trap ERR doesn't work with pipes - bash

Question

I am try to make a system backup script with trap "" ERR. I realized the trap doesn't get called when commands are part of pipes |.
Heres are some parts of my code that don't work with trap "" ERR ...
OpenFiles=$(lsof "$Source" | wc -l)
PackagesList=$(dpkg --get-selections | awk '!/deinstall|purge|hold/ {print $1}' | tee "$FilePackagesList")
How can I get this to work without using if [ "$?" -eq 0 ]; then, or similar coding ? Because this is the reason I declared a trap this way.
Here is the script ...
root#Lian-Li:~# cat /usr/local/bin/create_incremental_backup_of_system.sh
#!/bin/bash
# Create an incremental GNU-standard backup of important system-files.
# This script works with Debian Jessie and newer systems.
# Created for my lian-li NAS 2016-11-27.
MailTo="admin#example.com" # Mail Address of an admin
Source="boot etc root usr/local usr/lib/cgi-bin var/www"
BackupDirectory=/media/hdd1/backups/lian-li
SubDir="system.d"
FileTimeStamp=$(date "+%Y%m%d%H%M%S")
FileName=$(uname -n)
File="${BackupDirectory}/${SubDir}/${FileName}-${FileTimeStamp}.tgz"
FileIncremental="${BackupDirectory}/${SubDir}/${FileName}.gtar"
FilePackagesList="${BackupDirectory}/${SubDir}/installed_packages_on_${FileName}.txt"
# have2do ...
# Backup rotate
MailContent="None"
TimeStamp=$(date "+%F %T") # This format "2011-12-31 23:59:59" is needed to read the journal
exec 1> >(logger -i -s -t "$0" -p 3) 2>&1 # all error messages are redirected to syslog journal and after that to stdout
trap "BriefExit" ERR # Provide information for an admin (via sendmail) when an error occurred and exit the script
function BriefExit(){
rm -f "$File"
if [ "$MailContent" = "None" ]
then
case "$LANG" in
de_DE.UTF-8)
echo "Beende Skript, aufgrund vorherige Fehler." 1>&2
;;
*)
echo "Stopping script because of previous error(s)." 1>&2
;;
esac
MailContent=$(journalctl -p 3 -o "short" --since="$TimeStamp" --no-pager)
ScriptName="${0##*/}"
SystemName=$(uname -n)
MailSubject="${SystemName}: ${ScriptName}"
echo -e "Subject: ${MailSubject}\n\n${MailContent}\n" | sendmail "$MailTo"
fi
exit 1
}
if [ ! -d "${BackupDirectory}/${SubDir}" ]
then
mkdir -p "${BackupDirectory}/${SubDir}"
fi
LoopCount=0
OpenFiles=1
cd /
while [ "$OpenFiles" -ne 0 ]
do
if [ "$LoopCount" -le 180 ]
then
sleep 1
OpenFiles=$(lsof $Source | wc -l)
LoopCount=$(($LoopCount + 1))
else
echo "Closing Script. Reason: Can't create incremental backup, because some files are open." 1>&2
BriefExit
fi
done
tar -cpzf "$File" -g "$FileIncremental" $Source
chmod 0700 "$File"
PackagesList=$(dpkg --get-selections | awk '!/deinstall|purge|hold/ {print $1}' | tee "$FilePackagesList")
while read -r PackageName
do
case "$PackageName" in
minidlna)
# Code ...
;;
slapd)
# Code ...
;;
esac
done <<< "$PackagesList"
exit 0

Answer 1

This isn't a problem with ERR traps at all, or with command substitutions, but with pipelines.
false | true
returns true, unless the pipefail option is set.
Thus in OpenFiles=$(lsof "$Source" | wc -l), only a failure in wc will cause the pipeline to be considered a failure, or in PackagesList=$(dpkg --get-selections | awk '!/deinstall|purge|hold/ {print $1}' | tee "$FilePackagesList"), only a failure in tee will cause the command as a whole to be considered failed.
Put the command set -o pipefail at the top of your script if you want a failure from any pipeline component (as opposed to the last component alone) to cause the command as a whole to be considered failed -- and note the other caveats for ERR traps given in BashFAQ #105.

Answer 2

Another alternative is to look at the status for each stage in the pipeline:
# cat test_bash_return.bash
true | true | false | true
echo "${PIPESTATUS[#]}"
# ./test_bash_return.bash
0 0 1 0