Push command to the background in a cron script

bcnx · 02-03-2011, 03:44 PM

Hi all,

I'm writing a script that performs backups from partitions on remote servers. I used to do this with ssh, but that seems to be somewhat slow, so I switched to netcat.

I start a listener on the receiving server (which runs the script) like this:

netcat -l -p 7000 | dd of=imagefile &

and then I make the remote and start a dd piped to netcat.

This works quite well started manually and the listener is waiting in the background. However, when I have cron run the same script to automate it, the netcat listener is not started, probably because the "&" thing does not work (or there is something with the piping that behaves differently then).

Anyone any idea on how to accomplish this?

thx!!!!

B.

druuna · 02-03-2011, 04:01 PM

Hi,

& is special in cron. You need to escape it (\&).

But why background the process? It is not attached to any terminal when you run it from cron.

Hope this helps.

bcnx · 02-04-2011, 04:19 AM

It should be in background because the script will not continue otherwise until this process is stopped in foreground. And the next step is to tell the remote server to start dumping to the netcat listener. This will not happen as long as the listener is not pushed to the background.

So you are basically telling this is not possible because we run from cron?

thx!

B.

druuna · 02-04-2011, 05:16 AM

Hi,

Quote:

Originally Posted by bcnx

It should be in background because the script will not continue otherwise until this process is stopped in foreground. And the next step is to tell the remote server to start dumping to the netcat listener. This will not happen as long as the listener is not pushed to the background.

There is no foreground and/or background when you execute from cron, like I said before: The jobs run from cron are not connected to a terminal (simply said: they already run in the background).

Quote:

So you are basically telling this is not possible because we run from cron?

No I'm not.....

If you put the following line in your crontab: 10 * * * * netcat -l -p 7000 | dd of=imagefile the following will happen:

1) At ten past the hour crontab will launch the netcat command,
2) the netcat command will silently wait (execute a ps -ef | grep netcat to see the process) until called upon.

When you run dd if=imagefile | netcat <some.ip.number> 7000, the transfer is started and when finished all processes involved (on both machines) stop (re-run the ps -ef command shown earlier).

There's no need to push anything to the background.

The fetch command (dd if=imagefile | netcat <some.ip.number> 7000) can also be run from crontab on the other machine, just make sure it is started later then the first one.

Hope this helps.

bcnx · 02-05-2011, 05:39 AM

Hi,

makes sense. This compromises a bit the ambition to centralize things I need to create a separate cron rule for every LV that needs dd-ing), but I guess it will do.

thx!!!!

B.

druuna · 02-05-2011, 05:52 AM

You're welcome

druuna · 02-05-2011, 05:58 AM

Hi,

About the centralize part:

You can write a script with all the netcat statements in it and run that script from cron.

Inside the script you do have to put the netcat command into the background!

I.e:

Code:

#!/bin/bash

netcat -l -p 7000 | dd of=imagefile0 &
netcat -l -p 7001 | dd of=imagefile1 &
netcat -l -p 7002 | dd of=imagefile2

Safe the script (lvm.dd.sh for example), make it executable and let cron kick it of:

10 * * * * /full/path/to/lvm.dd.sh

At ten past the hour the script is launched, which in turn launches 3 netcat commands.

Hope this helps.

bcnx · 02-06-2011, 03:01 AM

I see, that centralizes a bit more. Even more handy would be to have a sort of daemonized listener that could be started from the original script. I was thinking along the lines of xinetd but I'm not sure this will work. Don't really think so because of the pipe.

thx!

B.