How to Safely Remove Old Kernels in Ubuntu When the Boot Partition is Full

During routine system maintenance or automated security patching on Ubuntu, package updates can suddenly fail with critical terminal errors: gzip: stdout: No space left on device, E: Write error - write (28: No space left on device), or update-initramfs: failed for /boot/initrd.img. This happens when Ubuntu accumulates redundant, historical Linux kernel images (each comprising vmlinuz, initrd.img, and System.map files) that completely exhaust the storage allocated to the dedicated /boot partition.

When /boot reaches 100% utilisation, standard package managers may refuse to install or remove software. Follow these systematic steps in Ubuntu to purge outdated kernels safely without compromising your active running kernel.

Step 1: Checking Your Active Running Kernel

Before deleting any kernel packages, identify the kernel currently powering your session. Never delete the running kernel:

uname -r

Note down the output version string (for example, 6.8.0-45-generic).

Step 2: Automated Cleanup Using apt autoremove

If the package manager is not completely blocked, Ubuntu’s built-in autoremove command can prune superseded kernels automatically while retaining the active kernel and the immediate previous fallback:

sudo apt autoremove --purge

If this command executes successfully and frees space in /boot, run df -h /boot to confirm disk recovery.

Step 3: Manually Removing Old Kernels When apt is Blocked

If /boot is completely full, apt commands may abort with unmet dependency errors. In this scenario, identify and purge packages directly using low-level dpkg:

1. List all installed kernel image packages:

   dpkg --list | grep -E "linux-image-[0-9]"

2. Identify older version numbers that do not match the output from uname -r.
3. Purge a specific obsolete kernel package using dpkg:

   sudo dpkg --purge linux-image-6.8.0-38-generic linux-modules-6.8.0-38-generic

4. Repeat for any other obsolete kernel versions until df -h /boot indicates at least 30% free space.

Step 4: Repairing Package Dependencies and Updating GRUB

Once sufficient disk space is reclaimed in /boot, finish the interrupted package configuration and update the GRUB bootloader menu:

sudo apt-get install -f
sudo update-grub

Your Ubuntu system will now install future security kernels smoothly without risk of bootloader partition exhaustion.