|
#REDIRECT [[cron]]
The '''<code>crontab</code>''' command, found in [[Unix]] and [[Unix-like]] [[operating system]]s, is used to schedule [[command]]s to be executed periodically. It reads a series of commands from [[standard input]] and collects them into a file known also known as a "crontab" which is later read and whose instructions are carried out.
[[Category:Unix programsSUS2008 utilities]] ▼
Generally, '''<code>crontab</code>''' uses a [[daemon (computer software)|daemon]], <code>crond</code>, which runs constantly in the background and checks once a minute to see if any of the scheduled jobs need to be executed. If so, it executes them. These jobs are generally referred to as '''cron jobs'''.
== Crontab files ==
The crontab files are where the lists of jobs and other instructions to the cron daemon are kept. Users can have their own individual crontab files and often there is a systemwide crontab file (usually in /etc or a subdirectory of /etc) which is also used but can only be edited by the system administrator(s).
Each line of a crontab file follows a particular format as a series of fields, separated by spaces and/or tabs. Each field can have a single value or a series of values.
=== Operators ===
There are several ways of specifying multiple values in a field:
*The comma (',') operator specifies a list of values, for example: "1,3,4,7,8"
*The dash ('-') operator specifies a range of values, for example: "1-6", which is equivalent to "1,2,3,4,5,6"
*The asterisk ('*') operator specifies all possible values for a field. For example, an asterisk in the hour time field would be equivalent to 'every hour'..
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.
=== Fields ===
The first five fields of the line are the date and time field which specify how frequently and when to execute a command.
<table border="0" cellpadding="2">
<tr>
<th>Field no.</th>
<th>Description</th>
<th>Permitted values</th>
</tr>
<tr>
<td>1</td>
<td>minute</td>
<td>0-59</td>
</tr>
<tr>
<td>2</td>
<td>hour</td>
<td>0-23</td>
</tr>
<tr>
<td>3</td>
<td>day of the month</td>
<td>1-31</td>
</tr>
<tr>
<td>4</td>
<td>month</td>
<td>1-12</td>
</tr>
<tr>
<td>5</td>
<td>day of the week</td>
<td>0-7</td>
</tr>
</table>
Note: For day of the week, both 0 and 7 are considered Sunday.
The sixth and subsequent fields (i.e., the rest of the line) specify the command to be run.
=== Disabling Email ===
Crontab always sends the user the output of the commands it runs via email, if any.
To silence any command, you may pipe its output to <code>/dev/null</code>. To stop receiving email output from crontab, append the following to any command. This will pipe [[stdout]] to null while piping [[stderr]] to [[stdout]], so you can still receive errors if they occur:
:<code>2>&1 >/dev/null</code>
== See also ==
*[[At (Unix command)|at]] (runs a job at a specified future time)
==External links==
* [http://adminschoice.com/docs/crontab.htm Unix crontab - Quick Reference at adminschoice.com]
* [http://www.kalab.com/freeware/cron/cron.htm Cron for Windows]
* [http://www.visualcron.com VisualCron] - Windows freeware version of Cron and alternative task scheduler
▲[[Category:Unix programs]]
[[fr:Crontab]]
[[pl:Crontab]]
[[pt:Crontab]]
[[de:Cron]]
| 