Cron Minitutorial

The cron daemon is where all timed events are initiated. It is executed upon system initialization and remains active while the system is operating in multi-user mode. Cron wakes up every minute and examines all the stored configuration files, called crontabs, to check each them for commands that may be scheduled to be executed at the current time. Some systems have limits to the number of tasks that can be scheduled during the one minute time period.

Besides starting commands each minute, some cron daemons also check to see if its spool directory's last modified time has been updated. If it has, cron will check the modification time on all crontabs and reread the ones that have been modified. Other cron daemons examine new crontab files when first initialized and when the commands crontab or at are executed. This reduces the overhead of checking for new or changed files at regularly scheduled intervals.

Cron searches the crontab spool directory for crontab files. These files are named after user accounts. For instance, if the system administrator is logged into the root accounts creates a crontab file, it will be named root and will be placed in the crontab spool directory. If Joe User, whose username is userj, creates a crontab file it is named userj in the crontab spool directory. When executing commands, any output is mailed to the owner of the crontab (or to the user named in the MAILTO environment variable in the crontab, if such exists).

The configuration files used to control the operation of cron are called crontab files or cron tables. These files contain information about the time, date and command to execute. Different versions of UNIX store cron and support files in different locations:

SunOS 5.X

 /etc/cron.d              main cron directory
 /etc/cron.d/FIFO         used as a lock file
 /etc/default/cron        contains cron default settings
 /var/cron/log            cron history information
 /var/spool/cron          spool area
 /etc/cron.d/logchecker   moves log file to /var/cron/olog if
                          log file exceeds system ulimit
 /etc/cron.d/queuedefs    queue  description  file  for   at,
                          batch, and cron
 /etc/cron.d/cron.allow   grant access to the cron facility 
 /etc/cron.d/cron.deny    revoke access if cron.alow file does not exists

Linux

 /var/spool/cron               main cron directory 
 /var/spool/cron/cron.allow    grant access to the cron facility 
 /var/spool/cron/cron.deny     revoke access if cron.alow file does not exists

HP-UX

 /var/adm/cron                 main cron directory
 /var/spool/cron/atjobs        Directory containing at and batch job
                               files
 /var/spool/cron/crontabs      Directory containing crontab files
 /var/adm/cron/log             Accounting information
 /var/spool/cron/queuedefs     queue  description  file  for   at,
                               batch, and cron
 /var/adm/cron/cron.allow      grant access to the cron facility
 /var/adm/cron/cron.deny       revoke access if cron.alow file does not exists

Cron table files, or crontabs, are text files which direct the cron daemon's activity. Each line or entry has six fields which are separated by space characters. The first five fields instruct the cron daemon as to when to execute the command, which is contained in the sixth field.

FIELD    VALUE
------------------
minute   00 to 59 
hour     00 to 23 (military time) 
day      1 to 31 
month    1 to 12 
weekday  0 to 6 (0=Sunday) Note: Linux uses sun, mon...

The first five fields can also use any one of the following formats.

• An asterisk character (*) to match all values.
• A single integer that matches that exact value.
• A comma separated list of integers, like 1,3,5 to match one of the listed values.
• A range of integers separated by a dash, like 4-6, to match the values within the range.

Here are sample entries along with a short explanation of when the operation will be performed.

0 * * * * echo "WAKE UP" 2>&1 /dev/console

This entry sends the string WAKE UP to the device /dev/console at the start of every hour on every day of every month.

0 0 * * *   calendar -

This entry runs the command calendar which reminds users of holidays and other events at the start of the hour at the start of the day on every day of the month.

10,20,30,40,50 * * * *  /adm/checkdaemon 2>&1 | /bin/mail -s "CRON:Check" root

This entry runs the command checkdaemon and mails the output of the command to root. The command is run 10, 20, 30 ,40, and 50 minutes after the hour on every day of every month.

The crontab command

The crontab files are not generated by editing a the crontab file in the crontab spool directory, instead the command crontab is used to edit, list, create or remove a crontab file for a user. The crontab command can be used by all the users on a system to create personal crontab as well as by the root account. Users are not allowed to view, edit or create crontab files for other users.

Additionally, the use of cron can be denied to users. This is done to prevent system unfriendly, or security compromising tasks to be performed. When the crontab command is invoked it examines the files cron.deny and cron.allow in the system's cron directory to grant or revoke the modification of the crontab spool file. If a username appears in the file cron.allow, the crontab command may be executed. If that file does not exist and the users name does not appear in the cron.deny file then cron can be used. If only an empty cron.deny exists, all users can use cron. If neither of these files exist, then only the root user can use cron.

The crontab command without options reads from standard input, so when executed it takes the information entered at the keyboard as input. This makes it easy to remove the existing crontab without really trying. If the crontab is run without options it should be exited with a "Control C" so that the existing crontab is unmodified. Entering a "Control D" will cause the current users' crontab to be replaced with no information, thereby erasing the existing crontab.

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CAUTION: 
If you type crontab and press Return without a filename, the standard input is read as the new crontab entries. Therefore, if you inadvertently enter crontab this way and you want to exit without destroying the contents of your current crontab file, press the Del key. Do not press the Ctrl-D key; if you do, your crontab file will only contain what you have currently typed.

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The edit option crontab -e  for the crontab command copies or creates the current user's crontab file. After editing is complete, the file is installed as the user's crontab file in the crontab spool directory. The default editor used by this command is ed. To specify an alternative, set the environment variable EDITOR. Not all systems' crontab  have an edit option. In this case, a file containing the crontab information can be created and read from by the crontab  command.

The list option, crontab -l, displays the contents of the current user's crontab file.

The remove option, crontab -r, empties the contents of the current user's crontab file.

The crontab command will accept an account name as the first argument if current user has superuser privileges.

Here is a sample session that adds a crontab entry for the current user, lists the crontab entry and then removes it.

#crontab -e                                (Create the crontab entry)
(within an editor enter) 
1 * * * * /usr/local/bin/runreport
# crontab -l                               (List the users' crontab file) 
1 * * * * /usr/local/bin/runreport  
# crontab -r                               (Remove the users' crontab file)

Using the crontab command without options to create the crontab file can be done by creating and editing a file. In this example, allcron.

#crontab allcron

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Created: May 16, 1997; Last modified: March 12, 2019