df very useful command in linux

report file system disk space usage
  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.
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 a sample output from my  system
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Filesystem           1K-blocks Used Available Use% Mounted on
/dev/hda1             19322864   3160228  15181068  18% /
varrun                  253748        92    253656   1% /var/run
varlock                 253748         4    253744   1% /var/lock
udev                    253748       116    253632   1% /dev
devshm                  253748         0    253748   0% /dev/shm
lrm                     253748     18856    234892   8% /lib/modules/2.6.15-23-386/volatile
/dev/hda2             19228308   4714944  13536612  26% /home