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markhawker
Posts: 511
Joined: 5/31/2004
From: Leeds, UK
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Cron Jobs? - 7/9/2004 13:21:24
Hi, what exactly are cron jobs? I understand you can automate server processes with them, schedule them to run at specific times, but what types of commands can you run? I've posted rather than read up on the matter as descriptions sometimes can be a bit wiry and it's best to hear 'from the horses mouth' so to speak.
Mark
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Mark Hawker
my business - http://www.thebubblejungle.com/
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ou812
Posts: 1589
Joined: 1/5/2002
From: San Diego
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RE: Cron Jobs? - 7/9/2004 14:38:04
cron jobs, or crontabs, are run from cron, a daemon. The cron allows crontab entries to be run at specified times (month, weekday, days of month, hours of day, hours, minutes etc.). So basically a crontab is a command or script, with whatever you want your script to do, that is executed by cron on a specific time(s).
Does that help?
-brian
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dpf
Posts: 7123
Joined: 11/12/2003
From: India-napolis
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RE: Cron Jobs? - 7/9/2004 14:50:32
quote:
from cron, a daemon
so what is "a daemon"?
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dpf
Posts: 7123
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From: India-napolis
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RE: Cron Jobs? - 7/9/2004 14:59:07
I see....sort of like my daughter when there is housework to be done! thanks
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Dan
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Shirley
Posts: 3127
Joined: 1/8/1999
From: Omaha, Ne USA
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RE: Cron Jobs? - 7/9/2004 16:01:59
quote:
cron is also a Unix/Linux command... not on Windows servers...
Windows 2003 server has scheduled tasks which accomplish the objective... to run a script or program at a scheduled time.
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Everything But Cake
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ellipisces
Posts: 849
Joined: 12/14/2003
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RE: Cron Jobs? - 7/12/2004 4:44:30
cron is a command line interface...
as such you can run almost any linux program from a cron job, (if any of you remember DOSKEY), and you can batch them so it could go like this...
connect to network drive n:
scan n: for virus
backup n:
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abbeyvet
Posts: 5095
From: Kilkenny Ireland
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RE: Cron Jobs? - 7/12/2004 11:52:02
The easiset way to explain this is by using an example.
First the Cron daemon.
Basically it is a little program that runs all the time on Linux servers, in the background, unnoticed and mostly doing nothing at all. Every so often (usually every minute), it checks to see if there is anything it should do.
If you want to do something on your site or on the server at a specific time every so-many days, you just need something that the daemon will see when it looks and obey.
This is called a crontab. It's just a file that has information about the minute, hour, day of month or day of week a specific command should be run. When the daemon sees this, it obediently causes the command to run at the given time.
In practical terms, this is very handy.
For example, on a site I manage we send out a newsletter to several thousand people every week. Some messages bounce, sometimes because an address is temporarily unavailable, sometimes because it has disappeared forever.
Dealing with this by hand is a big pain.
So, we have a little program that handles the bounces, resending mail twice in case of errors and then suspending the addresses from the list if they bounce more than twice - on the assumption that at that point they are dead.
This is the command that is in the crontab to make that all happen.
30 1 * * * /usr/bin/wget -O /dev/null -T 0 http://www.domain.com/mail/dailymail.php?pw=passowrd
That looks horrible - but break it down and it is simpler.
30 1 * * *
This bit says when the job should be done, in the order minute | hour | day of month | month | day of week
So my command will run at 1.30 am every day, as I have * * * for day of month, month, and day of week so they are ignored.
another example:
30 1 4 5 *
that could be used to send a birthday greeting to someone at 1.30 am on the 4th of May each year 
The next bits are pretty grim looking:
/usr/bin/wget
This is the command bit - wget is a little tool that grabs a web page, so basically this is saying "go to wget, which is in the directory /usr/bin, and tell it this......"
what it tells it is:
-O /dev/null
which if you are wget means "get the page but do not output it to a browser as you normally would, just run it".
-T 0
This tells it not to time out - in other words to stick with the task it is being given!
http://www.domain.com/mail/dailymail.php?pw=password
This tells it the page to get, and the password it needs to get into it.
Once that has been done the page dailymail.php will run on the server and the code in it will do whatever it has been written to do, which in this case is resend bounced messages.
It's important to note here that Cron did nothing much - all the work was done by the file dailymail.php. All the cron task did was find the file and run it at a specific time. That is what Cron is almost always used for - running a specified program or script at a specified time.
Some hosting setups allow you to enter your own cron tasks, but most do not. However your host will almost certainly be happy to set one up for you.
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:: InKK Design :: InKK Domains
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mrdance
Posts: 1
Joined: 7/27/2004
Status: offline
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RE: Cron Jobs? - 7/27/2004 12:00:18
I recommend using VisualCron. It like cron - but for windows.
You can read more here at it's homepage: www.visualcron.com
regards/ Henrik
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