`df` does a great job for an overview. But what if I want to set a variable in a shell script to the number of bytes available on a disk?

Example:

```
$ df
Filesystem            1K-blocks     Used Available Use% Mounted on
/dev/sda             1111111111  2222222  33333333  10% /
tmpfs                  44444444      555  66666666   1% /dev/shm

```

But I just want to return `33333333` (bytes available on `/`), not the whole `df` output.