It is a command to report filesystem disk space usage (on Linux).
Recently I checked my backup server's disk space. I issued df for that. The output was:
df: cannot read table of mounted filesystems: No such file or directory
Something wrong here. After some googling, I managed to get the solution. I issued this command:
grep -v rootfs /proc/mounts > /etc/mtab
After running this command, df worked again. One lesson learned. df relies on /etc/mtab for it to work. Now I can easily know how much disk space left for each partition. Thanks df!.