[ic] IC Jobs question

Geoff Sternecker geoffs at rdgi.com
Wed Oct 22 22:04:43 EDT 2003

found via 15 seconds on Google: Credit to the Authors, Not Me, Probably credit 
to RedHat..

Cron is a handy little utility that helps the system administrator automate 
repetative tasks.

Cron is controlled by a set of files called "crontabs". There is the master 
file in /etc/crontab (which in Red Hat Linux is set up a little different 
than other *nixes), along with crontab files for the users in 
/var/spool/cron/. In the latter directory, the files are given the same name 
as a user's login ID.

Crontab location:

In Red Hat Linux, it is a little easier for the sysadmin to set up cron jobs 
than in other distributions. The /etc/crontab file automatically executes 
items in several subdirectories at regular periods.


All the sysadmin needs to do is drop a shell script or a link to an executable 
in one of the directories and it will automatically be run at the appropriate 

Setting up a user-level crontab is somewhat different. The files in 
/var/spool/cron are not edited directly. Instead, a program called "crontab" 
is used to manipulate them. Depending on system security, all users, only 
some, or just the root user will be able to use crontab (see man crontab 
/etc/cron.allow and /etc/cron.deny for more information). SYNOPSIS crontab [ 
-u user ] file
crontab [ -u user ] { -l | -r | -e }

file       store the specified file as the current crontab
-u		user the crontab file being manipulated is for
-l		display the current crontab
-r		remove current crontab
-e		edit the current crontab (editor depends on system 
  		variables and will probably be vi unless your sysadmin
  		has changed it).

If you are not familiar with the systemwide default editor, it is probably 
best to create/edit the file with one you are familiar with and use the file 
option with the first command.

Crontab configuration: Blank lines, leading spaces, and tabs are ignored. 
Lines that start with a # are comments and are ignored. Comments are not 
allowed to be on the same line as cron commands; they will be assumed to be 
part of the command. Comments are not allowed on the same line as environment 
settings for similar reasons.

Environment settings take the format of

     name = value

(The spaces around the = are optional.)

Each cron command has 5 time and date fields, followed by a user name, and if
this is the system crontab file, it will be followed by a command.  Commands 
executed when the time specified by the time/date fields matches the
current time.

              field          allowed values
              -----          --------------
              minute         0-59
              hour           0-23
              day of month   0-31
              month          0-12 (or names, see below)
              day of week    0-7 (0 or 7 is Sun, or use names)

A  field  may  be an asterisk (*), which always stands for ``first to
last''.  So used in the hour field, it means 'every hour from 00:00 to

Example Crontab:

#      r----minute
#      |  r-----hour
#      |  |  r------day of the month
#      |  |  |  r------month
#      |  |  |  |  r------day of the week
#      |  |  |  |  |     |------ command to run ------------->
#      |  |  |  |  |     |
       5  0  *  *  *       $HOME/bin/daily.job >> $HOME/tmp/out 2>&1
# run five minutes after midnight, every day
       15 14 1  *  *     $HOME/bin/monthly

# run at 2:15pm on the first of every month -- output mailed to paul
       0  22 *  *  1-5   mail -s "It's 10pm" joe%Joe,%%Where are your kids?%

# print out the message at 4:05 every sunday.
       5  4  *  *  sun     echo "run at 5 after 4 every sunday"

If this file were saved as "paul.ct" then
      crontab -u paul paul.ct
would be used to store the crontab for the user paul.

More information about the interchange-users mailing list