I just had quite a nice scare: I was installing some packages on Pop!_OS 18.10 (Ubuntu), and I unknowingly caused my trackpad to stop working and I couldn’t even open a Terminal window to see what was happening.

The Symptoms

I had been installing some packages with apt. At one point, when I tried to take the next step, I tried to open a new window in Byobu Terminal. I pressed F2 and nothing happened. I tried again, and nothing happened. I couldn’t open a new window. I tried Alt+F2 and Ctrl+F2, but neither of these key combinations did anything. I closed my Byobu Terminal session and tried to open a new one, but launching Byobu did nothing. No window opened.

Oops.

Next, I tried launching the regular Terminal application. Again, no window. Nothing. I tried XTerm. Nothing. I couldn’t open a Terminal session at all! How the hell was I going to even get information to help me diagnose the problem?

I tried logging out, then logging in again. I couldn’t open any Terminal windows and, worse, I couldn’t use my trackpad! My trackpad didn’t move my cursor at all. This would make figuring out the problem very difficult.

I tried restarting the computer. No change: I couldn’t open a Terminal window and I couldn’t use my trackpad.

Getting Information

I couldn’t see any output from trying to start a Terminal session, so I had no idea how to even guess at the symptoms, let alone the cause. How could I start a shell and see the result of trying to start one? And how do I get around my windowing environment without a trackpad?

Fortunately, I had a USB mouse nearby, so I plugged that in and it just worked. This meant that I could at least browse the web on this computer to figure out what else to do. I searched the web and tried using Alt+F2 to run an arbitrary command (in Gnrome 3), so I ran gnome-terminal. This didn’t open a Terminal session and it showed me no output, so I didn’t learn anything about the problem.

A Trick

In desperation, I launched GVim, then tried using the :! command to run a shell. This finally gave me a clue:

Aha! apt fooled me.

The Trouble With autoremove

I had been trying to install some printer-related packages, and in the process of uninstalling them (they didn’t do what I needed), apt told me that it could autoremove these packages:

  • libncursesw5
  • libtinfo5

Since I didn’t know any better, I executed apt autoremove, which removed these packages, and my problems started there.

Thank You, etckeeper

Fortunately, etckeeper made it relatively easy both to recover and to document what went wrong. I could see a commit from when I removed the printer-related packages that I’d installed, and then a second commit from executing apt autoremove. This helped me connect the error message above with the packages that I’d removed. This gave me the idea that I could simply reinstall those packages and return everything to normal.

Opening A Shell… Any Shell

I couldn’t open my normal shell (zsh) to fix the problem, but could I open any shell? I opened the Preferences for the Terminal application, then cloned my Pop! profile in order to change the shell command that the terminal runs to /bin/bash. I set the new profile—I called it “Recovery”—as the default profile for the Terminal application, then launched it. Yes! I got a bash shell. Now I could try my idea for fixing the problem.

Even Better!

After reading https://askubuntu.com/a/809058/839487 I saw that I could have started gnome-terminal using bash instead of my shell by pressing Alt+F2 then running the command gnome-terminal -x bash. I hope I can remember this next time!

Fixing The Problem

I fixed the problem (some of it—keep reading) by reinstalling the packages that apt autoremove had removed.

I could open a shell again. Maybe everything had returned to normal. Before I did anything else, I documented what I learned where I might one day stumble upon it again: in the git log for /etc.

Documenting The Problem

I amended the relevant etckeeper commits, adding detail about what happened, as well as making it easier for the eye to notice that something interesting had happened. First, I decorated the one-line commit messages.

The last two commits contained the mistake (apt autoremove and the fix apt-get install...). I added simple text tags to make it easier to spot them. I also added some detail to the log messages.

I had documented my adventures for myself and could safely breathe, then begin describing my adventures for you, in case you should ever get yourself into a similar situation.

Verifying The Fix

I could start my shell again, but my trackpad still didn’t work, so I got all this information out of my head, then I tried to see whether my trackpad would work again, first by logging out, then by rebooting in case logging it didn’t suffice.

Curiously, when I logged out, my trackpad started to work again, but when I logged in, it stopped working. Moreover, each time I tried this, the cursor jumped to the same spot near the bottom-right corner of my display. I didn’t know what that meant yet, but it made me think that I had some configuration setting somewhere that needed to change. I hoped that rebooting would fix the problem, so I rebooted.

Sadly, that didn’t fix the entire problem. Good news: I can start my shell and Byobu. Bad news: my trackpad only works when I’m logged out. This points pretty clearly to a user-level configuration problem, which I can probably diagnose (and maybe fix) by looking at the changes to /etc or to my dotfiles when I reinstalled these packages. Fortunately, in the meantime, a USB mouse works, so I don’t have to fix this problem right now, even though I’ll want to fix it very soon.

Post Script: Fixing the Touchpad

I guess Linux calls it a “touchpad” and not a “trackpad”. Old habits die hard.

I don’t know how it happened, but somehow, in the middle of removing and reinstalling packages, the “on/off” toggle for the touchpad in Gnome switched to “off”. That’s all. I switched it back “on” through the Mouse & Touchpad settings, and the touchpad just started working again. Yay.