Ubuntu does not remove kernels when they install a new one, however the /boot partition is usually relatively small.
So after only a few kernel upgrades, you can get No Space Left On Device errors when trying to upgrade.
To determine which kernels can be removed run the following:
sudo dpkg --get-selections | grep 'linux-image.*-[0-9].*' | awk '{print $1}' | egrep -v "$(uname -r)"| while read n; do echo $n; done
To delete all these old kernels run the following.
NOTE: This will only keep the very latest kernel and will delete all others, so may not be what you want:
sudo dpkg --get-selections | grep 'linux-image.*-[0-9].*' | awk '{print $1}' | egrep -v "$(uname -r)"| while read n; do apt-get -y remove $n; done
Either remove them manually, or use this one liner to automatically remove them all.
export KERNEL="$(uname -r | grep -Po '([0-9\.\-]*[0-9])?')"; dpkg --get-selections | grep -E "linux-(header|image).*" | grep -iw install | sort | grep -v "$KERNEL" | grep -v "lts" | sed 's/install//g' | xargs dpkg -P
Here's the command by command explanation:
export KERNEL="$(uname -r | grep -Po '([0-9\.\-]*[0-9])?')"
The first portion sets the current kernel number in a variable KERNEL. It only takes the number, and greps out any additions like -generic or -server.
sudo dpkg --get-selections
The second portion first prints out all available packages.
grep -E "linux-(header|image).*"
The third portion greps for all packages with either linux-header or linux-image in the name.
grep -iw install
The fourth portion greps out only installed packages.
sort
The fifth portion sorts the output.
grep -v "$KERNEL" | grep -v "lts"
The sixth portion filters out the current kernel and the LTS kernel package. Removing those will cause problems.
sed 's/install//g'
The seventh part strips off the install part.
xargs dpkg -P
The last part actually removes the packages. xargs send all the package names to dpkg. Then dpkg -P purges the packages. That means, removing them and removing their configs.