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Here's a crontab example that shows how to run a command from the cron daemon once every day 0 1 * * * /var/www/backup.sh
crontab: invalid option -- - crontab: usage error: unrecognized option usage: crontab [-u user] file crontab [-u user] [ -e | -l | -r ] (default operation is replace, per 1003.2) -e (edit user's crontab) -l (list user's crontab) -r (delete user's crontab) -i (prompt before deleting user's crontab)
Like most Linux users, you may find it necessary to schedule repetitive tasks to be run at a certain time. Such tasks can occur as frequently as once a minute, to as infrequently as once a year. This scheduling can be done by using the ``cron'' facilities. The cron facilities as implemented in Linux are fairly similar to those available in other Unix implementations. However, Red Hat has adopted a slightly different way of scheduling tasks than is usually done in other distributions of Linux. Just as in other distributions, scheduling information is placed in the system ``crontab'' file (locating in the ``/etc/'' directory), using the following format:
You can specify each time component as an integer number (eg. 1 through 12 for the months January through December), or specify one or more components as ``*'' characters which will be treated as wildcards (eg. * in the month component means the command will run at the given day and time in every month. Here are some examples:
Note that commands which produce output to standard out (ie. a terminal) such as the examples above using ``echo'' will have their output mailed to the ``root'' account. If you want to avoid this, simply pipe the output to the null device as follows:
In addition to the standard ``crontab'' entries, Red Hat adds several directories:
As their names suggest, executable files can be placed in any of these directories, and will be executed on an hourly, daily, or weekly basis. This saves a bit of time when setting up frequent tasks; just place the executable script or program (or a symbolic link to one stores elsewhere) in the appropriate directory and forget about it. From :http://tldp.org/LDP/lame/LAME/linux-admin-made-easy/using-cron.html Examples of crontabs
As you can see from the /etc/crontab file, it uses the run-parts script to execute the scripts in the /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, and/etc/cron.monthly files on an hourly, daily, weekly, or monthly basis respectively. The files in these directory should be shell scripts. If a cron tasks needs to be executed on a schedule other than hourly, daily, weekly, or monthly, it can be added to the /etc/cron.d directory. All files in this directory use the same syntax as /etc/crontab. The cron daemon checks the etc/crontab file, the etc/cron.d/ directory, and the /var/spool/cron directory every minute for any changes. If any changes are found, they are loaded into memory. Thus, the daemon does not need to be restarted if a crontab file is changed. Users other than root can configure cron tasks by using the crontab utility. All user-defined crontabs are stored in the /var/spool/cron directory and are executed using the usernames of the users that created them. To create a crontab as a user, login as that user and type the command crontab -e to edit the user's crontab using the editor specified by the VISUAL or EDITOR environment variable. The file uses the same format as /etc/crontab. When the changes to the crontab are saved, the crontab is stored according to username and written to the file /var/spool/cron/username.
The main configuration file for cron, /etc/crontab, contains the following lines:
The first four lines are variables used to configure the environment in which the cron tasks are run. The value of the SHELL variable tells the system which shell environment to use (in this example the bash shell), and the PATH variable defines the path used to execute commands. The output of the cron tasks are emailed to the username defined with the MAILTOvariable. If the MAILTO variable is defined as an empty string (MAILTO=""), email will not be sent. The HOME variable can be used to set the home directory to use when executing commands or scripts. Each line in the /etc/crontab file has the format:
For any of the above values, an asterisk (*) can be used to specify all valid values. For example, an asterisk for the month value means execute the command every month within the constraints of the other values. A hyphen (-) between integers specifies a range of integers. For example, 1-4 means the integers 1, 2, 3, and 4. A list of values separated by commas (,) specifies a list. For example, 3, 4, 6, 8 indicates those four specific integers. The forward slash (/) can be used to specify step values. The value of an integer can be skipped within a range by following the range with /<integer>. For example, 0-59/2 can be used to define every other minute in the minute field. Step values can also be used with an asterisk. For instance, the value */3 can be used in the month field to run the task every third month. Any lines that begin with a hash mark (#) are comments and are not processed.
Linux Command: crontab usage: crontab [-u user] file crontab [-u user] [ -e | -l | -r ] (default operation is replace, per 1003.2) -e (edit user's crontab) -l (list user's crontab) -r (delete user's crontab) -i (prompt before deleting user's crontab)
Lines that can be in the crontab file.
minute (0-59), hour (0-23), day of the month (1-31), month of the year (1-12), day of the week (0-6 with 0=Sunday). Examples crontab -e = edits the crontab file to be used. 0 12 14 2 * mailx john%Happy Birthday!%Time for lunch. #crontab -e */1 * * * * php /var/www/html/poller.php > /dev/null 2>&1 0 1 * * * nice -n 15 /var/www/backup.sh
I want to cat a file and view the crontab entries and not use
/var/spool/cron/ |
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