/usr/lib/zfs-linux/scrub is in zfsutils-linux 0.6.5.6-0ubuntu8.
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 9 | #!/bin/sh -eu
# Scrub all healthy pools.
zpool list -H -o health,name 2>&1 | \
awk 'BEGIN {FS="\t"} {if ($1 ~ /^ONLINE/) print $2}' | \
while read pool
do
zpool scrub "$pool"
done
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