How to fix E: Not enough free disk space. The upgrade has aborted in APT

Run df -h to see which partitions are full. Pay special attention to /boot, /var, and / (root). You can also use sudo du -h --max-depth=1 /var | sort -h | tail -n 10 to identify the largest directories consuming space.

Remove cached .deb files that are no longer needed:

sudo apt-get clean

For a more aggressive cleanup that also removes partial package files:

sudo apt-get autoclean

This typically frees up several hundred MB to several GB, depending on your system history.

Old kernel versions consume significant space in /boot. Remove them with:

sudo apt-get autoremove

For a more thorough cleanup with headers:

sudo apt-get autoremove --purge

If autoremove doesn't remove enough, you can manually purge specific old kernels:

sudo apt-get purge linux-headers-X.X.X-XX-generic linux-image-X.X.X-XX-generic

Replace X.X.X-XX with the kernel version you want to remove (use uname -r to see your current kernel).

Large log files in /var/log can consume significant disk space. Check which logs are large:

ls -lhS /var/log | head -20

If journalctl is using excessive space, trim it:

sudo journalctl --vacuum-size=64M

You can also rotate or delete individual large log files:

sudo truncate -s 0 /var/log/syslog

Beyond kernels, remove other unused packages:

sudo apt-get autoremove --purge

You can also check for config files from removed packages:

sudo dpkg --list | grep '^rc' | awk '{print $2}' | xargs sudo apt-get remove --purge

Check disk usage again:

df -h

Ensure you have at least 5-10% free space on all partitions, especially /boot (which should have at least 500MB free). For a safe upgrade, aim for at least 20% free space.

Once you've freed up sufficient space, retry the upgrade:

sudo apt-get update
sudo apt-get upgrade

Or for a full distribution upgrade:

sudo do-release-upgrade