Unix Crontab

Introduction

cron is a utility that you can use to schedule and automate tasks. By defining items in the cron table, called crontab, you can schedule any script or program to run on almost any sort of schedule. A few examples:

Download RSS feeds daily at 6:30am,
Run a program 5 minutes after midnight on mondays, wednesdays and fridays,
Schedule something to run every five minutes, or
Do something just once a month.

Basics

Each user has their own crontab, the scheduled scripts run as that user so take this in account with regards to permissions. To edit the crontab use the following command: $ crontab -e

You can list what your currnet crontab is using the following command: $ crontab -l

Crontab Format The following is the format entries in a crontab must be. Note all lines starting with # are ignored, comments.

MIN   HOUR   MDAY  MON  DOW   COMMAND
5     *      *     *    *    echo 'Hello'

Item	Definition	Valid Values
MIN	Minute	0-60
HOUR	Hour [24-hour clock]	0-23
MDAY	Day of Month	1-31
MON	Month	1-12 OR jan,feb,mar,apr …
DOW	Day of Week	0-6 OR sun,mon,tue,wed,thu,fri,sat
COMMAND	Command to be run	Any valid command-line

Examples

Here are a few examples, to see what some entries look like, you can mix and match entries to setup elaborate schedules as needed.

Run a cron command at a specific time on weekdays

# Run command at 7:00am each weekday [mon-fri]
00 07 * * 1-5  mail_pager.script 'Wake Up'

2. Schedule command on a specific date and time each month

# Run command on 1st of each month, at 5:30pm
30 17 1 * *   pay_rent.scrip

3. Run command at multiple times each day

# Run command at 8:00am,10:00am and 2:00pm every day
00 8,10,14 * * *   do_something.script

4. Run a command every 5 minutes, during working hours

# Run command every 5 minutes during market hours
*/5 6-13 * mon-fri   get_stock_quote.script

5. Run a command every 3 hours every day

# Run command every 3-hours while awake
0 7-23/3 * * *   drink_water.script

Special Characters in Crontab

You can use an asterisk in any category to mean for every item, such as every day or every month.

You can use commas in any category to specify multiple values. For example: mon,wed,fri

You can use dashes to specify ranges. For example: mon-fri, or 9-17

You can use forward slash to specify a repeating range. For example: */5 for every five minutes, hours, days

Commands

The commands you enter are any shell command, but cron needs to be able to find them. So it is often best to give a full path. So you want to use something like

13 * * * * /home/user/bin/my-hourly-script.sh

You can also chain commands, so if you want to change to a directory before running script use && between commands. You could also use a semi-color ; but with && the commands will fail if the directory does not exist and not run the command.

13 10 * * * cd /home/user/bin && my-ten-oclock.sh

Special Entries

There are several special entries, some which are just shortcuts, that you can use instead of specifying the full cron entry.

The most useful of these is probably @reboot which allows you to run a command each time the computer gets reboot. This could be useful if you want to start up a server or daemon under a particular user, or if you do not have access to the rc.d/init.d files.

Example Usage:

# restart freevo servers
@reboot freevo webserver start
@reboot freevo recordserver start

The complete list:

Entry	Description	Equivalent To
@reboot	Run once, at startup.	None
@yearly	Run once a year	0 0 1 1 *
@annually	(same as @yearly)	0 0 1 1 *
@monthly	Run once a month	0 0 1 * *
@weekly	Run once a week	0 0 * * 0
@daily	Run once a day	0 0 * * *
@midnight	(same as @daily)	0 0 * * *
@hourly	Run once an hour	0 * * * *

Miscelleanous Issues

Script Output If there is any output from your script or command it will be sent to that user's e-mail account, on that box. Using the default mailer which must be setup properly.

You can set the variable MAILTO in the crontab to specify a separate e-mail address to use. For example:

MAILTO="admin@mydomain.com"

Redirect Output to /dev/null You can redirect the output from a cron script to /dev/null which just throws it away. By redirecting to /dev/null you will not receive anything from the script, even if it is throwing errors.

* * * * * /script/every_minute.pl > /dev/null 2>&1

Timezone If you want to run cron at a different timezone than your system time. You can set the TZ parameter in /etc/default/cron. For example, I want it to run in Pacific Time zone, so I set:

TZ="America/Los_Angeles"

Missed Schedule Time Cron does not run a command if it was missed. Your computer must be running for cron to run the job at the time it is scheduled. For example, if you have a 1:00am scheduled job and your computer was off at that time, it will not run the missed job in the morning when you turn it on.