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Top Forums UNIX for Dummies Questions & Answers differnce between working of at and crontab commands Post 70841 by solaris-ninja on Wednesday 4th of May 2005 03:17:51 PM
Old 05-04-2005
Quote:
Originally Posted by Just Ice
at is usually for non-regular, one-time-only jobs while cron is usually for regular, perpetual jobs ...

at samples:
Code:
# harass the console user at 7:05 pm
at 1905
echo "This is my console!" > /dev/console
# reboot the box 1 hour from now
at now + 1 hour
sync; sync; reboot

cron samples:
Code:
# reboot host every weekday @ 8:30 pm
30 20 * * 0-5 (sync; sync; init 6) > /dev/null 2>&1
# annoy the console user every weekday @ 8:00 am
00 08 * * 0-5 echo "This is my concole!" > /dev/console

however, you can also get a job to go regularly with an at command by adding something like this in your script ...
Code:
at now + 1 week << GO
/path/to/this/script > /dev/null 2>&1
GO

... and a "one-time" job in cron ... (will only run when all time conditions are met) ...
Code:
# run the job at 1:01 am Monday, January 1
01 01 01 01 1 echo "This is my console!" > /dev/console

btw, it would be best if you get into checking the man pages while you're just starting ... see "man at" and "man crontab" ...

Very interesting answer. Good Job. Smilie
 

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CRONTAB(1)						    BSD General Commands Manual 						CRONTAB(1)

NAME
crontab -- maintain crontab files for individual users (V3) SYNOPSIS
crontab [-u user] file crontab [-u user] { -l | -r | -e } DESCRIPTION
The crontab utility is the program used to install, deinstall or list the tables used to drive the cron(8) 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 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. The format of these files is one username per line, with no leading or trailing whitespace. Lines of other formats will be ignored, and so can be used for comments. The first form of this command is used to install a new crontab from some named file or standard input if the pseudo-filename '-' is given. The following options are available: -u Specify 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(1) can confuse crontab and that if you are running inside of su(1) you should always use the -u option for safety's sake. -l Display the current crontab on standard output. -r Remove the current crontab. -e Edit the current crontab using the editor specified by the VISUAL or EDITOR environment variables. The specified editor must edit the file in place; any editor that unlinks the file and recreates it cannot be used. After you exit from the editor, the modified crontab will be installed automatically. FILES
/var/cron/allow List of users allowed to use crontab /var/cron/deny List of users prohibited from using crontab /var/cron/tabs Directory for personal crontab files DIAGNOSTICS
A fairly informative usage message appears if you run it with a bad command line. SEE ALSO
crontab(5), cron(8) STANDARDS
The crontab command conforms to IEEE Std 1003.2 (``POSIX.2'') with the exception that the dangerous variant of calling crontab without a file name in the first form of the command is not allowed by this implementation. The pseudo-filename '-' must be specified to read from standard input. The new command syntax differs from previous versions of Vixie Cron, as well as from the classic SVR3 syntax. AUTHORS
Paul Vixie <paul@vix.com> BSD
May 13, 2010 BSD
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