What is /etc/apt/apt.conf.d/50unattended-upgrades.ucf-old and can I safely delete it?

Question

I've recently upgraded from Ubuntu 14.04 to 16.04. A few things didn't make it across, for example the ttf-mscorefonts-installer (no idea why), and while I was trying to install them that I got a warning.

The command...

sudo apt-get install <package>

...as well as producing the usual behavior, also produced multiple instances of the warning...

N: Ignoring file '50unattended-upgrades.ucf-old' in directory '/etc/apt/apt.conf.d/' as it has an invalid filename extension

There is a file with the same name but without the .ucf-old extension in the same directory. Is this a relic of the upgrade process? Can I just delete the "old" file?

Answer 1

The file 50unattended-upgrades.ucf-old is created by the utility ucf ["update configuration files"] which interacts with the user during the upgrade, asking whether to overwrite or retain a copy of old configuration files. The one with the suffix .ucf-old is the copy. It is retained for reference and can be deleted safely.

From the ucf manpage:

During the course of operations, when working with configuration files, ucf optionally creates copies of versions of the configuration file in question. For example, a file with the suffix ucf-old holds the old version of a configuration file replaced by [the utility] ucf. [...] the maintainer scripts should consider purging copies of the configuration file with these extensions during purge.

The behavior of ucf is described on askubuntu.