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!!! Introduction Scheduling under Linux is provided by the cron(8) daemon. It reads a file called a crontab(5), but you don't necessarily need to edit any cron table directly. !!! Running commands at standard intervals On most systems, the easiest thing you can do if you simply want your command to be run once an hour, or once a day, or once a week, etc, is to put a script in the corresponding one of the __/etc/cron.{hourly,daily,weekly,monthly}__ etc directories. This is a simple [Shell] script, not a crontab(5) file. On most systems, all the "daily" scripts will be run sometime in the wee hours, such as 4am. !!! Running commands at custom times/intervals If you need to run commands at custom times or intervals, you need to know the crontab(5) line format described in the ManPage. Read that now, if you don't know about it yet. The main, system wide crontab(5) is __/etc/crontab__, but on most [LinuxDistribution]s, there is a __/etc/cron.d__ directory where you should put single line crontab files, eg: # /etc/cron.d/exim: crontab fragment for exim # Run queue every 15 minutes 08,23,38,53 * * * * mail if [[ -x /usr/sbin/exim -a -f /etc/exim/exim.conf ]; then /usr/sbin/exim -q ; fi This will be run every 15 minutes (8 past the hour, 23 past the hour etc) on every hour, every day, etc, as user mail. The initial fields are delimited by spaces, up to the the command (__if [[] ... fi__), which is taken as a whole to the end of the line. (The __if__ bit is a sanity check to ensure a runnable exim binary and an __exim.conf__ file exist.) Users can also have their own cron tables, with commands run under their respective [UID]. These should be manipulated using the crontab(1) command. __crontab -e__ will launch your configured interactive TextEditor on your user crontab, which generally resides under __/var/spool/cron__. !!! Running commands at standard intervals, revisited, and more With Vixie Cron (used in [*BSD] and some [LinuxDistribution]s such as [Debian] and RedHat) you can use __@reboot__ instead of the time fields. Cron will then run that command at system startup. This allows regular users to launch their own daemons, such as fetchmail(1). This feature first appeared [in FreeBSD 4.1 | http://www.freebsd.org/cgi/man.cgi?query=crontab&apropos=0&sektion=5&manpath=FreeBSD+4.1-RELEASE&format=html].
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