One of the most useful utilities in FreeBSD is cron. This utility runs in the background and regularly checks /etc/crontab for tasks to execute and searches /var/cron/tabs for custom crontab files. These files are used to schedule tasks which cron runs at the specified times. Each entry in a crontab defines a task to run and is known as a cron job.

Two different types of configuration files are used: the system crontab, which should not be modified, and user crontabs, which can be created and edited as needed. The format used by these files is documented in crontab(5). The format of the system crontab, /etc/crontab includes a who column which does not exist in user crontabs. In the system crontab, cron runs the command as the user specified in this column. In a user crontab, all commands run as the user who created the crontab.

User crontabs allow individual users to schedule their own tasks. The root user can also have a user crontab which can be used to schedule tasks that do not exist in the system crontab.

Here is a sample entry from the system crontab, /etc/crontab:

# /etc/crontab - root's crontab for FreeBSD
#
# $FreeBSD$
# 
SHELL=/bin/sh
PATH=/etc:/bin:/sbin:/usr/bin:/usr/sbin 
#
#minute	hour	mday	month	wday	who	command 
#
*/5	*	*	*	*	root	/usr/libexec/atrun  
Lines that begin with the # character
	  are comments.  A comment can be placed in the file as a
	  reminder of what and why a desired action is performed.
	  Comments cannot be on the same line as a command or else
	  they will be interpreted as part of the command; they must
	  be on a new line.  Blank lines are ignored.
 
The equals (=) character is used to
	  define any environment settings.  In this example, it is
	  used to define the SHELL and
	  PATH.  If the SHELL is
	  omitted, cron will use the
	  default Bourne shell.  If the PATH is
	  omitted, the full path must be given to the command or
	  script to run.
 
This line defines the seven fields used in a system
	  crontab: minute, hour,
	  mday, month,
	  wday, who, and
	  command.  The minute
	  field is the time in minutes when the specified command will
	  be run, the hour is the hour when the
	  specified command will be run, the mday
	  is the day of the month, month is the
	  month, and wday is the day of the week.
	  These fields must be numeric values, representing the
	  twenty-four hour clock, or a *,
	  representing all values for that field.  The
	  who field only exists in the system
	  crontab and specifies which user the command should be run
	  as.  The last field is the command to be executed.
 
This entry defines the values for this cron job.  The
	  */5, followed by several more
	  * characters, specifies that
	  /usr/libexec/atrun is invoked by
	  root every five
	  minutes of every hour, of every day and day of the week, of
	  every month.
Commands can include any number of switches.  However,
	  commands which extend to multiple lines need to be broken
	  with the backslash “\” continuation
	  character.

To create a user crontab, invoke crontab in editor mode:

% crontab -e

This will open the user's crontab using the default text editor. The first time a user runs this command, it will open an empty file. Once a user creates a crontab, this command will open that file for editing.

It is useful to add these lines to the top of the crontab file in order to set the environment variables and to remember the meanings of the fields in the crontab:

SHELL=/bin/sh
PATH=/etc:/bin:/sbin:/usr/bin:/usr/sbin
# Order of crontab fields
# minute	hour	mday	month	wday	command

Then add a line for each command or script to run, specifying the time to run the command. This example runs the specified custom Bourne shell script every day at two in the afternoon. Since the path to the script is not specified in PATH, the full path to the script is given:

0	14	*	*	*	/usr/home/dru/bin/mycustomscript.sh

env -i SHELL=/bin/sh PATH=/etc:/bin:/sbin:/usr/bin:/usr/sbin HOME=/home/dru LOGNAME=dru /usr/home/dru/bin/mycustomscript.sh

When finished editing the crontab, save the file. It will automatically be installed and cron will read the crontab and run its cron jobs at their specified times. To list the cron jobs in a crontab, use this command:

% crontab -l
0	14	*	*	*	/usr/home/dru/bin/mycustomscript.sh

To remove all of the cron jobs in a user crontab:

% crontab -r
remove crontab for dru? y