This one-liner removes all not running installed kernels from the system.
After couple U/K/Ebuntu updates it saves 1GB 
apt-get remove –purge -y $(dpkg -l | grep -E “(linux-image|linux-headers)” | grep ^ii |grep -v `uname -r|cut -f1-2 -d-|tr – .` | cut -f3 -d’ ‘ )
