df [OPTION]... [FILE]...
This manual page documents the GNU version of df. df displays the
amount of disk space available on the file system containing each file
name argument. If no file name is given, the space available on all
currently mounted file systems is shown. Disk space is shown in 1K
blocks by default, unless the environment variable POSIXLY_CORRECT is
set, in which case 512-byte blocks are used.
If an argument is the absolute file name of a disk device node contain-
ing a mounted file system, df shows the space available on that file
system rather than on the file system containing the device node (which
is always the root file system). This version of df cannot show the
space available on unmounted file systems, because on most kinds of
systems doing so requires very nonportable intimate knowledge of file
system structures.