fortypoundhead.com

CRONTAB

Posted On 2007-04-27 by FortyPoundHead
Keywords: Command Reference
Tags: Linux Commandline Linux
Views: 1533


Schedule a command to run at a later time

SYNTAX
crontab [ -u user ] file
crontab [ -u user ] { -l | -r | -e }

Key
-l List - display the current crontab entries.

-r Remove the current crontab.

-e Edit the current crontab using the editor specified by the
VISUAL or EDITOR environment variables.
After you exit from the editor, the modified crontab will be installed automatically.
Crontab is the program used to install, deinstall or list the tables used to drive the cron daemon in Vixie Cron.
Each user can have their own crontab, and though these are files in /var, they are not intended to be edited directly.

If the -u option is given, it specifies the name of the user whose crontab is to be tweaked. If this option is not given, crontab examines "your" crontab, i.e., the crontab of the person executing the command. Note that su can confuse crontab and that if you are running inside of su you should always use the -u option for safety''s sake.

cron file is used to install a new crontab from some named file or standard input if the pseudo-filename `-'' is given.

Each line in the cron table follows the following format: 7 fields left to right

Field Meaning
1 Minute (0-59)
2 Hour (2-24)
3 Day of month (1-31)
4 Month (1-12, Jan, Feb, ...)
5 Day of week (0-6) 0=Sunday, 1=Monday ...
or Sun, Mon, Tue, Wed, Thur, Fri
6 User that the command will run as
7 Command to execute


There are several ways of specifying multiple values in a field:


  • The comma ('','') operator specifies a list of values, for example: "1,3,4,7,8"

  • The dash (''-'') operator specifies a range of values, for example: "1-6", which is equivalent to "1,2,3,4,5,6"

  • The asterisk (''*'') operator specifies all possible values for a field. e.g. every hour or every day.


There is also an operator which some extended versions of cron support, the slash (''/'') operator, which can be used to skip a given number of values. For example, "*/3" in the hour time field is equivalent to "0,3,6,9,12,15,18,21"; "*" specifies ''every hour'' but the "/3" means that only the first, fourth, seventh...and such values given by "*" are used.

Cron will email to the user all output of the commands it runs, to silence this, redirect the output to a log file or to /dev/null

Example

Run /usr/bin/somecommand at 12.59 every day and supress the output (redirect to null)

59 12 * * * simon /usr/bin/somecommand >> /dev/null 2>&1

Permissions
If the allow file exists, then you must be listed therein in order to be allowed to use this command. If the allow file does not exist but the deny file does exist, then you must not be listed in the deny file in order to use this command. If neither of these files exists, then depending on site-dependent configuration parameters, only the super user will be allowed to use this command, or all users will be able to use this command.


About the Author

FortyPoundHead has posted a total of 1974 articles.

 


Comments On This Post

No comments on this post yet!


Do you have a thought relating to this post? You can post your comment here. If you have an unrelated question, you can use the Q&A section to ask it.

Or you can drop a note to the administrators if you're not sure where you should post.


Your IP address is:54.80.58.75

Before you can post, you need to prove you are human. If you log in, this test goes away.



Recent Forum Posts

List of Shady Characters
dwirch posted on April 25, 2017 at about 16:39 in Webmaster Stuff

Job Spammer: Bilal Uddin
dwirch posted on April 25, 2017 at about 11:00 in Spammers

Bug Fix: Contact Form Error
dwirch posted on April 21, 2017 at about 11:38 in Site News

Bug Fix: Ophion Time Tracker
dwirch posted on April 9, 2017 at about 11:30 in Site News

Job Spammer: Yogesh Kapadne
dwirch posted on March 31, 2017 at about 8:04 in Spammers

Job Spammer: Sathya Narayana
dwirch posted on March 15, 2017 at about 7:18 in Spammers