Thursday, 15 February 2007

Linux: Format and examples of the crontab command

Whatever there is to find out about crontab is right there in the man pages. Paul Vixie explains everything crystal clear. Here are the main points to remember.

The cron(8) daemon examines cron entries once every minute.

Display of the user's crontab file is performed by crontab -l while editing is done with the -e switch. (Make sure that the EDITOR environment variable is set to your favorite editor program)

cronatb entries are date time fields followed by the command to be executed at that time. The time and date fields are:

field allowed values
minute 0-59
hour 0-23
day of month 1-31
month 1-12 (or names, see below)
day of week 0-7 (0 or 7 is Sun, or use names)

A field may be an asterisk (*), which always stands for ``first-last''.

Ranges of numbers are allowed. Ranges are two numbers separated with a hyphen. The specified range is inclusive. For example, 8-11 for an ``hours'' entry specifies execution at hours 8, 9, 10 and 11.

Lists are allowed. A list is a set of numbers (or ranges) separated by commas. Examples: ``1,2,5,9'', ``0-4,8-12''.

Step values can be used in conjunction with ranges. Following a range with ``/'' specifies skips of the number's value through the range. For example, ``0-23/2'' can be used in the hours field to specify command execution every other hour (the alternative in the V7 standard is ``0,2,4,6,8,10,12,14,16,18,20,22''). Steps are also permitted after an asterisk, so if you want to say ``every two hours'', just use ``*/2''.

Names can also be used for the ``month'' and ``day of week'' fields. Use the first three letters of the particular day or month (case doesn't matter). Ranges or lists of names are not allowed.


# use /bin/sh to run commands, no matter what /etc/passwd says
# mail any output to `paul', no matter whose crontab this is
# run five minutes after midnight, every day
5 0 * * *       $HOME/bin/daily.job >> $HOME/tmp/out 2>&1
# run at 2:15pm on the first of every month -- output mailed to paul
15 14 1 * *     $HOME/bin/monthly
# run at 10 pm on weekdays, annoy Joe
0 22 * * 1-5   mail -s "It's 10pm" joe%Joe,%%Where are your kids?%
23 0-23/2 * * * echo "run 23 minutes after midn, 2am, 4am ..., everyday"
5 4 * * sun     echo "run at 5 after 4 every sunday"

Finally here is my DB Server crontab which backs up the entire database every day at 6:00pm and backups up the archive logs at past 35 every two hours.

oracle@poulcheria:~> crontab -l
# DO NOT EDIT THIS FILE - edit the master and reinstall.
# (/tmp/crontab.21301 installed on Fri Feb  2 09:45:47 2007)
# (Cron version -- $Id: crontab.c,v 2.13 1994/01/17 03:20:37 vixie Exp $)
0 18 * * * /home/oracle/backup_jobs/perform_backup
35 */2 * * * /home/oracle/backup_jobs/perform_archlog_backup

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