This script uses smartctl to report current temperature and health of all disks as a JSON array. It can be used on FreeNAS systems with telegraf's exec input.

diskstatus.sh

#!/usr/bin/env sh

# This script uses smartctl to report current temperature and health of all disks as a JSON array.
# It can be used on FreeNAS systems with telegraf's exec input.
#
# Example telegraf.conf input:
#
# [[inputs.exec]]
#     name_override = "diskhealth"
#     commands = ["/path/to/diskstatus.sh"]
#     timeout = "10s"
#     data_format = "json"
#     tag_keys = ["disk","health"]

# Location of smartctl executable.
SMARTCTL=/usr/local/sbin/smartctl

# Returns a list of disks.
DISKS=$(/sbin/sysctl -n kern.disks | cut -d= -f2)

for DISK in ${DISKS}
do
  TEMP=$(${SMARTCTL} -l scttemp /dev/${DISK} | grep '^Current Temperature:' | awk '{print $3}')
  HEALTH=$(${SMARTCTL} -H /dev/${DISK} | grep 'test result:' | cut -d: -f2 | sed 's/^[ \t]*//')

  if [ -z != ${TEMP} ] && [ -z != ${HEALTH} ]
  then
    JSON=$(echo ${JSON}{\"disk\":\"${DISK}\",\"health\":\"${HEALTH}\",\"temperature\":${TEMP}},)
  fi
done

# Remove trailing comma on last item.
JSON=$(echo ${JSON} | sed 's/,$//')

echo [${JSON}] >&1