[CentOS] df to get total disk usage on all filesystems?

Stephen Harris wrote:
> And, remember, that the output of "df" might have changed in between
> times you ran "df" and you ran the "awk" command; there's only 7Mbytes
> difference.  Did someone delete a 7Mbyte file?  Send email?  Finish a
> print job?  Or... could be plenty of reasons for the used amount to
> go down.
>
> The awk script works correctly, as can be tested easily:
>
>   $ cat h
>   Filesystem         1024-blocks      Used Available Capacity Mounted on
>   /dev/mapper/VolGroup00-LogVol00 274405432  18584656 241656808       8% /
>   /dev/sda2               101105     19096     76788      20% /boot
>   tmpfs                  1682508         0   1682508       0% /dev/shm
>
>   $ cat h | awk '/^\/dev\// { avail += $3/1024 } END { printf("%d Mb used\n",avail)}'
>   18167 Mb used
>
>   
But the whole system used only 18 MB ? That's not true.