10. Crontab

Execute scheduled commands
Can redirect Stderr and Stdout
Cron will email you not redirected output

10.1. Files

/etc/crontab
/etc/cron.allow - Allow User-Level Cron
/etc/cron.deny - Deny User-Level Cron
cat /var/log/syslog - Logs

10.2. Directories

/etc/cron.d/
/etc/cron.daily/
/etc/cron.hourly/
/etc/cron.monthly/
/etc/cron.weekly/
/var/spool/cron/crontabs/

10.3. Output Redirect

1 > /dev/null - stdout to /dev/null
2 > /dev/null - stderr to /dev/null
1>/dev/null 2>&1 - stdout to /dev/null and stderr to the same as stdout

10.4. Commands

crontab -e
crontab -l

$ export EDITOR=/usr/bin/vim
$ crontab -e

10.5. Variables

PATH=/usr/sbin:/usr/bin:/sbin:/bin

10.6. Syntax

* any value
, value list separator
- range of values
/ step values

10.7. Time

minute: 0-59
hour: 0-23
day of month: 0-31
month: JAN-DEC / 0-12
day of week: SUN-SAT / 0-7 (Sunday = 0 or 7)

10.8. Recurrence

@hourly - Run once an hour 0 * * * *
@daily - Run once a day 0 0 * * *
@weekly - Run once a week 0 0 * * 0
@monthly - Run once a month 0 0 1 * *
@yearly - Run once a year, 0 0 1 1 *
@reboot - Run once, at startup
@midnight - Same as @daily
@annually - Same as @yearly

10.9. Example

SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# Example of job definition:
# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  * user-name command to be executed
17 *        * * *   root    cd / && run-parts --report /etc/cron.hourly
25 6        * * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
47 6        * * 7   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52 6        1 * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )

10.10. Use Case - 0x01

/var/spool/cron/crontabs/
crontab -e

@midnight       /bin/echo 'at midnight'
@daily          /bin/echo 'at midnight'
@weekly         /bin/echo 'at midnight on Sunday'

10.11. Use Case - 0x02

/var/spool/cron/crontabs/
crontab -e

00 5 * * * /usr/bin/updatedb

10.12. Use Case - 0x03

/var/spool/cron/crontabs/
crontab -e

4 * * *       /bin/echo 'five past four a.m.'
*/10 * * * *     /bin/echo 'every ten minutes'
05-10 4 * * *    /bin/echo 'every minute from 5-10 past four a.m.'
* 4 * * *        /bin/echo 'every minute at 4 a.m.'
14 * * *      /bin/echo 'at 2 p.m.'
0 1 * *       /bin/echo 'at midnight of first day of month'
0 1 JAN *     /bin/echo 'at midnight of first day of January'
0 1 1 *       /bin/echo 'at midnight of first day of January'
0 * * SAT,SUN /bin/echo 'at midnight on weekends'
0 * * 0,6     /bin/echo 'at midnight on weekends'

10.13. Use Case - 0x04

/var/spool/cron/crontabs/
crontab -e

# Book Python
00  * * * * *     /Users/mwatney/book-python/.venv-py310/bin/python /Users/mwatney/book-python/bin/make-notes.py 1>/dev/null 2>&1
05  * * * * *     /Users/mwatney/book-python/.venv-py310/bin/python /Users/mwatney/book-python/bin/make-assignments.py 1>/dev/null 2>&1