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这是很好的学习嵌入式LINUX的文章

<div class="CALLOUTLIST">
<dl compact="COMPACT">
<dt><a href="term.html#CO-TTYS-LINE1COL1"><img src="./imagelib/callouts/1.png" hspace="0"
vspace="0" border="0" alt="(1)" /></a></dt>

<dd>The first field normally specifies the name of the terminal special file as it is
found in <tt class="FILENAME">/dev</tt>.</dd>

<dt><a href="term.html#CO-TTYS-LINE1COL2"><img src="./imagelib/callouts/2.png" hspace="0"
vspace="0" border="0" alt="(2)" /></a></dt>

<dd>The second field is the command to execute for this line, which is usually <a
href="http://www.FreeBSD.org/cgi/man.cgi?query=getty&sektion=8"><span
class="CITEREFENTRY"><span class="REFENTRYTITLE">getty</span>(8)</span></a>. <tt
class="COMMAND">getty</tt> initializes and opens the line, sets the speed, prompts for a
user name and then executes the <a
href="http://www.FreeBSD.org/cgi/man.cgi?query=login&sektion=1"><span
class="CITEREFENTRY"><span class="REFENTRYTITLE">login</span>(1)</span></a> program.</dd>

<dd>
<p>The <tt class="COMMAND">getty</tt> program accepts one (optional) parameter on its
command line, the <var class="REPLACEABLE">getty</var> type. A <var
class="REPLACEABLE">getty</var> type configures characteristics on the terminal line,
like bps rate and parity. The <tt class="COMMAND">getty</tt> program reads these
characteristics from the file <tt class="FILENAME">/etc/gettytab</tt>.</p>
</dd>

<dd>
<p>The file <tt class="FILENAME">/etc/gettytab</tt> contains lots of entries for terminal
lines both old and new. In almost all cases, the entries that start with the text <var
class="LITERAL">std</var> will work for hardwired terminals. These entries ignore parity.
There is a <var class="LITERAL">std</var> entry for each bps rate from 110 to 115200. Of
course, you can add your own entries to this file. The <a
href="http://www.FreeBSD.org/cgi/man.cgi?query=gettytab&sektion=5"><span
class="CITEREFENTRY"><span class="REFENTRYTITLE">gettytab</span>(5)</span></a> manual
page provides more information.</p>
</dd>

<dd>
<p>When setting the <var class="REPLACEABLE">getty</var> type in the <tt
class="FILENAME">/etc/ttys</tt> file, make sure that the communications settings on the
terminal match.</p>
</dd>

<dd>
<p>For our example, the Wyse-50 uses no parity and connects at 38400&nbsp;bps. The
286&nbsp;PC uses no parity and connects at 19200&nbsp;bps.</p>
</dd>

<dt><a href="term.html#CO-TTYS-LINE1COL3"><img src="./imagelib/callouts/3.png" hspace="0"
vspace="0" border="0" alt="(3)" /></a></dt>

<dd>The third field is the type of terminal usually connected to that tty line. For
dial-up ports, <var class="LITERAL">unknown</var> or <var class="LITERAL">dialup</var> is
typically used in this field since users may dial up with practically any type of
terminal or software. For hardwired terminals, the terminal type does not change, so you
can put a real terminal type from the <a
href="http://www.FreeBSD.org/cgi/man.cgi?query=termcap&sektion=5"><span
class="CITEREFENTRY"><span class="REFENTRYTITLE">termcap</span>(5)</span></a> database
file in this field.</dd>

<dd>
<p>For our example, the Wyse-50 uses the real terminal type while the 286 PC running <b
class="APPLICATION">Procomm</b> will be set to emulate at VT-100.</p>
</dd>

<dt><a href="term.html#CO-TTYS-LINE1COL4"><img src="./imagelib/callouts/4.png" hspace="0"
vspace="0" border="0" alt="(4)" /></a></dt>

<dd>The fourth field specifies if the port should be enabled. Putting <var
class="LITERAL">on</var> here will have the <tt class="COMMAND">init</tt> process start
the program in the second field, <tt class="COMMAND">getty</tt>. If you put <var
class="LITERAL">off</var> in this field, there will be no <tt class="COMMAND">getty</tt>,
and hence no logins on the port.</dd>

<dt><a href="term.html#CO-TTYS-LINE1COL5"><img src="./imagelib/callouts/5.png" hspace="0"
vspace="0" border="0" alt="(5)" /></a></dt>

<dd>The final field is used to specify whether the port is secure. Marking a port as
secure means that you trust it enough to allow the <tt class="USERNAME">root</tt> account
(or any account with a user ID of 0) to login from that port. Insecure ports do not allow
<tt class="USERNAME">root</tt> logins. On an insecure port, users must login from
unprivileged accounts and then use <a
href="http://www.FreeBSD.org/cgi/man.cgi?query=su&sektion=1"><span
class="CITEREFENTRY"><span class="REFENTRYTITLE">su</span>(1)</span></a> or a similar
mechanism to gain superuser privileges.</dd>

<dd>
<p>It is highly recommended that you use ``insecure'' even for terminals that are behind
locked doors. It is quite easy to login and use <tt class="COMMAND">su</tt> if you need
superuser privileges.</p>
</dd>
</dl>
</div>
</div>
</div>

<div class="SECT3">
<h3 class="SECT3"><a id="TERM-HUP" name="TERM-HUP">20.3.2.2 Force <tt
class="COMMAND">init</tt> to Reread <tt class="FILENAME">/etc/ttys</tt></a></h3>

<p>After making the necessary changes to the <tt class="FILENAME">/etc/ttys</tt> file you
should send a SIGHUP (hangup) signal to the <tt class="COMMAND">init</tt> process to
force it to re-read its configuration file. For example:</p>

<pre class="SCREEN">
<samp class="PROMPT">#</samp> <kbd class="USERINPUT">kill -HUP 1</kbd>
</pre>

<div class="NOTE">
<blockquote class="NOTE">
<p><b>Note:</b> <tt class="COMMAND">init</tt> is always the first process run on a
system, therefore it will always have PID 1.</p>
</blockquote>
</div>

<p>If everything is set up correctly, all cables are in place, and the terminals are
powered up, then a <tt class="COMMAND">getty</tt> process should be running on each
terminal and you should see login prompts on your terminals at this point.</p>
</div>
</div>

<div class="SECT2">
<h2 class="SECT2"><a id="TERM-DEBUG" name="TERM-DEBUG">20.3.3 Troubleshooting Your
Connection</a></h2>

<p>Even with the most meticulous attention to detail, something could still go wrong
while setting up a terminal. Here is a list of symptoms and some suggested fixes.</p>

<div class="SECT3">
<h3 class="SECT3"><a id="AEN28916" name="AEN28916">20.3.3.1 No Login Prompt
Appears</a></h3>

<p>Make sure the terminal is plugged in and powered up. If it is a personal computer
acting as a terminal, make sure it is running terminal emulation software on the correct
serial port.</p>

<p>Make sure the cable is connected firmly to both the terminal and the FreeBSD computer.
Make sure it is the right kind of cable.</p>

<p>Make sure the terminal and FreeBSD agree on the bps rate and parity settings. If you
have a video display terminal, make sure the contrast and brightness controls are turned
up. If it is a printing terminal, make sure paper and ink are in good supply.</p>

<p>Make sure that a <tt class="COMMAND">getty</tt> process is running and serving the
terminal. For example, to get a list of running <tt class="COMMAND">getty</tt> processes
with <tt class="COMMAND">ps</tt>, type:</p>

<pre class="SCREEN">
<samp class="PROMPT">#</samp> <kbd class="USERINPUT">ps -axww|grep getty</kbd>
</pre>

<p>You should see an entry for the terminal. For example, the following display shows
that a <tt class="COMMAND">getty</tt> is running on the second serial port <var
class="LITERAL">ttyd1</var> and is using the <var class="LITERAL">std.38400</var> entry
in <tt class="FILENAME">/etc/gettytab</tt>:</p>

<pre class="SCREEN">
22189  d1  Is+    0:00.03 /usr/libexec/getty std.38400 ttyd1
</pre>

<p>If no <tt class="COMMAND">getty</tt> process is running, make sure you have enabled
the port in <tt class="FILENAME">/etc/ttys</tt>. Also remember to run <tt
class="COMMAND">kill -HUP 1</tt> after modifying the <tt class="FILENAME">ttys</tt>
file.</p>

<p>If the <tt class="COMMAND">getty</tt> process is running but the terminal still does
not display a login prompt, or if it displays a prompt but will not allow you to type,
your terminal or cable may not support hardware handshaking. Try changing the entry in
<tt class="FILENAME">/etc/ttys</tt> from <var class="LITERAL">std.38400</var> to <var
class="LITERAL">3wire.38400</var> remember to run <tt class="COMMAND">kill -HUP 1</tt>
after modifying <tt class="FILENAME">/etc/ttys</tt>). The <var
class="LITERAL">3wire</var> entry is similar to <var class="LITERAL">std</var>, but
ignores hardware handshaking. You may need to reduce the baud rate or enable software
flow control when using <var class="LITERAL">3wire</var> to prevent buffer overflows.</p>
</div>

<div class="SECT3">
<h3 class="SECT3"><a id="AEN28949" name="AEN28949">20.3.3.2 If Garbage Appears Instead of
a Login Prompt</a></h3>

<p>Make sure the terminal and FreeBSD agree on the bps rate and parity settings. Check
the <tt class="COMMAND">getty</tt> processes to make sure the correct <var
class="REPLACEABLE">getty</var> type is in use. If not, edit <tt
class="FILENAME">/etc/ttys</tt> and run <tt class="COMMAND">kill -HUP 1</tt>.</p>
</div>

<div class="SECT3">
<h3 class="SECT3"><a id="AEN28956" name="AEN28956">20.3.3.3 Characters Appear Doubled;
the Password Appears When Typed</a></h3>

<p>Switch the terminal (or the terminal emulation software) from ``half duplex'' or
``local echo'' to ``full duplex.''</p>
</div>
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