/usr/sbin/mount.unionfs is in unionfs-fuse 1.0-1ubuntu1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | #!/bin/bash
label=$1
mpoint=$2
shift 3
dirs=${@##*,dirs=}
dirs=${dirs/,/:}
mount.fuse "unionfs#$dirs" "$mpoint" -o ${@%%,dirs=*}
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