I've got two devices on my LAN: a Raspbian jessie and an Ubuntu 14.04. The latter has some nfs shared folders, which are available from Raspbian at startup, set up in its /etc/fstab file as:192.168.1.10:/mnt/nfs/HDD /mnt  nfs     defaults,nofail,noatime          0       0The problem is coming up when Ubuntu is offline and I try to run df on raspbian... infinite loop. No answer.Does df have any way to ignore non-available devices? To show only those file systems that are currently available.
Ignore unmounted file systems
nfs;disk usage;devices;fstab
The automounter was designed exactly for this kind of problem. It automatically mounts drives (local or remote) only when they are needed, and unmounted them when they are no longer being used.Install autofs on your NFS client and comment out (or remove) the entries in /etc/fstab. Edit /etc/auto.master and ensure that there is a line like this uncommented in the file/net /etc/auto.net --timeout=120Do not just uncomment the line /net -hosts as this requires NIS installed and configured to work - which is highly unlikely.Restart the automounter with service autofs restart. You will now have access to your remote NFS filesystems under the /net directory. In your specific instance the path will be /net/192.168.1.10/mnt/nfs/HDD. You can then symlink that into your filesystem as if it were mounted:ln -s /net/192.168.1.10/mnt/nfs/HDD /mnt/hddSome notesMy personal preference is to tweak the entries in /etc/auto.master so that items are managed underneath deeper less visible directories such as /var/autofs/net and /var/autofs/misc rather than /net and /misc, but for your specific situation I've left the configuration as standard as possible.If you want to adjust the mount options for the NFS remote filesystem you will need to edit the file /etc/auto.net as options cannot be passed from auto.master.