Free space on hard drive going up and down too much On my laptop with macOS Sierra 10.12.5 (but it was happening with previous versions of the OS too), I get pinged by the system that I have only a few GBs left, I clean up, reboot, go from 4 GB left to 30 GB left, and within a couple of days, not doing anything special (not downloading movies or anything space-hungry), I get told again I have only 4 GB left ... I went through that cycle a few times now, and I'm wondering what could be causing that behavior, and how I could stop it. Any idea or pointers to what I should look at on my HDD to try and understand this? (sleep images, sparse bundles...)

Note: this question is not about using a disk inventory app to find the large files that take up too much space on my HDD. What I am interested in is why I got from 30 GB after boot to 4 GB without installing anything on my Mac. I want to understand which process is doing that and stop it.
Install Homebrew (www.brew.sh)
In the teminal, use this command to install ncdu

brew install ncdu


then, run it

cd ~; sudo ncdu


You will get a nice overview of the size of all folders in your home directory, largest topmost. It should be easy to track your gigabytes.

If this does not give results, you can run ncdu for your whole disc as well. Just go to /Volumes/DISKNAME e.g.

cd /Volumes/MACBOOT; sudo ncdu


You should also make sure that no application is simply "eating ram" by running

top -o mem


In the top lines, if there are applications that have a "G" in the "mem" column, they use gigabytes of ram. Make sure this is adequate. You might have an application with a memory leak installed.