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I have a script that uses at command. And sometimes I receiving emails as this one:

Date: Mon,  3 Jul 2017 14:18:00 +0300 (EEST)
From: root <root@localhost.org>
To: root@localhost.org
Subject: Output from your job      173

Another app is currently holding the xtables lock. Perhaps you want to use the -w option?

Of course I would use this option to prevent messages like this, but first I want to know what is the meaning of this option. Unfortunately -w option is not listed in man at.

Does anyone know what exactly this option means?

11

The message is from your job, not from at (and at also does not have a -w option BTW). By default, at sends STDOUT and STDERR from any job via mail, unless specified otherwise.


Presumably, your job was a iptables task -- based on the message regarding xtables. And the -w (--wait) option of iptables waits for the xtables lock to be released, as suggested in the message.

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    Thanks for the fast answer, Heemayl! Apparently I must think more before asking. :) – pa4080 Jul 3 '17 at 12:00
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    @pa4080 It happens, no problem :) – heemayl Jul 3 '17 at 12:01
4

The error seems to be related to iptables command

man iptables

-w, --wait Wait for the xtables lock. To prevent multiple instances of the program from running concurrently, an attempt will be made to obtain an exclusive lock at launch. By default, the program will exit if the lock cannot be obtained. This option will make the program wait until the exclusive lock can be obtained.

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    Thanks for the fast answer, Yaron! But Heemayl's answer appears first (we talk about 6 seconds) and I've accepted it. – pa4080 Jul 3 '17 at 12:07
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    @pa4080 - happy to help :-) – Yaron Jul 3 '17 at 12:14
  • @pa4080 :As general rule, it is worth waiting a little while before accepting an answer, in case a better one comes along (admittedly, that seems unlikely here where the solution is obvious once it is pointed out). There is nothing to stop you then accepting the first one (but you might also want to consider accepting the best written one). – Martin Bonner supports Monica Jul 3 '17 at 12:37

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