How do I fix this "E: Type '*' is not known on line * in source list ..." update error?

The error indicates a malformed entry in a source file, which causes the update process to abort. To fix it, you have to either fix the entry (if you know what the right entry should look like) or remove it altogether (that's what I'm going to describe, as it's the fastest way to enable you to update your system again).

1. First, you need to open the file containing the bad entry. The filename is given in the error message, in your example /etc/apt/sources.list.d/some-ppa.list. Open a terminal, and type

   sudo nano /etc/apt/sources.list.d/some-ppa.list
   

   and press Enter. After entering your password, you should now see the file opened in the text editor nano.

2. Now you need to locate the malformed entry. It should be on the line number given in the error message - in your case that would be line 1.

3. This line should be incomplete and start with the unknown type the update process is complaining about (here ain). Just delete the whole line, and save/close the file with Ctrl+X.

4. That's it. You should now be able to successfully run the update process!

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If that entry is all that's in the file, then you can just delete the file. To do this, just open a terminal and run the following command:

sudo rm /etc/apt/sources.list.d/some-ppa.list

You're experiencing Bug #789859 in the software-properties package.

The quick and immediate fix is to open up the file that the error message mentions (in this case /etc/apt/sources.list.d/the-board-team-dev-snapshots-natty.list) and remove the line that says ain. This is achieved like so:

1. Press AltF2
2. Enter: gksudo gedit /etc/apt/sources.list.d/the-board-team-dev-snapshots-natty.list
3. Remove the line that says ain
4. Save the file

When done, everything should work.

To avoid experiencing this bug in the future, please visit the bug's Launchpad page and mark the bug as affecting you.