"*\1" vs "*" in crontab file

m4rtin · 04-22-2011, 10:30 AM

I tried to set up the /etc/contab file under Debian. I would like to run one script every minute. At first I set a following line:

Code:

1 *	*	*	*	root  date > /tmp/date

..however, it looks like the cron is not creating a new /tmp/date file every minute. So I modified this line to look like this:

Code:

*\1	*	*	*	*	root  date > /tmp/date

..and now I get a new /tmp/date every minute. Why doesn't this 1 in minute field mean "after every 1 minute"?

TobiSGD · 04-22-2011, 10:34 AM

AFAIK, if you just specify the 1, the script will be run every first minute of the hour, like 00:01, 01:01, 02:01 , ..., 23:01.
It is not run every minute.

SL00b · 04-22-2011, 11:37 AM

Tobi is correct. If you specify 1, it'll run on the first minute of every hour. If you specify */1, it'll run every minute. You could have just specified * and achieved the same result, because cron checks its tables every minute to see if it has a job to run.

Likewise, if you specify 5, it'll run on the fifth minute, whereas */5 means every five minutes.

m4rtin · 04-22-2011, 11:38 AM

Quote:

Originally Posted by TobiSGD

AFAIK, if you just specify the 1, the script will be run every first minute of the hour, like 00:01, 01:01, 02:01 , ..., 23:01.
It is not run every minute.

ok, I see. So to run for example something after every two hours, one should specify:

Code:

*	*/2	*	*	*	root  date > /tmp/date

..otherwise:

Code:

*	2	*	*	*	root  date > /tmp/date

..it would be 02:00 every day?

SL00b · 04-22-2011, 11:39 AM

Correct.

m4rtin · 04-22-2011, 12:34 PM

SL00b, TobiSGD: thanks!

One more thing regarding crontab- in Mark G. Sobell "Practical Guide to Linux Commands, Editors, and Shell Programming, A (2nd Edition)" on page 651 there is a following example:

Code:

00 02 * * 0,4 $HOME/bin/twice.week

..and the explanation is "twice.week runs at 12:02 AM on Sunday and Thursday (days 0 and 4)". Is this a mistake? This should be "twice.week runs at 02:00 AM on Sunday and Thursday (days 0 and 4)", shouldn't it?

TobiSGD · 04-22-2011, 12:49 PM

Quote:

Originally Posted by m4rtin

SL00b, TobiSGD: thanks!

One more thing regarding crontab- in Mark G. Sobell "Practical Guide to Linux Commands, Editors, and Shell Programming, A (2nd Edition)" on page 651 there is a following example:

Code:

00 02 * * 0,4 $HOME/bin/twice.week

..and the explanation is "twice.week runs at 12:02 AM on Sunday and Thursday (days 0 and 4)". Is this a mistake? This should be "twice.week runs at 02:00 AM on Sunday and Thursday (days 0 and 4)", shouldn't it?

Yes, I would consider this a mistake in the book. In fact I have an entry like "0 2 * * *" in my root's crontab on the fileserver that syncs every night at 02:00 AM my Slackware mirror.

SL00b · 04-22-2011, 12:54 PM

Yeah... in fact, looking at the previous example, I think I may have answered incorrectly.

Code:

*	  2	*	*	*	root  date > /tmp/date

If I'm interpreting correctly, this would not run every day at 2:00. This would run every minute from 2:00 to 2:59.

m4rtin · 04-22-2011, 07:12 PM

TobiSGD, thanks! That's what I thought as well. I sent an email to Mark G. Sobell

Quote:

Originally Posted by SL00b

Yeah... in fact, looking at the previous example, I think I may have answered incorrectly.

Code:

*	  2	*	*	*	root  date > /tmp/date

If I'm interpreting correctly, this would not run every day at 2:00. This would run every minute from 2:00 to 2:59.

I tested this out and this:

Code:

*	  2	*	*	*	root  date > /tmp/date

..indeed means "every minute from 2:00 to 2:59"