Deleting the old kernels lost on your hard drive

Spring Cleaning

© Lead Image © Amy Walters, 123RF.com

© Lead Image © Amy Walters, 123RF.com

Article from Issue 218/2019
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When you update the kernel, the old version remains on the disk. If you clean up, the reward is several hundred megabytes of free disk space.

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.

Turning Back the Clock

The Linux kernel is responsible for many tasks, including managing hardware resources, enabling access to individual system components, and providing interfaces for system communication. Various hardware embodiments can sometimes impair this harmonious relationship. In the worst case, the system will not boot after a kernel update. If there are problems after the update, the old kernel might help, at least until the next kernel update. For this reason, most distributions keep one, but more likely several, of the previous kernel packages on disk.

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").

Kernel-Related Packages

Kernel-related packages are packages with files that are directly related to the version of the kernel. These files include, for example, the kernel modules, the RAM disks used at boot time, the kernel headers, and sometimes a few add-ons that depend directly on the kernel version. Which of these components are in which packages is determined by the distribution.

The software manager remembers which packages are installed. For example, if you have installed the kernel headers, the new package with kernel headers will be installed on your disk each time the kernel is updated. The system keeps the previous version along with the actual kernel.

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.)

Caution

Problems and data loss can occur when using the package management system. Therefore, make a backup before deleting old kernel versions.

Some distributions even let you delete the current kernel version. Please pay attention to the screen output. Do not delete the last working kernel. Uninstall older kernel versions only if the system is working perfectly.

Use the abstracted commands of the package manager, for example the yum command instead of rpm for CentOS. The abstracted commands display an overview before the actions are executed and let you cancel.

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.

Table 1

Determining Package Names

Distribution

Command

Examples of Package Names

CentOS

rpm -qa "kernel*"

kernel-3.10.0-862.9.1, kernel-headers-3.10.0-862.9.1, kernel-devel-3.10.0-862.9.1

Fedora

rpm -qa "kernel*"

kernel-4.16.3-301, kernel-modules-4.16.3-301, kernel-devel-4.16.3-301

Debian

apt list --installed linux-*

linux-image-4.9.0-6-amd64, linux-headers-4.9.0-4-amd64

Ubuntu LTS

apt list --installed linux-*

linux-image-4.15.0-30-generic, linux-headers-4.15.0-30-generic, linux-image-extra-4.15.0-15-generic

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