So that means I have to do an apt-cache depends ( and download the dependencies ) for each dependence of the desired program ? 😭

That's what I do, but as Steve_V rightly said if you have an active connection apt can do it for you.

There is a way to have apt resolve depends using local packages, and that's if you have set up an offline repo of your own.

I did this years ago when gnome 2 was transitioning to gawdawful 3, I downed the entire repo including all gnome 2 packages and set myself up an offline repo in case I ever wanted more programs in the gnome 2 system I was using at the time.

You cannot use apt / apt-get / aptitude to install a deb file directly.

AFAIK aptitude can't install from a file, but apt/apt-get has supported this for years. It'll even resolve dependencies for local packages (but not using local packages, at least not automatically, same as dpkg).

So that means I have to do an apt-cache depends ( and download the dependencies ) for each dependence of the desired program ? 😭

The One such tool for this is the package manager, dpkg.

To install offline, get the packages, including all of the dependencies along with the dependencies of the "recommends", if you feel you will need those as well (you most likely will), and put them all in one directory within your $HOME, cd to that directory, then:

# dpkg -i *deb

I would like to easily install some software/packages to an offline rpi devuan

I've heard of apt-offline that seem good for this purpose.
obviously it need to be installed to the offline machine.

so on the online machine I did

$ apt-cache depends apt-offline
apt-offline
  Depends: <python3:any>
    python3
  Depends: apt
  Depends: less
  Depends: python3-magic
  Recommends: debian-archive-keyring
  Recommends: python3-debianbts
  Recommends: python3-apt

then to get the .deb files,  and move them the offline machine.

$ apt-get download python3-magic apt-offline

once the deb file on the offline machine I did:

$ apt install /path/python3-magic_2%3a0.4.20-3_all.deb
success #I don't remember the ouput by heart :)

$ apt install /path/apt-offline_1.8.2-2_all.deb
#error, see here after

apt wanted to install automatically 9 other packages (iso-codes python-apt-common - python3-apt python3-debianbts etc... )
Of course as this machine is offline, it failed...

but why now, it want to dw other packages that are not listed with apt-cache depends ??????

Thanks.