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

Mon Aug 11 19:00:09 UTC 2008
Sean Carolan <scarolan at gmail.com>

Is there a flag for the df command to get the total disk space used on
all filesystems as one number?  I have a server with a lot of mounted
shares.  I'm looking for a simple way to measure rate of data growth
across all shares as one total value.