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The first step is get some space in the partition “/boot” because without that it’s impossible to do anything.
So go to /boot and remove some “initrd” files as they are the biggest ones. A few of them will be enough.
After that a good point is to ensure there is no partial installation pending to finish:
apt-get -f install
Now it’s a good idea to purge all kernels except the running one:
dpkg -l 'linux-*' | sed '/^ii/!d;/'"$(uname -r | sed "s/\(.*\)-\([^0-9]\+\)/\1/")"'/d;s/^[^ ]* [^ ]* \([^ ]*\).*/\1/;/[0-9]/!d' | xargs sudo apt-get -y purge
To avoid that in future before filling the partition, it’s a good idea to install and run periodically: purge-old-kernels. Installation and example of use are:
apt-get install bikeshed
# keep three old kernels:
purge-old-kernels --keep 3
# if you want to put that in the crontab use that command
purge-old-kernels --keep 3 -qy
If you’re a Grub user don’t forget to run:
Personally I have a nightmare with that problem and Ubuntu, especially with version 12.04 which is installed in a lot of servers that I manage. I repeated the previous process a lot of times and in the end, I decided to document it because I always have to go to Google and find the proper steps to solve that problem.