02 Crontab

Cron jobs are cool and easy, once you figure them out. (Like everything on

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.


# 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:


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
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