Issuing the df Command

While using /etc/mtab or /proc/mounts lets you know what file systems are currently mounted, it does little beyond that. Most of the time you are more interested in one particular aspect of the file systems that are currently mounted -- the amount of free space on them.
For this, we can use the df command. Here is some sample output from df:
 Filesystem 1k-blocks Used Available Use% Mounted on /dev/sda3 8428196 4280980 3719084 54% / /dev/sda1 124427 18815 99188 16% /boot /dev/sda4 8428196 4094232 3905832 52% /home none 644600 0 644600 0% /dev/shm 
Several differences from /etc/mtab and /proc/mount are immediately obvious:
  • An easy-to-read heading is displayed
  • With the exception of the shared memory file system, only disk-based file systems are shown
  • Total size, used space, free space, and percentage in use figures are displayed
That last point is probably the most important because every system administrator eventually has to deal with a system that has run out of free disk space. With df it is very easy to see where the problem lies.