duplicacy-backup.sh

#!/usr/bin/env bash

## Configs ##
ZFSSNAPSHOTNAME="duplicacy"

mountSnapshots () {
  mkdir -p "/mnt/$ZFSSNAPSHOTNAME"

  mount -t zfs "$ZFSSNAPSHOTBASE@$ZFSSNAPSHOTNAME" "/mnt/$ZFSSNAPSHOTNAME"

  while read -r line
  do
    MOUNTPATH=$(echo "$line" | sed "s/$ESCAPEDZFSSNAPSHOTBASE\///" | sed "s/@$ZFSSNAPSHOTNAME//")
    if [ -d "/mnt/$ZFSSNAPSHOTNAME/$MOUNTPATH" ]; then
      mount -t zfs "$line" "/mnt/$ZFSSNAPSHOTNAME/$MOUNTPATH"
    fi
  done < <(zfs list -t snapshot | grep "$ZFSSNAPSHOTNAME" | grep "$ZFSSNAPSHOTBASE" | grep -v "$ZFSSNAPSHOTBASE/\." | grep -o "$ZFSSNAPSHOTBASE/[0-9,a-z,A-Z,/,[:space:]]*@$ZFSSNAPSHOTNAME" | sed 's/[[:space:]]*$//')

}

unmountSnapshots () {
  while read -r line
  do
    MOUNTPATH=$(echo "$line" | sed "s/$ESCAPEDZFSSNAPSHOTBASE\///" | sed "s/@$ZFSSNAPSHOTNAME//")
    if [ -d "/mnt/$ZFSSNAPSHOTNAME/$MOUNTPATH" ]; then
      umount "/mnt/$ZFSSNAPSHOTNAME/$MOUNTPATH"
    fi
  done < <(zfs list -t snapshot | grep "$ZFSSNAPSHOTNAME" | grep "$ZFSSNAPSHOTBASE" | grep -v "$ZFSSNAPSHOTBASE/\." | grep -o "$ZFSSNAPSHOTBASE/[0-9,a-z,A-Z,/,[:space:]]*@$ZFSSNAPSHOTNAME" | sed 's/[[:space:]]*$//' | tail -r)

  umount "/mnt/$ZFSSNAPSHOTNAME"

  rmdir "/mnt/$ZFSSNAPSHOTNAME"
}


exitCleanup () {
  trap SIGINT

  cd "/"
  unmountSnapshots

  zfs destroy -r "$ZFSSNAPSHOTBASE@$ZFSSNAPSHOTNAME"
  rm $PIDFILE

  exit

}

createPIDFile () {
  if [ -f $PIDFILE ]
  then
    PID=$(cat $PIDFILE)
    ps -p $PID > /dev/null 2>&1
    if [ $? -eq 0 ]
    then
      echo "Process already running"
      exit 1
    else
      ## Process not found assume not running
      echo $$ > $PIDFILE
      if [ $? -ne 0 ]
      then
        echo "Could not create PID file"
        exit 1
      fi
    fi
  else
    echo $$ > $PIDFILE
    if [ $? -ne 0 ]
    then
      echo "Could not create PID file"
      exit 1
    fi
  fi
}

if [ $# -eq 1 ]
  then
    BASENAME=$(basename $1)
    RUNNAME="$ZFSSNAPSHOTNAME-$BASENAME"
    ZFSSNAPSHOTBASE=$(zfs list | grep "$BASENAME" | grep -v "$BASENAME/" | awk '{print $1}')
    ESCAPEDZFSSNAPSHOTBASE=$(echo "$ZFSSNAPSHOTBASE" | sed 's/\//\\\//')
    PIDFILE=/var/lock/$RUNNAME.pid
    ISOTIME=$(date -u +"%Y-%m-%dT%H:%M:%SZ")
    YEAR=$(date -u +"%Y")

    createPIDFile

    zfs snapshot -r "$ZFSSNAPSHOTBASE@$ZFSSNAPSHOTNAME"

    trap "exitCleanup" INT
    
    mountSnapshots

    # Do tha backup dance!
    cd "/mnt/$ZFSSNAPSHOTNAME"
    duplicacy -log backup -threads 10 -stats >> "/var/log/$RUNNAME.log"
    cd "/"

    unmountSnapshots

    zfs destroy -r "$ZFSSNAPSHOTBASE@$ZFSSNAPSHOTNAME"
    rm $PIDFILE
else
  echo "Argument should be: path remote"
fi

No you can just ignore the second parameter

How does it know where to send the backups?

root@freenas:~ # ./duplicacy_freebsd_x64_2.2.3 backup --help
NAME:
   duplicacy backup - Save a snapshot of the repository to the storage

USAGE:
   duplicacy backup [command options]

OPTIONS:
   -hash                        detect file differences by hash (rather than size and timestamp)
   -t <tag>                     assign a tag to the backup
   -stats                       show statistics during and after backup
   -threads <n>                 number of uploading threads
   -limit-rate <kB/s>           the maximum upload rate (in kilobytes/sec)
   -dry-run                     dry run for testing, don't backup anything. Use with -stats and -d
   -vss                         enable the Volume Shadow Copy service (Windows and macOS using APFS only)
   -vss-timeout <timeout>       the timeout in seconds to wait for the Volume Shadow Copy operation to complete
   -storage <storage name>      backup to the specified storage instead of the default one
   -enum-only                   enumerate the repository recursively and then exit

root@freenas:~ #

I don't see where the '-storage' option has been defined.

After reading the docs again I am assuming I do an 'init' in the root of the data to be backed up and the .duplicity directory is then available from within the ZFS snapshot. The preferences file would then have the storage URL and other metadata to allow the backup to run.