Rev 11/24/21 (see Notes)
According to an article entitled
Snap Packages {Comprehensive Guide for Ubuntu Users} (https://phoenixnap.com/kb/snap-packages), Snaps can be installed on installations without an internet connection, as follows:
To download a snap package, use the following command:
snap download <package_name>
Download a snap package.
The system downloads two files to your $HOME directory – a .assert and a .snap file.
Note: It is advisable to also download and install the “core” and “gnome-3-26-1604” snaps if they are not already present on the target system. This is because some GNOME snaps require them to function properly.
1. Copy the downloaded files to the $HOME directory of the machine on which you want to install the app.
2. Install the packages using the following commands:
sudo snap ack <package_name.assert>
sudo snap install <package_name.snap>
[end of excerpt]
So, I looked into whether it would be possible to install snapd on Ubuntu on Userland on an Android phone, and found that it apparently is, although it's a 34MB download and I decided to put it off for a while. But assuming that it would be able to perform the procedure described above, it would be possible to download snaps with an Android phone and install them on a Linux installation. But snaps are typically massive, so you'd need a high-speed connection to download them in a reasonable amount of time.
I can think of several scenarios in which APT-Offline would still come in handy, and there will most likely be a demand for it for a long time to come. I enjoy using it because it's like an intricate toy that performs functions which nothing else can perform. With the instructions I've provided, those who want to use it should be able to figure out how to use it and adapt it to their situation. The instructions are long, but the idea is to read the introduction, and then start using APT-Offline to make some desired change to some installation, and refer to the instructions only as needed. There's nothing like using APT-Offline to learn how to use it.
Unfortunately as of this writing, it appears that APT-Offline doesn't perform the install-operation, or at least it doesn't on my installation. I'll try upgrading my system and see of that fixes it, but if not, the solution would probably be to make a fresh installation and get a temporary direct internet connection to set it up without installing APT-Offline, because if it doesn't perform the install-op, yet somehow ended up getting released, it might contain malware, and can't be trusted.
To get a temporary internet connection, you need to have access to AC power at your access point, and if all else fails, you could use an DC-to-AC inverter plugged into a car's cigarette lighter, or an uninterruptible power supply. You might also need a cardboard hood over the monitor to block direct sunlight.
If after using your secure installation for a while you realize that you want to install something else on it, perhaps you could find it in the form of a Snap and install it as described above, because if APT-Offline can't perform the install-op, the only other definitely secure procedure at that point would be to create a fresh installation, get a temporary direct internet connection for it, and set it up again.
Creating new installations isn't a big deal if you don't store any data on them (for details, see my review of the Gigabyte Brix GB-BLCE-4105R mini-PC, at http://toggwu.blogspot.com/2021/11/cheap-air-gap-pc.html), although the only flash drives which I've used and which work well are Sandisk Ultra-FITs, which must be plugged into USB-3 ports to provide the functionality required for an installation. The vast majority of U-Fs have plastic connector-shrouds, and in one such case which I was constantly plugging and unplugging the drive, part of the shroud broke off, although it didn't have any effect on its usability (Lexar manufactures a model without a shroud.) The U-Fs sometimes run on the hot side, although Sandisk apparently doesn't consider this to be a problem. 15GB U-Fs have slow write-speeds compared to 64GB U-Fs, so naturally the installation-process takes much longer on 15GB U-Fs compared to 64GB U-Fs. Because U-Fs are physically tiny, I attach tags to them to label them and to make it easier to find them if they get dropped. I also use combinations of brightly-colored zip ties (such as yellow, and dayglo green and orange) to make them more visible and to give each one a color-code to differentiate them from each other.
I also tried a Lexar 32GB USB-3 drive, and although it had a very high write-speed when I was erasing it before performing the installation, it had a very slow write-speed during installation, and the installation booted extremely slowly and didn't function correctly. So, Lexars are OK for storing data, but unusable as bootable drives. I've also tried two installations on a 15GB Kingston USB-3 Datatraveler, but the installations didn't respond to my inputs very quickly, although they worked much better than the installation on the Lexar. I like the fact that the Kingston has an all-metal connector (which is the same as the drive's body), and that it ran cool. So, in a pinch, Kingstons could be used. I plan on trying Verbatim Metal Executive USB 3.0 flash drives, because I've found Verbatims to be very reliable, which is obviously a requirement for a bootable drive.
I use cheap Chinese drives with LED transfer-indicators and caps for uncritical data (which is nevertheless backed-up multiple times), because I like their no-nonsense physical design and the fact that they're fairly easy to label. For example, in some cases I simply mark them with a Sharpie, and then put clear tape over the label. To change the label, I remove the tape and wipe the drive with alcohol. In other cases, I create a window in a piece of wide clear tape by placing a piece of narrow clear tape face-to-face across the wide tape, and then cutting the wide tape to certain dimensions and wrapping it around the drive so that the window is on one side of the drive, and then I put a label under the window and cinch the tape to hold the label in place. But these drives have a high failure rate because they use low-quality chip grade-outs from the edges of wafers, as opposed to the high-quality chips from the center of the wafer, which are used for more expensive drives.
Notes
Rev 11/24/21 - Corrected claim that Lexar 32GB USB-3 drives make good bootable drives, which was based on an erronoeous recollection. I tried one such drive again, I realized that it's actually unusable as a bootable drive. Also made other revisions to section on flash drives.
