Automating Tasks with cron services

Contents:

• Overview of the Cron daemon
• Cron table, crontab, file format
• The crontab command

Overview of the cron daemon

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 Bill Wilson, whose username is bill, creates a crontab file it is named bill 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 and may use a slightly different format.

IRIX 5.3

 /usr/lib/cron             main cron directory
 /usr/lib/cron/FIFO        used as a lock file
 /usr/lib/cron/log         accounting information
 /usr/spool/cron           spool area
 /usr/lib/cron/queuedefs   queue description file for at,
                           batch and cron 
 /usr/lib/cron/cron.allow  grant access to the cron facility 
 /usr/lib/cron/cron.deny   revoke access to the cron facility

 /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 to the cron facility

 /var/spool/cron             main cron directory
 /var/spool/cron/FIFO        FIFO for sending messages to cron
 /var/spool/cron/cron.allow  grant access to the cron facility 
 /var/spool/cron/cron.deny   revoke access to the cron facility

 /var/spool/cron               main cron directory 
 /var/spool/cron/cron.allow    grant access to the cron facility 
 /var/spool/cron/cron.deny     revoke access to the cron facility

 /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 to the cron facility

The cron table, crontab, file format

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.

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

0 0 * * *   calendar -

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

Which of these is the correct definition for this entry?

15 5 * * 1    find /var/preserve/. -mtime +7 -exec rm -f {} \;

1. At 5:15 of every day
2. At 5:15 on Mondays
3. On the first day of the month at 5:15.

The crontab command

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.

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)

#vi allcron 
#crontab allcron

Terms used: multi-user mode, cron, scheduling, task scheduling.