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</div> <!-- #EndLibraryItem --><h1> crontab (cron table)</h1>
<p>Schedule a command to run at a later time</p>
<pre>SYNTAX
crontab [ -u <em>user</em> ] <em>file</em>
crontab [ -u <em>user</em> ] { -l | -r | -e }
Key
-l List - display the current crontab entries.
-r Remove the current crontab.
-e Edit the current crontab using the editor specified by the
VISUAL or EDITOR environment variables.
After you exit from the editor, the modified crontab will be installed automatically.
</pre>
<p> <span class="body">Crontab is the program used to install, deinstall or list
the tables used to drive the <a href="cron.html">cron</a> daemon in Vixie Cron.<br>
Each user can have their own crontab, and though these are
files in /var, they are not intended to be edited
directly.<br>
<br>
If the -u option is given, it specifies the name of the
user whose crontab is to be tweaked. If this option is
not given, crontab examines "your" crontab, i.e., the
crontab of the person executing the command. Note that <a href="su.html">su</a> can confuse crontab and that if you are running
inside of <a href="su.html">su</a> you should always use the -u option for
safety's sake.</span><br>
<br>
<span class="telety">cron <em>file</em> </span>is used to install a new
crontab from some named file or standard input if the
pseudo-filename `-' is given.</p>
<p>Each line in the cron table follows the following format: 7 fields left to right </p>
<table border="1" bordercolor="#333333" cellpadding="5" cellspacing="0">
<tbody><tr><th align="left" class="body">Field</th><th align="left" class="body">Meaning</th>
</tr><tr>
<td width="30" align="center" class="body">1</td><td class="body">Minute (0-59)</td>
</tr>
<tr>
<td align="center" class="body">2</td><td class="body">Hour (2-24)</td>
</tr>
<tr>
<td align="center" class="body">3</td><td class="body">Day of month (1-31)</td>
</tr>
<tr>
<td align="center" class="body">4</td>
<td class="body">Month (1-12, Jan, Feb, ...)</td>
</tr>
<tr>
<td align="center" class="body">5</td>
<td class="body">Day of week (0-6) 0=Sunday, 1=Monday ... <br>
or Sun, Mon, Tue, Wed, Thur, Fri </td>
</tr>
<tr>
<td align="center" class="body">6</td><td class="body">User that the command will run as</td>
</tr>
<tr>
<td align="center" class="body">7</td><td class="body">Command to execute</td>
</tr>
</tbody></table>
<p>There are several ways of specifying multiple values in a field:</p>
<p> <span class="body">• The comma (',') operator specifies a list of values, for example: "1,3,4,7,8"<br>
• The dash ('-') operator specifies a range of values, for example: "1-6", which is equivalent to "1,2,3,4,5,6"<br>
• The asterisk ('*') operator specifies all possible values for a field. e.g. every hour or every day.</span></p>
<p>There is also an operator which some extended versions of cron support, the slash ('/') operator, which can be used to skip a given number of values. For example, "*/3" in the hour time field is equivalent to "0,3,6,9,12,15,18,21"; "*" specifies 'every hour' but the "/3" means that only the first, fourth, seventh...and such values given by "*" are used.</p>
<p>Cron will email to the <span class="telety">user</span> all output of the commands it runs, to silence this, redirect the output to a log file or to /dev/null</p>
<p><strong>Example</strong></p>
<p>Run <span class="telety">/usr/bin/somecommand</span> at 12.59 every day and supress the output (redirect to null) <br>
<br>
<span class="telety">59 12 * * * simon /usr/bin/somecommand >> /dev/null 2>&1</span></p>
<p><strong>Permissions</strong><br>
If the <span class="telety">allow</span> file exists, then you must be listed therein in order to be allowed to use this command. If the allow file does not exist but the deny file does exist, then you must not be listed in the deny file in order to use this command. If neither of these files exists, then depending on site-dependent configuration parameters, only the super user will be allowed to use this command, or all users will be able to use this command.</p>
<p><i>"Wisdom is the power to put our time and our knowledge to the
proper use" - Thomas
J. Watson </i></p>
<p><b>Related</b>:<br>
<br>
<a href="cron.html">cron</a> - daemon to execute scheduled commands<br>
<a href="nice.html">nice</a> - Change job scheduling priority<br>
<a href="builtin.html">builtin</a> - Run a shell builtin<br>
<a href="chroot.html">chroot</a> - Run a command with a different root directory<br>
<a href="exec.html">exec</a> - Execute a command<br>
<a href="nohup.html">nohup</a> - Run a command immune to hangups<br>
<a href="source.html">.source</a> - Run commands from a file <br>
<a href="su.html">su</a> - Run a command with substitute user and group id<br>
<a href="watch.html">watch</a> - Execute/display a program periodically<br>
Equivalent Windows command: <a href="../term_tools/nt/schtasks.html">schtasks</a> - Schedule a command to run at a later time</p>
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