bash: Help clean up code and shorten output from error notification

[oshunluvr]

In a bash script, I do mounting and unmounting of a folder. I have the script send messages to my desktop via notify-send. I wish to trim this output slightly and have my code robust enough to notify me of the condition.

The relevant variables in the script:

backup_folder="/mnt/root_backup/"
dismount="true"
NOTIFYUSER="myuser"
NOTIFYDISPLAY=":0.0"​

In this instance, the backup folder is mounted and the current script requests it to unmount via this command:

# Unmount backup folder if requested."
if [ "$dismount" = true ] ; then
umount "$backup_folder"
su $NOTIFYUSER -c "export DISPLAY=${NOTIFYDISPLAY}; /usr/bin/notify-send -i ksnapshot -t 0 'Daily Backup:' 'Backup unmounted.'"
echo "Backup unmounted"
fi

The output in the notification looks like:

Daily Backup:
Backup unmounted.

which is what I want when it works.

Note that there is no error condition checking, so it reports "backup unmounted" regardless whether it actually worked or not.
The last couple of days, I had left a console window open at the backup folder, so it did not actually unmount it The script redirects the errors to a log file, so I could see what had happened. I decide to "upgrade" my script to tell my directly when this occurs.

The replacement code for the above code snippet:

# Unmount backup folder if requested."
if [ "$dismount" = true ] ; then
  umount "$backup_folder"
fi
# Check to see if unmount worked
if [[ -d "$backup_folder" ]]
  then
# umount did not work
    nodismount=$((umount "$backup_folder" 2>&1) | grep -oP '(?<=umount: ).*')
    su $NOTIFYUSER -c "export DISPLAY=${NOTIFYDISPLAY}; /usr/bin/notify-send -i ksnapshot -t 0 'Daily Backup:' 'Backup unmount failed. $nodismount'"
  else
# umount worked
    su $NOTIFYUSER -c "export DISPLAY=${NOTIFYDISPLAY}; /usr/bin/notify-send -i ksnapshot -t 0 'Daily Backup:' 'Backup unmounted.'"
fi

##Script complete.
exit 0​

Output notification:

Daily Backup:
Backup unmount failed. /mnt/root_backup: target is busy.

1. This does what I want except it's doing the same thing twice. It seems like I could shorten the code a bit by not having to separately create the message content via running the umount command twice.

2. The notification is a bit long. I'd like to trim "Backup unmount failed. /mnt/root_backup: target is busy" to "Backup unmount failed: target is busy" but I can't seem to be able to figure out how to sed or grep my way there.