/usr/lib/os-probes/mounted/90solaris is in os-prober 1.74ubuntu1.
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 10 11 12 13 14 15 16 17 18 19 | #!/bin/sh
# Attempt to check if solaris is installed in this system
# looking at the /etc/system parameters file and /etc/vfstab.
set -e
. /usr/share/os-prober/common.sh
partition="$1"
dir="$2"
type="$3"
if [ -f "$dir/etc/system" ] && [ -f "$dir/etc/vfstab" ]; then
        label="$(count_next_label Solaris)"
	result "$partition:Solaris/IA32:$label:chain"
	exit 0
else
	exit 1
fi
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