All is flux; nothing stays still. This adage also applies to long-term Linux versions such as Ubuntu LTS and CentOS. The most important reasons for regular updates are security and troubleshooting. And the kernel develops, too. Recent patches for Spectre and Meltdown are good examples of important kernel changes that had to arrive through system updates.

But what happens to the old kernel after an update? First of all, the kernel is a normal software package. It is thus managed through the package management system for your Linux variant. In the case of a Debian or Ubuntu system, this means Apt; in the case of Red Hat-based systems, RPM and its friends Yum and DNF.

If you take a closer look, you'll see that there is no update for the kernel package: The package manager simply adds the new version to the system without deleting the previous version. Often several old versions will remain on the disk (see the box "Turning Back the Clock"), which requires a large amount of disk space.

This article shows how to remove old kernel images from your system. The reference systems are Debian 9, Fedora 28, Ubuntu LTS 16.04, and CentOS 7.5. By default, these distributions limit the number of kernel versions installed on the system – at least if they have be installed via a package update. For manually installed versions, or for additional kernel-related packages, the techniques described in this article might not apply (see the box entitled "Kernel-Related Packages").

For the purposes of this article, the term "kernel package" refers to all packages belonging to the kernel, including kernel-related packages. As Figure 1 shows, these packages occupy a significant amount of disk space.

Figure 1: The kernel headers also need space (Debian).

Manual Removal

Handling the software installed on the system is the package manager's task. The basic procedure is similar for all distributions: First, determine the version of the running and working kernel. You need to be sure this current version stays on the system. Then display all kernel packages. The last step is to delete the packages that are no longer required. (Be sure you observe the instructions in the "Caution" box.)

Determining the Current Version

The GRUB bootloader usually displays a selection of installed kernel versions at boot time. For Red Hat-based versions, this output is direct; for Debian the list appears in the Advanced Options area (Figure 2); for Ubuntu, you need to press the Shift key to see the list.

Figure 2: Debian provides a complete list of installed kernel versions in the Advanced Options area.

The uname -r command, which is available with all distributions, displays the version of the running kernel. The output for a Debian system is as follows:

$ uname -r
4.9.0-6-rt-amd64

Package Service

Distributions differ in the name and contents of kernel packages. Table 1 gives an overview of the commands you might need to determine the package names, organized by distribution. The commands listed in the table provide the complete names. The names of the packages alone indicate whether they are kernel packages or components.