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This on an iMac running Big Sur. (I asked over at AskDifferent, but no one knew the answer.)

This script is set up to run by launchd once a day to get battery data from a NUT (Network UPS Tools) Server running on a Raspberry Pi:

#!/bin/bash
# Shell script to query upsmon regarding ups battery status
log=/Users/mnewman/ups/ups.log
email="[email protected]"
echo `date` "UPS Check">> $log
# get UPS data from Raspsky
bat=`/usr/local/bin/upsc apcups@raspsky | \
grep -E 'battery.charge:|input.voltage:'`
# write USB data to the log
echo $bat >> $log
# send an email
echo "$bat" | /usr/bin/mail -s "UPS Status" $email
exit 0

The script runs fine from the command line. It runs from launchd except for the email line which fails silently - nothing in the console log and nothing in stderr. The email just doesn't get sent.

However, if I run the mail command in verbose mode:

echo "$bat" | /usr/bin/mail -v -s "UPS Status" $email

it runs fine.

Or, if I add another command after the email line:

curl -sS https://api.prowlapp.com/publicapi/add  \
        -F apikey=$apikey -F application=$(hostname) -F event="UPS"  \
        -F description="$bat"

it runs fine.

Can anyone explain what's doing on here?

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  • "I asked over at AskDifferent, but no one knew the answer" – Please read about cross-posting. Commented Jan 16, 2022 at 22:28
  • What happens if you quote the email variable in the mail command? /usr/bin/mail -s"UPS" "$email" Commented Jan 17, 2022 at 0:13
  • Quoting the email variable doesn't help. No email. No stderr output, nothing in the console log. -- I have other scripts that use an identical command to send mail and they all seem to work fine. Commented Jan 17, 2022 at 0:52
  • Check logs. Either some security measure is blocking mail from running or mail is failing to contact a mail agent to deliver it. Commented Jan 17, 2022 at 5:44

1 Answer 1

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I learned this elsewhere. It turns out that:

<key>AbandonProcessGroup</key><true/>

has to be added to the launchd plist file. When a job is finished, launchd kills any remaining processes with the same process group ID as the job. This setting disables that behavior. The problem is that mail launches a daughter process to send mail which might not be finished when the script is done executing. If launchd kills that daughter process, the mail will not be sent. That's what was happening in the situation I describe in the OP. My guess is that by running mail in verbose mode or by adding additional commands after the mail line kept the script alive long enough for mail to finish sending the email.

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