Using "At" to schedule Linux jobs, thanks to: Job scheduling with cron and at

There is lots out there about cron, but I like this explanation of "at", and sometimes it's what you need (esp. if your cron is temporarily hosed):

Listing 5. Using the at command

[ian@lyrebird ~]$ at -f mycrontest.sh -v 10:25 Sat Jul 7 10:25:00 2007 job 5 at Sat Jul 7 10:25:00 2007

Listing 6. Job output from at

From ian@lyrebird.raleigh.ibm.com Sat Jul 7 10:25:00 2007 Date: Sat, 7 Jul 2007 10:25:00 -0400 From: Ian Shields Subject: Output from your job 5 To: ian@lyrebird.raleigh.ibm.com It is now 10:25:00 on Saturday

Time specifications can be quite complex. Listing 7 shows a few examples. See the man page for at or the file /usr/share/doc/at/timespec or a file such as /usr/share/doc/at-3.1.10/timespec, where 3.1.10 in this example is the version of theat package.

Listing 7. Time values with the at command

[ian@lyrebird ~]$ at -f mycrontest.sh 10pm tomorrow job 14 at Sun Jul 8 22:00:00 2007 [ian@lyrebird ~]$ at -f mycrontest.sh 2:00 tuesday job 15 at Tue Jul 10 02:00:00 2007 [ian@lyrebird ~]$ at -f mycrontest.sh 2:00 july 11 job 16 at Wed Jul 11 02:00:00 2007 [ian@lyrebird ~]$ at -f mycrontest.sh 2:00 next week job 17 at Sat Jul 14 02:00:00 2007

Being nice

The nice value for a job is a measure of how nice it is to other users. See our tutorial LPI exam 101 prep: GNU and UNIX commands for information on the niceand renice commands.

The at command also has a -q option. Increasing the queue increases the nice value for the job. There is also a batchcommand, which is similar to the at command except that jobs are run only when the system load is low enough. See the man pages for more details on these features.

...
Listing 11. Displaying and removing jobs with atq and atrm

[ian@lyrebird ~]$ atq 16 Wed Jul 11 02:00:00 2007 a ian 17 Sat Jul 14 02:00:00 2007 a ian 14 Sun Jul 8 22:00:00 2007 a ian 15 Tue Jul 10 02:00:00 2007 a ian [ian@lyrebird ~]$ atrm 16 14 15 [ian@lyrebird ~]$ atq 17 Sat Jul 14 02:00:00 2007 a ian ...

Linux tip: Job scheduling with cron and at

'via Blog this'

Crontab – Cron Quick Reference | Admin's Choice

Crontab – Quick Reference | Admin's Choice:
...

3. Crontab file
Crontab syntax :
A crontab file has five fields for specifying day , date and time followed by the command to be run at that interval.

*     *   *  *   *  command to be executed -     -    -   -  - |     |     |   |    | |     |     |   |    +----- day of week (0 - 6) (Sunday=0) |     |     |   +------- month (1 - 12) |     |     +--------- day of month (1 - 31) |     +----------- hour (0 - 23) +------------- min (0 - 59)

* in the value field above means all legal values as in braces for that column.
The value column can have a * or a list of elements separated by commas. An element is either a number in the ranges shown above or two numbers in the range separated by a hyphen (meaning an inclusive range).
Notes
A. ) Repeat pattern like /2 for every 2 minutes or /10 for every 10 minutes is not supported by all operating systems. If you try to use it and crontab complains it is probably not supported.

B.) The specification of days can be made in two fields: month day and weekday. If both are specified in an entry, they are cumulative meaning both of the entries will get executed .

4. Crontab Example
A line in crontab file like below removes the tmp files from /home/someuser/tmp each day at 6:30 PM.

30     18     *     *     *         rm /home/someuser/tmp/*

Changing the parameter values as below will cause this command to run at different time schedule below :

min	hour	day/month	month	day/week	Execution time
30	0	1	1,6,12	*	– 00:30 Hrs  on 1st of Jan, June & Dec.
0	20	*	10	1-5	–8.00 PM every weekday (Mon-Fri) only in Oct.
0	0	1,10,15	*	*	– midnight on 1st ,10th & 15th of month
5,10	0	10	*	1	– At 12.05,12.10 every Monday & on 10th of every month
:

Note : If you inadvertently enter the crontab command with no argument(s), do not attempt to get out with Control-d. This removes all entries in your crontab file. Instead, exit with Control-c.

5. Crontab Environment
cron invokes the command from the user’s HOME directory with the shell, (/usr/bin/sh).
cron supplies a default environment for every shell, defining:
HOME=user’s-home-directory
LOGNAME=user’s-login-id
PATH=/usr/bin:/usr/sbin:.
SHELL=/usr/bin/sh

Users who desire to have their .profile executed must explicitly do so in the crontab entry or in a script called by the entry.

6. Disable Email
By default cron jobs sends a email to the user account executing the cronjob. If this is not needed put the following command At the end of the cron job line .

>/dev/null 2>&1

7. Generate log file
To collect the cron execution execution log in a file :

30 18 * * * rm /home/someuser/tmp/* > /home/someuser/cronlogs/clean_tmp_dir.log

...

'via Blog this'