Email sent to root: "Output from from your job 1843" - "Killed"

I run a Sheevaplug (small ARM server) with Debian 9. It does not have any third-party repos enabled in sources.list / sources.list.d. I have a backup script which runs as root, and uses at. I think something broke on Sep 13, because I am getting these emails that look like they come from at. They are daily, like my backups. The body of the message just says Killed. I can't think what would be sending SIGKILL to my process! Without gathering any more information than I have now, can you think of *any* reason this would happen? It can't be from the OOM killer (Out of Memory condition), because I have a full kernel log in dmesg which does not show any OOM messages. The at job is #!/bin/sh # at uses sh shell set -e cd /d/backup/jenkins-desktop/ for i in */; do nice ionice -c 3 rdiff-backup "$i" ../jenkins-desktop.rdiff/"$i" done I doubt it's systemd SystemCallFilter=, and that would send SIGSYS by default. I see that a couple of rlimits send SIGKILL. But I'm not doing anything to set rlimits myself; also it looks like in both cases you would be killed by SIGXCPU first, which defaults to fatal and should show "CPU time limit exceeded". I have looked in journalctl --since=-2d -p notice and there are no errors, only some success messages from anacron. --- Return-path: Envelope-to: root@brick Delivery-date: Thu, 13 Sep 2018 02:14:15 +0100 Received: from root by brick with local (Exim 4.89) (envelope-from ) id 1g0GD0-0000Xr-Bz for root@brick; Thu, 13 Sep 2018 02:14:14 +0100 Subject: Output from your job 1843 To: root@brick Message-Id: From: root Date: Thu, 13 Sep 2018 02:14:14 +0100 X-IMAPbase: 1541805998 113 Status: O X-UID: 1 Killed