Cron - Revision history

Shane at 08:01, 8 April 2013

2013-04-08T08:01:16Z

Shane at 08:25, 23 March 2013

2013-03-23T08:25:07Z

Shane: /* Examples */

2013-03-17T08:56:52Z

Examples

Shane: categorizing

2013-02-24T23:27:22Z

categorizing

Shane at 22:37, 24 February 2013

2013-02-24T22:37:52Z

Shane at 04:42, 10 August 2012

2012-08-10T04:42:26Z

Shane: Created page with '''Cron'' is a system service which is used to invoke programs or scripts in a periodic manner. The ''crontab'' is the file which contains the schedule of what programs should be…'

2012-08-10T04:41:35Z

Created page with '''Cron'' is a system service which is used to invoke programs or scripts in a periodic manner. The ''crontab'' is the file which contains the schedule of what programs should be…'

New page

''Cron'' is a system service which is used to invoke programs or scripts in a periodic manner. The ''crontab'' is the file which contains the schedule of what programs should be called when. Cron itself is a system daemon which runs in the background, wakes up periodically, and runs anything in the schedule whose time has come. ''crontab'' is also the command you run to edit the schedule file.

The ''crontab'' command can run run with ''-l'' option to show the current schedule, and with ''-e'' option to launch an editor to modify the schedule. The ''$VISUAL'' environment variable determines which editor is launched. For example, set ''$VISUAL'' to '/usr/bin/vi' to run vi to edit the schedule.

Each line in the ''crontab'' file can be a comment, a variable declaration, or an event line.

== Comments ==

Comments begin with a comment mark ''#'', and must be the first character on the line.

== Variable declarations ==

Variable declarations are of the form

name=value

Unlike bash scripts, you can get away with putting spaces around the ''='' sign.

== Event lines ==

Each event line specifies a time and a date, and a command which is to be executed, in the format

minute hour date month day command

The first five fields can be numbers or ranges, in the format described below. Note that you can specify either the ''date'' (i.e. within the month) or the ''day'' (of the week), but not both. The other field should be set to ''*''.

The sixth field is a command with parameters. Percent signs, unless escaped with a ''\'' backslash, will be turned into newlines and everything after the first one of these will be fed into the command's STDIN stream.

It is also possible to execute shell scripts or run various applications with cron. For example. if you want it to start an alarm at 6AM every weekday morning, here is the crontab line you would use:

0 6 * * 1-5 /home/user/alarm.sh

Normally, the crontab file contains a ''MAILTO'' variable that directs output (STDOUT and STDERR) to an email to the address (e.g. ''MAILTO=dave''). If this is not working, the script may quit unexpectedly when its output has nowhere to go.

=== Range format ===

{|
|*
|Any number
|-
|*/5
|Any number, in steps of 5
|-
|1-6
|Any number between 1 and 6 (inclusive)
|-
|0-30/5
|Any number between 0 and 30, in steps of 5
|-
|1,4,9
|1, 4 or 9
|}

=== Months ===

Months can be specified in numbers or in words.
 1 = jan
 2 = feb
 ...
 12 = dec

=== Days of the week ===

Days of the week also can be specified in numbers or words.
 0 = Sunday
 1 = Monday
 2 = Tuesday
 ...
 6 - Saturday
 7 - Sunday

== Examples ==

# fetch e-mail every ten minutes
*/10 * * * * fetchmail
# send myself a birthday greeting
0 9 7 28 * mail -s'Happy Birthday' ajs318%Many Happy Returns - you old fart!%.%%
# back up my recipe database every Monday
30 5 * * 1 mysqldump --opt recipes > /home/ajs318/backups/recipes.sql

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		+	'''Note: This article, Cron, will ''not'' be on any quizzes or assignments in CSE 330, but it is useful for background information.'''
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	''Cron'' is a system service which is used to invoke programs or scripts in a periodic manner. The ''crontab'' is the file which contains the schedule of what programs should be called when. Cron itself is a system daemon which runs in the background, wakes up periodically, and runs anything in the schedule whose time has come. ''crontab'' is also the command you run to edit the schedule file.		''Cron'' is a system service which is used to invoke programs or scripts in a periodic manner. The ''crontab'' is the file which contains the schedule of what programs should be called when. Cron itself is a system daemon which runs in the background, wakes up periodically, and runs anything in the schedule whose time has come. ''crontab'' is also the command you run to edit the schedule file.

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	# back up my recipe database every Monday		# back up my recipe database every Monday
	30 5 * * 1 mysqldump --opt recipes > /home/ajs318/backups/recipes.sql		30 5 * * 1 mysqldump --opt recipes > /home/ajs318/backups/recipes.sql
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