Modify Date

There is a better way to get the modify timestamp of a file – check the -c option of stat:
OLDDISKSTAT_EPOCH=$(stat $HISTFILE -c %Y)

Furthermore the perfdata documentation states, that you should add the warning and crit levels:
echo “${OUTPUT}summary: $TPS io/s, read $SECTORS_READ sectors (${KBYTES_READ_PER_SEC}kB/s), write $SECTORS_WRITE sectors (${KBYTES_WRITTEN_PER_SEC}kB/s) in $TIME seconds | tps=${TPS};${WARN_TPS};${CRIT_TPS};; read=${BYTES_READ_PER_SEC};${WARN_READ};${CRIT_READ};; write=${BYTES_WRITTEN_PER_SEC};${WARN_WRITE};${CRIT_WRITE};;”