rrdtool dump filename.rrd rrdtool info filename.rrd
rrdtool graph /dev/null -s 3600 DEF:ierr=filename.rrd:ds1:AVERAGE VDEF:ioct_a=ierr,AVERAGE PRINT:ierr:%lf
this command will get the avg value of ds1 (ds1 is the dataset – you can see it in rrdtool info ) , assign it to variable ierr and print out
#3600 => one hour