Cron jobs are cool and easy, once you figure them out. (Like everything on
Linux!)
There are two cron utilities: /etc/crontab, and the crontab command. I
like /etc/crontab because it puts everything in one place. Only root can
edit /etc/crontab. The 'cat' command is a nice safe way to view files:
~~~~~~~~~~~~
$ cat /etc/crontab
# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file.
# This file also has a username field, that none of the other crontabs do.
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
# m h dom mon dow user command
25 6 * * * root test -e /usr/sbin/anacron || run-parts
--report /etc/cron.daily
47 6 * * 7 root test -e /usr/sbin/anacron || run-parts
--report /etc/cron.weekly
52 6 1 * * root test -e /usr/sbin/anacron || run-parts
--report /etc/cron.monthly
#
#shutdown the system every evening at midnight, with five minutes warning
00 0 * * * root /sbin/shutdown -h +5
~~~~~~~~~~~~
Ignore the anacron stuff, or whatever else your system may have stuffed in
there. This is the important bit:
# m h dom mon dow user command
this is what all those things mean:
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)
The asterisk * means "all of them." So this command shuts down the system
every night at midnight:
# m h dom mon dow user command
00 0 * * * root /sbin/shutdown -h +5
Suppose we just want it to shut down on weekends?
#shutdown at 1:05am Saturdays and Sundays
01 05 * * 7,0 root /sbin/shutdown -h +5
Sometimes Sunday is 7, you'll have to try it to see what works:
01 05 * * 6,7 root /sbin/shutdown -h +5
It would be nice to use 'sat,sun', but you can't list names. Day-of-the-week
and months can use names, use the first three letters: sat, sun, jan, feb.
Case does not matter.
You can use ranges: 1-4 means 1,2,3, and 4
Ranges and lists can be mixed: 1,3,5,6-10
Step values are kewl. Step values follow ranges:
10-23/2 means every second hour in the range
*/2 in the dow field means every other day
See man 5 crontab for yet more crontab fun! Remember the man -f command to
find all man files on a subject:
$ man -f crontab
crontab (1) - maintain crontab files for individual users (V3)
crontab (5) - tables for driving cron
***crontab command***
This lets users set their own cron jobs. First look to see if there are any
existing crontabs for you:
$ crontab -l
To create or edit one:
$ crontab -e
This will open in the default editor, which is defined either in your .bashrc
or .bash_profile. Create a new cron job, just like in /etc/crontab, with one
exception: there is no user field.
Only root can edit crontabs for other users:
# crontab -u carla -e
Use -r to delete a crontab. When you're editing a crontab, ignore the filepath
displayed in your editor, it looks something like this:
/tmp/crontab.STYJqN/crontab
Do not try to name the file, simply save & close, and it will automagically
end up in the right place, /var/spool/cron.
crontab (5), crontab (1), cron (8)
--
~~~~~~~~~~~~~~~~~~~~~~~~~
Carla Schroder
www.tuxcomputing.com
this message brought to you
by Libranet 2.8 and Kmail
~~~~~~~~~~~~~~~~~~~~~~~~~
Linux!)
There are two cron utilities: /etc/crontab, and the crontab command. I
like /etc/crontab because it puts everything in one place. Only root can
edit /etc/crontab. The 'cat' command is a nice safe way to view files:
~~~~~~~~~~~~
$ cat /etc/crontab
# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file.
# This file also has a username field, that none of the other crontabs do.
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
# m h dom mon dow user command
25 6 * * * root test -e /usr/sbin/anacron || run-parts
--report /etc/cron.daily
47 6 * * 7 root test -e /usr/sbin/anacron || run-parts
--report /etc/cron.weekly
52 6 1 * * root test -e /usr/sbin/anacron || run-parts
--report /etc/cron.monthly
#
#shutdown the system every evening at midnight, with five minutes warning
00 0 * * * root /sbin/shutdown -h +5
~~~~~~~~~~~~
Ignore the anacron stuff, or whatever else your system may have stuffed in
there. This is the important bit:
# m h dom mon dow user command
this is what all those things mean:
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)
The asterisk * means "all of them." So this command shuts down the system
every night at midnight:
# m h dom mon dow user command
00 0 * * * root /sbin/shutdown -h +5
Suppose we just want it to shut down on weekends?
#shutdown at 1:05am Saturdays and Sundays
01 05 * * 7,0 root /sbin/shutdown -h +5
Sometimes Sunday is 7, you'll have to try it to see what works:
01 05 * * 6,7 root /sbin/shutdown -h +5
It would be nice to use 'sat,sun', but you can't list names. Day-of-the-week
and months can use names, use the first three letters: sat, sun, jan, feb.
Case does not matter.
You can use ranges: 1-4 means 1,2,3, and 4
Ranges and lists can be mixed: 1,3,5,6-10
Step values are kewl. Step values follow ranges:
10-23/2 means every second hour in the range
*/2 in the dow field means every other day
See man 5 crontab for yet more crontab fun! Remember the man -f command to
find all man files on a subject:
$ man -f crontab
crontab (1) - maintain crontab files for individual users (V3)
crontab (5) - tables for driving cron
***crontab command***
This lets users set their own cron jobs. First look to see if there are any
existing crontabs for you:
$ crontab -l
To create or edit one:
$ crontab -e
This will open in the default editor, which is defined either in your .bashrc
or .bash_profile. Create a new cron job, just like in /etc/crontab, with one
exception: there is no user field.
Only root can edit crontabs for other users:
# crontab -u carla -e
Use -r to delete a crontab. When you're editing a crontab, ignore the filepath
displayed in your editor, it looks something like this:
/tmp/crontab.STYJqN/crontab
Do not try to name the file, simply save & close, and it will automagically
end up in the right place, /var/spool/cron.
crontab (5), crontab (1), cron (8)
--
~~~~~~~~~~~~~~~~~~~~~~~~~
Carla Schroder
www.tuxcomputing.com
this message brought to you
by Libranet 2.8 and Kmail
~~~~~~~~~~~~~~~~~~~~~~~~~