df is the POSIX command for determining the amount of free disk space on a mounted filesystem. It presents its results in a simple columnar format:

Filesystem           1k-blocks      Used Available Use% Mounted on
/dev/hda1              3526172   1139560   2207492  35% /
/dev/hda2             55337152       840  52525344   1% /home
otherbox:/home        53937632  21984112  29213632  43% /automount/otherbox.home
With the -h option, available in GNU df, the figures become more comprehensible:
Filesystem            Size  Used Avail Use% Mounted on
/dev/hda1             3.4G  1.1G  2.1G  35% /
/dev/hda2              53G  840k   50G   1% /home
otherbox:/home         51G   21G   27G  43% /automount/otherbox.home
The only other useful option is to list filenames or directories, each of which has the available space information printed for the filesystem it is associated with.

Although df is a very simple command, it is quite convenient, especially when working with large files. Its output, especially when the -h option is not used, is easily manipulable by other programs, and it fits with the Unix two-letter naming convention for convenience of typing. Probably the most convenient feature is that it defaults to displaying all filesystems, so that it is very easy to get a summary of the state of your machine. Nevertheless, it is not nearly as useful as its counterpart, du, as discussed at that node.

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