use Perl

$ df
Filesystem    1K-blocks     Used Available Capacity  Mounted on
/dev/disk0s2   33554432 29457252   3841180    89%    /
devfs               111      111         0   100%    /dev
fdesc                 1        1         0   100%    /dev
/dev/disk0s3   16777216 16046752    730464    96%    /Volumes/Stuff
map -hosts            0        0         0   100%    /net
map auto_home         0        0         0   100%    /home
$ df
Filesystem    1K-blocks     Used Available Capacity  Mounted on
/dev/disk0s2   33554432 29457252   3841180    89%    /
devfs               111      111         0   100%    /dev
fdesc                 1        1         0   100%    /dev
/dev/disk0s3   20840448 16046808   4793640    77%    /Volumes/Stuff
map -hosts            0        0         0   100%    /net
map auto_home         0        0         0   100%    /home


The first and the second df are truthfully one after the other on in the same shell, on the same machine, without rebooting. The only thing I did was to run Disk Utility, and drag the size of the partition to make it bigger. Live. Without dismounting the disk. Crazy.