Adding new repositories without getting everything from those repositories

Wanted to be able to install selected things from Trisquel, such as IceCat, without changing everything over to Trisquel (yet).

To get access to Trisquel packages, add them to your sources list, add their keyring, and update:

https://trisquel.info/en/wiki/packages-repository

vi /etc/apt/sources.list

sudo apt-get install trisquel-keyring

sudo dpkg -L trisquel-keyring
(to find the path for the next command)

sudo apt-key add /usr/share/keyrings/trisquel-archive-keyring.gpg

Now update will work:

sudo apt-get update

And now this will work:

sudo apt-get install icecat

All great, except now my updates wanted to upgrade (sidegrade?) from Ubuntu to Trisquel.

Answer is to use pinning.

Thanks to http://www.argon.org/~roderick/apt-pinning.html for the precise syntax.

Note that the "o=" way of defining the origin used here http://www.debian.org/doc/manuals/apt-howto/ch-apt-get.en.html and here https://help.ubuntu.com/community/PinningHowto and echoed here http://www.jaredlog.com/?p=1820 and here https://wiki.debian.org/AptPreferences did not work for me.

Didn't work (but should have?):

Package:  *
Pin: release o=us.archive.trisquel.info
Pin-Priority: 400

Did work:

Package:  *
Pin: origin us.archive.trisquel.info
Pin-Priority: 400

Also the above worked immediately when testing with, say, apt-cache policy php5, no apt-get update needed.