B>EOF</B>
characters typed as the first characters on an input line before
<B>bash</B>
exits.  If the variable exists but does not have a numeric value, or
has no value, the default value is 10.  If it does not exist,
<B>EOF</B>
signifies the end of input to the shell.  This is only in effect for
interactive shells.
</DD>
<DT><STRONG>TMOUT</STRONG></DT>
<DD>If set to a value greater than zero, the value is interpreted as the
number of seconds to wait for input after issuing the primary prompt.
<B>Bash</B>
terminates after waiting for that number of seconds if input does
not arrive.
</DD>
<DT><STRONG>FCEDIT</STRONG></DT>
<DD>The default editor for the
<B>fc</B>
builtin command.
</DD>
<DT><STRONG>FIGNORE</STRONG></DT>
<DD>A colon-separated list of suffixes to ignore when performing
filename completion (see
<B>READLINE</B>
below).  A filename whose suffix matches one of the entries in 
<B>FIGNORE</B>
is excluded from the list of matched filenames.  A sample
value is ``.o:~''.
</DD>
<DT><STRONG>INPUTRC</STRONG></DT>
<DD>The filename for the readline startup file, overriding the default
of
<I>~/.inputrc</I>
(see
<B>READLINE</B>
below).
</DD>
<DT><STRONG>notify</STRONG></DT>
<DD>If set,
<B>bash</B>
reports terminated background jobs immediately, rather than waiting
until before printing the next primary prompt (see also the
<B>-b</B>
option to the
<B>set</B>
builtin command).
</DD>
<DT><STRONG>history_control</STRONG></DT>
<DD>
</DD>
<DT><STRONG>HISTCONTROL</STRONG></DT>
<DD>If set to a value of
<I>ignorespace</I>,
lines which begin with a
<B>space</B>
character are not entered on the history list.  If set to
a value of
<I>ignoredups</I>,
lines matching the last history line are not entered.
A value of
<I>ignoreboth</I>
combines the two options.
If unset, or if set to any other value than those above,
all lines read
by the parser are saved on the history list.
</DD>
<DT><STRONG>command_oriented_history</STRONG></DT>
<DD>If set,
<B>bash</B>
attempts to save all lines of a multiple-line
command in the same history entry.  This allows
easy re-editing of multi-line commands.
</DD>
<DT><STRONG>glob_dot_filenames</STRONG></DT>
<DD>If set,
<B>bash</B>
includes filenames beginning with a `.' in the results of pathname
expansion.
</DD>
<DT><STRONG>allow_null_glob_expansion</STRONG></DT>
<DD>If set,
<B>bash</B>
allows pathname patterns which match no
files (see
<B>Pathname Expansion</B>
below)
to expand to a null string, rather than themselves.
</DD>
<DT><STRONG>histchars</STRONG></DT>
<DD>The two or three characters which control history expansion
and tokenization (see
<B>HISTORY EXPANSION</B>
below).  The first character is the
<I>history expansion character</I>,
that is, the character which signals the start of a history
expansion, normally `<B>!</B>'.
The second character is the
<I>quick substitution</I>
character, which is used as shorthand for re-running the previous
command entered, substituting one string for another in the command.
The default is `<B>^</B>'.
The optional third character is the character
which signifies that the remainder of the line is a comment, when found
as the first character of a word, normally `<B>#</B>'.  The history
comment character causes history substitution to be skipped for the
remaining words on the line.  It does not necessarily cause the shell
parser to treat the rest of the line as a comment.
</DD>
<DT><STRONG>nolinks</STRONG></DT>
<DD>If set, the shell does not follow symbolic links when executing
commands that change the current working directory.  It uses the
physical directory structure instead.  By default,
<B>bash</B>
follows the logical chain of directories when performing commands
which change the current directory, such as 
<B>cd</B>.
See also the description of the <B>-P</B> option to the <B>set</B>
builtin (
<B>SHELL BUILTIN COMMANDS</B>
below).
</DD>
<DT><STRONG>hostname_completion_file</STRONG></DT>
<DD>
</DD>
<DT><STRONG>HOSTFILE</STRONG></DT>
<DD>Contains the name of a file in the same format as
<I>/etc/hosts</I>
that should be read when the shell needs to complete a
hostname.  The file may be changed interactively; the next
time hostname completion is attempted
<B>bash</B>
adds the contents of the new file to the already existing database.
</DD>
<DT><STRONG>noclobber</STRONG></DT>
<DD>If set,
<B>bash</B>
does not overwrite an existing file with the
<B>&#62;</B>,
<B>&#62;&amp;</B>,
and
<B>&#60;&#62;</B>
redirection operators.  This variable may be overridden when 
creating output files by using the redirection operator
<B>&#62;|</B>
instead of
<B>&#62;</B>
(see also the <B>-C</B> option to the
<B>set</B>
builtin command).
</DD>
<DT><STRONG>auto_resume</STRONG></DT>
<DD>This variable controls how the shell interacts with the user and
job control.  If this variable is set, single word simple
commands without redirections are treated as candidates for resumption
of an existing stopped job.  There is no ambiguity allowed; if there is
more than one job beginning with the string typed, the job most recently
accessed is selected.  The
<I>name</I>
of a stopped job, in this context, is the command  line used to
start it.
If set to the value
<I>exact</I>,
the string supplied must match the name of a stopped job exactly;
if set to
<I>substring</I>,
the string supplied needs to match a substring of the name of a
stopped job.  The
<I>substring</I>
value provides functionality analogous to the
<B>%?</B>
job id (see
<B>JOB CONTROL</B>
below).  If set to any other value, the supplied string must
be a prefix of a stopped job's name; this provides functionality
analogous to the
<B>%</B>
job id.
</DD>
<DT><STRONG>no_exit_on_failed_exec</STRONG></DT>
<DD>If this variable exists, a non-interactive shell will not exit if
it cannot execute the file specified in the
<B>exec</B>
builtin command.  An interactive shell does not exit if
<B>exec</B>
fails.
</DD>
<DT><STRONG>cdable_vars</STRONG></DT>
<DD>If this is set, an argument to the
<B>cd</B>
builtin command that
is not a directory is assumed to be the name of a variable whose
value is the directory to change to.
</DD>
</DL><BLOCKQUOTE></BLOCKQUOTE>

<P>
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