Tracking migration progress of 14705 files (or any other arbitrary pre-known number)...
watch -n 10 'PROGRESS=$(drush @your-alias sqlq "SELECT COUNT(*) FROM file_managed") && echo "$PROGRESS => "$(awk "BEGIN {printf \"%.2f%\n\", $PROGRESS/14705 * 100}")'
Which outputs something like:
12074 => 82.11%
Updated every 10 seconds.