4.4.4. Cron and crontab
The cron system is managed by the cron
daemon. It gets information about which programs and when they
should run from the system's and users' crontab entries. Only the
root user has access to the system crontabs, while each user should
only have access to his own crontabs. On some systems (some) users
may not have access to the cron facility.
At system startup the cron daemon searches /var/spool/cron/ for crontab entries which are
named after accounts in /etc/passwd, it
searches /etc/cron.d/ and it searches
/etc/crontab, then uses this information
every minute to check if there is something to be done. It executes
commands as the user who owns the crontab file and mails any output
of commands to the owner.
On systems using Vixie cron,
jobs that occur hourly, daily, weekly and monthly are kept in
separate directories in /etc to keep an
overview, as opposed to the standard UNIX cron function, where all
tasks are entered into one big file.
Example of a Vixie crontab
[[email protected] /etc]# more crontab
# commands to execute every hour
01 * * * * root run-parts /etc/cron.hourly
# commands to execute every day
02 4 * * * root run-parts /etc/cron.daily
# commands to execute every week
22 4 * * 0 root run-parts /etc/cron.weekly
commands to execute every month
42 4 1 * * root run-parts /etc/cron.monthly
You could also use the crontab -l command to display crontabs.
Some variables are set, and after that there's the actual
scheduling, one line per job, starting with 5 time and date fields.
The first field contains the minutes (from 0 to 59), the second
defines the hour of execution (0-23), the third is day of the month
(1-31), then the number of the month (1-12), the last is day of the
week (0-7, both 0 and 7 are Sunday). An asterisk in these fields
represents the total acceptable range for the field. Lists are
allowed; to execute a job from Monday to Friday enter 1-5 in the
last field, to execute a job on Monday, Wednesday and Friday enter
Then comes the user who should run the processes which are
listed in the last column. The example above is from a Vixie cron
configuration where root runs the program run-parts on regular intervals, with the appropriate
directories as options. In these directories, the actual jobs to be
executed at the scheduled time are stored as shell scripts, like
this little script that is run daily to update the database used by
the locate command:
[email protected] cron.daily]$ cat slocate.cron
renice +19 -p $$ >/dev/null 2>&1
/usr/bin/updatedb -f "nfs,smbfs,ncpfs,proc,devpts" -e \
Users are supposed to edit their crontabs in a safe way using
the crontab -e
command. This will prevent a user from accidentally opening more
than one copy of his/her crontab file. The default editor is
Chapter 6, but you can use any text editor, such as gvim or gedit if you feel more
comfortable with a GUI editor.
When you quit, the system will tell you that a new crontab is
This crontab entry reminds billy to go to his sports
club every Thursday night:
billy:~> crontab -l
# DO NOT EDIT THIS FILE - edit the master and reinstall.
# (/tmp/crontab.20264 installed on Sun Jul 20 22:35:14 2003)
# (Cron version -- $Id: chap4.xml,v 1.27 2006/10/26 15:37:52 tille Exp $)
38 16 * * 3 mail -s "sports evening" billy
After adding a new scheduled task, the system will tell you that
a new crontab is installed. You do not need to restart the
cron daemon for the changes to take effect.
In the example, billy added a new line pointing to a
billy:~> crontab -e
45 15 * * 3 mail -s "sports evening" billy
4 4 * * 4,7 /home/billy/bin/backup.sh
<--write and quit-->
crontab: installing new crontab
The backup.sh script is executed every
Thursday and Sunday. See
Section 7.2.5 for an
introduction to shell scripting. Keep in mind that output of
commands, if any, is mailed to the owner of the crontab file. If no
mail service is configured, you might find the output of your
commands in your local mailbox, /var/spool/mail/<your_username>, a plain text
||Who runs my commands?
You don't have to specify the user who should run the commands.
They are executed with the user's own permissions by default.