I'm interested how to take a screenshot of the gdm3 login screen. I found this guide, but it seems no longer to work or not to work with gdm3. I opened a tty on the login screen and logged in there, but the command shown in the guide didn't work.
In the comments is mentioned that the guide needs an update, but I found no easy instruction how to do this.
In this link is shown a script, but it is very old, too, so I'm not sure if it would help
My approach is as follows:
I often use GNOME Machines to test things before installing anything on my PC’s hard drive. I always keep my virtual machine running Zorin 18.1 Core up to date. Before each test, I create a copy. I can then do whatever I like with this copy and take screenshots at any time 
Yes, in virtual machines you can take the screenshot with the host system, that's practical.
I'm looking for a solution when you have no VM.
Since I only have 8 GiB of RAM, a Zorin VM with 4 GiB of RAM allocated was always very slow. That's why I don't have a VM right now, and my storage space is very limited, too, because of a small drive. Often I make my tests in live sessions.
You can try this
on Login screen open TTY
sudo apt install gnome-screenshot
sudo DISPLAY=:0 gnome-screenshot -f ~/gdm-login.png
It showed "Authorization required, but no authorization protocol specified
(gnome-screenshot:15199): GTK-Warning: ...cannot open display :0"
(running on X11).
To find the right display
echo $DISPLAY
No output. But everything is very small and hard to read on my 14'' laptop, and I have to type as on an English keyboard. Do you know if I can change the input source to German in the tty3?
Edit: AI told me to run
sudo loadkeys de
and now I can type German.
For others who are wondering how to enter a tty:
I used Ctrl+Alt+F3 to open the tty, then entered my username and (if not autologin is set) the password, and to leave the tty I entered exit and then pressed Ctrl+Alt+F1.
As it was the first time for me in a tty, that was new and I had to play a bit around to find working keys.
You could make a Picture with Your Smartphone of the Screen.
The photo was not very good and clear, so I thought, I explore if there is a possibility to take a screenshot.
When you press the print key on the login screen, a screenshot is taken, but I don't find the screenshot after I'm logged in. In root's home folder it is not. Where is it saved? And paste is greyed out although there is a message on the login screen that the screenshot is put to the clipboard.
I tried to change the command of the first guide from gdm to gdm3, but it seems not to have worked. The screenshot file was created, but the tty terminal ended with an empty black screen with a blinking cursor. I tried to convert the image xwd in online converters because image magick showed errors, but the conversion always failed, perhaps because the file was empty. There was no click sound to hear.
I don't think that it is intended to make a Screenshot on the Login Screen.
How do You know that it is taken?
Normally, Screenshots are saved in ~/Pictures/Screenshots
You can hear the sound and get the message. In the home folder of the user it is not saved, probably because the user is not logged in.
I wondered if it would help to install another screenshot tool, set the defaut place where the images are saved to a folder in root directory and set a custom shortcut in the db database for all users to run this screenshot tool when pressing print key. But I doubt that it would help. It seems to be more a permissions problem that the screenshot tool gets no access to the screen.
You used the Script from the itsfoss Artice, right?
chvt 7; sleep 5s; DISPLAY=:0 XAUTHORITY=/var/lib/gdm/:0 xwd -root -out ~/screenshot.xwd; convert ~/screenshot.xwd ~/screenshot.png; rm ~/screenshot.xwd
And You replaced there gdm with gdm3, yes? Did You checked this Directory for the Picture? I mean /var/lib/gdm3 and You could also check there the hidden Folders.
Also maybe this could help (older Instructions but who knows):
Yes, in that command I replaced gdm with gdm3.
But the screenshots which I'm looking for are the ones which I took by pressing the print key, not running the script.
Yes, I had seen this and had added it to my first post, but it looks complicated, so I haven't tested it yet.
I would be glad if someone knows what is worth to try. Many old guides don't work. And some of these things can crash the session or show a black screen, and often I don't know what to do then and I'll have to start all over again. But I found out that pressing ctl+Alt+F1 can help to see the gdm screen again.
Perhaps I'll try it tomorrow. I'm tired now.
I tested the script of ao2.it now, but in the tty always is shown "Saving to disk is disabled" after running the script command.
I had put the lines to /etc/gdm3/greeter.dconf-defaults, but they had no effect.
I also removed the locked disable-save-to-disk settings from /usr/share/gdm/dconf/locks
and
/usr/share/gdm/dconf/00-upstream-settings
When You should need a Picture to show other Users, You could use as a Compromise a Picture that @zenzen posted a While ago. When neccessary, I use it, too:
I wanted to take screenshots from my experiments with gdm, and the camera of my smartphone is really bad.
I would really like to know why there is still shown that saving to disk is disabled. I ran sudo dconf update after making the changes.
@Ponce-De-Leon Do you have an account at It's FOSS and if yes, could you please ask Abhishek Prakash for an update of the guide in the comments?
I don't have an Account there. But You can post there without one. To post, You need to type in Your Mail and a User Name:
You can see in the red marked Area what You don't have without an Account but You can post without it.
Some years back, when I wanted to produce a lot of screenshots of GDM when I themed Gnome, I launched an instance in Gnome Boxes, then easily was able to screenshot the login screen.
