freeing up space on ubuntu

This post has lots of useful information in the answers. I removed libre-office and old kernels to get 1.5 GB. For the removing the old kernels, I followed
1. uname -r to know current kernel
2. sudo apt-get remove kernelname1 kernelname2 etc after doing
sudo apt-get clean
sudo apt-get autoclean
sudo apt-get autoremove
sudo apt-get install localpurge
and
rm -Rf /tmp
etc.

The listing of all packages with sizes with
dpkg-query -W --showformat='${Installed-Size} ${Package}\n' | sort -nr | less
is also a nice trick to use.