Export a Docker volume to a .tar / Import into Docker volume from a .tar / Duplicate volumes (via tar)

export-import-volumes.sh

# Usage: export_volume volume_name
#        export_volume volume_name > file.tar
#
# prints to STDOUT a volume in .tar format
export_volume() {
  # Quit if volume name is not specified
  vol=${1:?}

  # If there is no such container, a new container will be created, we don't need that
  docker volume inspect $vol > /dev/null || (echo "Container specified for export doesn't exist" && return 1)

  # Export container contents to STDOUT in an uncompressed tar format
  docker run --rm -i -v $vol:/vol alpine tar -C /vol -c .
}

# Usage: import_volume volume_name
#        cat file.tar | import_volume volume_name
#
# Unpacks a tar archive passed in to STDIN to volume_name volume.
# The volume may not exist in advance, otherwise must be empty.
import_volume() {
  # Quit if volume name is not specified
  vol=${1:?}

  # If the container does not exist, create it
  docker volume inspect $vol > /dev/null 2>&1 || docker volume create $vol > /dev/null

  # If the container is not empty, quit (don't mess it up)
  docker run --rm -v $vol:/vol alpine sh -c "test \"\`ls -A /vol\`\"" && return 1

  # Finally, untar into a volume
  docker run --rm -i -v $vol:/vol alpine tar -C /vol -xf -
}

# Usage: duplicate_volume from_volume_name to_volume_name
duplicate_volume() {
  source_volume=${1:?}
  destination_volume=${2:?}
  export_volume $source_volume | import_volume $destination_volume
}