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<!--TITLE=Linux in Plain English//-->
<!--AUTHOR=Patrick Volkerding//-->
<!--AUTHOR=Kevin Reichard//-->
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<P><FONT SIZE="+1"><B>sockdown......<TT>Shuts Down a Socket</TT></B></FONT></P>
<IMG SRC="images/05-152i.jpg"><P><B>sockdown</B> <I>option(s)</I></P>
<P><FONT SIZE="+1"><B><I>PURPOSE</I></B></FONT></P>
<P>The <B>sockdown</B> command performs a <B>shutdown</B> system call on one of its file descriptors specified by <I>fd</I>. The possible values for <I>how</I> are:</P>
<TABLE WIDTH="100%"><TR>
<TD WIDTH="20%"><B>0</B>
<TD WIDTH="80%">convert to write-only file descriptor
<TR>
<TD><B>writeonly</B>
<TD>symbolic for same as above
<TR>
<TD><B>1</B>
<TD>convert to read-only file descriptor
<TR>
<TD><B>readonly</B>
<TD>symbolic for same as above
<TR>
<TD VALIGN="TOP"><B>2</B>
<TD>complete shutdown—no reads or writes allowed in the future
<TR>
<TD><B>totally</B>
<TD>symbolic for same as above
</TABLE>
<P><FONT SIZE="+1"><B>talk......<TT>Talk to Another User</TT></B></FONT></P>
<P><B>talk</B> <I>username [tty]</I></P>
<P><FONT SIZE="+1"><B><I>PURPOSE</I></B></FONT></P>
<P>The <B>talk</B> commands allows you to chat interactively with another user currently logged on the system. When both sides are running the <B>talk</B> command, the screen splits, with one user’s typing appearing in one half of the screen, and the other user’s typing appearing in the other half of the screen.</P>
<P>The <I>username</I> can be someone on your own system. If you want to chat with a user on another system, then you must specify the username as <I>user@host</I>. If a user has more than one terminal going, you can specify a terminal with <I>tty</I>.</P>
<P>To quit <B>talk</B>, type <B>Ctrl-D</B>.</P>
<P><FONT SIZE="+1"><B><I>RELATED COMMAND</I></B></FONT></P>
<DL>
<DD><B>write</B>
</DL>
<P><FONT SIZE="+1"><B>telnet......<TT>Remote Connection</TT></B></FONT></P>
<P><B>telnet</B> <I>option(s) hostname</I></P>
<P><FONT SIZE="+1"><B><I>PURPOSE</I></B></FONT></P>
<P>The <B>telnet</B> command launches a connection to a remote host using the Telnet protocol. From there, you can use the remote host as if it were your own machine.</P>
<P><FONT SIZE="+1"><B><I>OPTIONS</I></B></FONT></P>
<TABLE WIDTH="100%">
<TR>
<TD WIDTH="30%"><B>-a</B>
<TD WIDTH="70%">Attempts automatic login.
<TR>
<TD><B>-d</B>
<TD>Turns on debugging mode.
<TR>
<TD><B>-e <I>escapechar</I></B>
<TD>Sets the <I>escapechar</I>.
<TR>
<TD VALIGN="TOP"><B>-l <I>user</I></B>
<TD>Sends <I>user</I> as the variable USER to the remote system; used with <B>-a</B>.
<TR>
<TD><B>-n <I>tracefile</I></B>
<TD>Recording trace information in <I>tracefile</I>.
</TABLE>
<P><FONT SIZE="+1"><B>uucp......<TT>UNIX-to UNIX Copy</TT></B></FONT></P>
<P><B>uucp</B> <I>option(s) sourcefile destinationfile</I></P>
<P><B>uucp</B> <I>option(s) sourcefile destinationdirectory</I></P>
<P><FONT SIZE="+1"><B><I>PURPOSE</I></B></FONT></P>
<P>The <B>uucp</B> command copies files between systems. See the online-manual pages for a list of the available options.</P>
<P><FONT SIZE="+1"><B>write......<TT>Write Another User</TT></B></FONT></P>
<P><B>write</B> <I>user</I></P>
<P><FONT SIZE="+1"><B><I>PURPOSE</I></B></FONT></P>
<P>The <B>write</B> command sends a message to another user, who can choose whether or not to carry on a conversation.</P>
<P><FONT SIZE="+1"><B>ytalk......<TT>Chat Program</TT></B></FONT></P>
<P><B>ytalk</B> <I>option(s) username</I></P>
<P><FONT SIZE="+1"><B><I>PURPOSE</I></B></FONT></P>
<P>The <B>ytalk</B> command is a multuser chat program. It can also be used to chat with <B>talk</B> users.</P>
<P><FONT SIZE="+1"><B><I>OPTIONS</I></B></FONT></P>
<TABLE WIDTH="100%">
<TR>
<TD WIDTH="20%"><B>-x</B>
<TD WIDTH="80%">Disables the X Window System interface.
<TR>
<TD><B>-s</B>
<TD>Starts the <B>ytalk</B> window in a shell.
</TABLE>
<P><FONT SIZE="+1"><B><I>RELATED COMMAND</I></B></FONT></P>
<DL>
<DD><B>talk</B>
</DL>
<H3><A NAME="Heading6"></A><FONT COLOR="#000077">System-Administration Commands</FONT></H3>
<P>These are the commands that make your Linux system run more smoothly, ranging from scheduling system chores to dealing directly with PC hardware.
</P>
<P><FONT SIZE="+1"><B>at......<TT>Run Command at Specific Time</TT></B></FONT></P>
<P><B>at</B> <I>option(s) time date</I></P>
<P><B>at -c</B> <I>job-ID</I></P>
<P><FONT SIZE="+1"><B><I>PURPOSE</I></B></FONT></P>
<P>The <B>at</B> command runs a command or a set of commands at a specific <I>time</I> and/or on a specific <I>date</I>. These must be self-sufficient commands that require no input from you or another user. Normally, these commands are system-administration commands, relating to system backups, electronic-mail delivery, and so on.</P>
<P>Commands and times for the <B>at</B> command are entered directly at the command line. You first enter the <B>at</B> command and then the time the command is to be run, followed by the command to be run. When you’ve finished, press <B>Ctrl-D</B>. At the specified time, <B>at</B> runs the command; if there is any output from the command, it’s sent to you as electronic mail.</P>
<P>You have many options when it comes to setting a job. You can specify a time using <B>HHMM</B> notation or <B>HH:MM</B> notation (in military time), with the assumption that the command will be run starting with the first upcoming instance. You can also indicate <B>AM</B> or <B>PM</B>. You can also use words like midnight, noon, or teatime (4 p.m.) to specify a time. Commands can also be set up to be run on specific days, using the forms <B>MMDDYY</B>, <B>MM/DD/YY</B>, or <B>DD,MM,YY</B>. (Note the use of slashes and commas.)</P>
<P>Intervals can also be set at specific times from the present. For instance, you could tell <B>at</B> to run a command in two weeks using <B>today + 2 weeks</B> as the specifier. Other intervals include <B>minutes</B>, <B>hours</B>, and <B>days</B>. You can combine times and dates in the following manner: <B>at 4pm + 3 days</B>. Finally, you can combine a time with tomorrow: <B>at 1pm tomorrow</B>.</P>
<P>Not all users have access to this command. The <B>root</B> user always has access to the <B>at</B> command. In addition, the superuser can specify which users have access to the <B>at</B> command, as well as explicitly deny access to specific users, through usernames in the <B>/etc/at.allow</B> and <B>/etc/at.deny</B> files. If <B>/etc/at.allow</B> exists, only those usernames in the file can run the <B>at</B> command; if it doesn’t exist, then <B>/etc/at.deny</B> is checked, with the result that anyone <I>not</I> mentioned in <B>/etc/at.deny</B> can use the <B>at</B> command.</P>
<P><FONT SIZE="+1"><B><I>EXAMPLE</I></B></FONT></P>
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<PRE>
$ at 1am
ls
Ctrl-D
</PRE>
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