Real time cmd tool to show HDD space remaining

As Julie said, you can use df to display free space, passing it either the mount point or the device name:

df --human-readable /home
df --human-readable /dev/sda1

You'll get something like this:

Filesystem Size  Used Avail Use% Mounted on
/dev/sda1  833G  84G  749G  10%  /home

To run it continuously, use watch. Default update interval is 2 seconds, but you can tweak that with --interval:

watch --interval=60 df --human-readable /dev/sda1

df is a simple command line utility that shows you disk usage, including free space.

Check man df for details.

If you don't like the idea of dedicating a whole terminal to watching the output of df, you could consider a tool such as conky. There are countless examples of using conky to monitor everything from HDD usage, HDD temp, ram usage, local weather, news headlines... you name it.