When Something Breaks

A method

Most problems give way to the same few steps:

1. What changed last? An update, a new program, an edited file, new hardware. Problems usually start right after something changed, and undoing it is often the fix.
2. Read the exact error. Not “it didn’t work”: the actual message, word for word.
3. Check the journal: journalctl -b -p err for this boot, -b -1 for the last one (module 3) ¹.
4. Change one thing at a time, and check after each. Two changes at once means you won’t know which helped, or which made it worse.
5. Keep notes of what you tried. You’ll need them if you ask for help, and next time it happens.

A frozen desktop

If the screen stops responding, the rest of the system is often fine. Linux has text consoles behind the desktop: Ctrl+Alt+F3 switches to one (on some laptops, Ctrl+Alt+Fn+F3), and Ctrl+Alt+F1 or F2 switches back ². Log in there with your user name and password, and you have a shell:

me@garden-laptop:~$ top
me@garden-laptop:~$ kill 4321
me@garden-laptop:~$ sudo systemctl reboot

top shows what’s using the processor and memory; press M to sort by memory, and note the PID of anything running away (Shell course, module 6). kill asks it to stop, and kill -9 forces it. If it’s hopeless, reboot from here, which is far kinder to your files than holding the power button.

A program or service misbehaving

me@garden-laptop:~$ systemctl --failed
me@garden-laptop:~$ systemctl status cups
me@garden-laptop:~$ journalctl -b -u cups

Start with systemctl status, which shows the state and the last log lines, then the full journal for the unit (module 3) ^{3 1}. For a desktop app, start it from a terminal: error messages it would normally hide appear there.

A full disk

Odd failures, such as programs failing to save, the desktop failing to log in, or updates failing, are often a full disk. df -h shows it; module 4’s du and ncdu find what filled it, and sudo apt clean and a journal vacuum free space quickly.

Updates interrupted

If an update is cut off by a power cut or a crash, packages can be left half-installed, and apt refuses to continue until they’re sorted out. Two commands fix most cases:

me@garden-laptop:~$ sudo dpkg --configure -a
me@garden-laptop:~$ sudo apt --fix-broken install

dpkg --configure -a finishes configuring every package that was unpacked but not yet set up ⁴. apt --fix-broken install (-f) then tries to correct broken dependencies ⁵. After that, sudo apt update && sudo apt upgrade to finish the job.

It won’t boot

The GRUB menu. Ubuntu’s boot loader, GRUB, has a menu that’s usually hidden. To show it, press Escape (on UEFI computers, perhaps several times) or hold Shift (on older BIOS computers) just after the maker’s logo ⁶. If you see the Ubuntu logo, you’ve missed it; restart and try again.

An older kernel. In the menu, Advanced options for Ubuntu lists each installed kernel ⁶. If problems began after an update, try the previous kernel. If it boots, the new kernel is the problem; wait for its next update or report it.

Recovery mode. Each kernel also has a (recovery mode) entry, which starts only basic services and offers a menu; Drop to root shell prompt gives you a root shell with no desktop and no password ⁶. The root file system starts read-only, so before changing anything ⁶:

root@garden-laptop:~# mount -o remount,rw /
root@garden-laptop:~# mount --all

From there you can undo whatever broke it. Common fixes:

• A bad /etc/fstab line, from module 4: nano /etc/fstab and put a # at the start of the line to turn it into a comment ⁷.
• Interrupted updates: dpkg --configure -a, as above.
• A full disk: clear space as in module 4.

Then reboot.

The last resort

If none of that works, the live USB from module 1 still boots, from the stick, with the computer’s disk untouched ⁸. Choose Try Ubuntu, open Files, and the internal disk is listed; open it and copy your files to an external drive. Then you can reinstall without fear, restore from your backups, and clone your dotfiles to have your setup back in minutes. This is where all the earlier modules pay off.

Asking for help

Ubuntu’s community answers questions on Ask Ubuntu and the Ubuntu Discourse ⁸. A question that gets answers includes:

• your release (lsb_release -a) and, for hardware problems, the model;
• the exact error message, and the relevant lines from journalctl, pasted as text, not a photo;
• what you were doing, what you expected, and what happened;
• what you’ve already tried.

And remember last lesson: before pasting a suggested sudo command, understand what it does.

Your turn

So

Ask what changed, read the exact error, check the journal for this boot and the last, and change one thing at a time. A frozen desktop: Ctrl+Alt+F3, top, kill, or systemctl reboot. A broken service: systemctl status, journalctl -u. A full disk: df, then du. Interrupted updates: dpkg --configure -a and apt --fix-broken install. Won’t boot: the GRUB menu (Escape or Shift), an older kernel under Advanced options, or recovery mode’s root shell, after mount -o remount,rw /. Last resort: the live USB, your files, and your backups. Ask for help with your release, the exact message, and what you tried.