Run uname -r and compare its complete output with the kernel release you intended to install. That tells you which kernel is running now; a successfully installed package or a reboot-required notice does not, by itself, confirm that the system has started using the new kernel.
Check the kernel that is running now
- Identify the exact kernel release you intended to install. Use the package manager and package names appropriate for your Linux distribution; Debian/Ubuntu and RHEL-family systems do not use identical commands or naming.
- Open a terminal and run:
uname -r - Compare the output with the intended release. Ubuntu’s kernel documentation says this command prints the loaded kernel version and flavor. Compare the whole string, including any distribution build revision, flavor such as
genericor a cloud flavor, and architecture-specific suffix. Do not assume that the numerically newest-looking part alone is enough to establish a match. - If the output is the release you expected, the machine is running that kernel. If it is an older release, the intended kernel is not currently running. A new kernel can be installed on disk while the older kernel continues running in memory.
More than one kernel can be installed at once, so “the package is installed” and “the machine booted that package” are different states. Ubuntu’s guidance is to reboot after installing a kernel and then use uname -r to verify the loaded version.
What reboot-required notices can—and cannot—tell you
Reboot indicators are useful supporting evidence, but they do not replace checking the running kernel directly. Their behavior and reliability depend on the distribution and package tools.
Ubuntu and Debian-derived systems
Debian Policy defines a convention in which a program can create /run/reboot-required and list affected packages in /run/reboot-required.pkgs. These files indicate that a package signaled a reboot is needed to fully apply changes; they do not prove that a reboot has occurred. Debian Policy also explicitly says the convention does not guarantee when—or whether—the requested reboot will happen. See Debian Policy §9.12.
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On Ubuntu, pro system reboot-required can query whether installed updates signaled a reboot, and /var/run/reboot-required.pkgs often lists packages involved. On typical systems, /var/run refers to the same runtime location as /run. Ubuntu’s cloud-update guidance notes that updated files can be on disk while older code remains in memory; rebooting is the straightforward way to have processes and services use all updated versions, although service restarts can be appropriate in some cases.
An absent marker is not universal proof that no reboot is needed. For this kernel-specific question, compare uname -r with the intended installed release.
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Red Hat lists kernel packages among updates that require a reboot for full use and describes needs-restarting -r as a reboot hint. It is not a direct check of the currently loaded kernel version. In a documented RHEL 9 case, dnf needs-restarting -r can incorrectly report that a reboot is unnecessary after kernel-core is installed or updated without the kernel meta-package. Red Hat describes both the general hint and the exception in its guidance on identifying packages that require a reboot and its RHEL 9 kernel-core false-negative case.
After a kernel update on RHEL, use uname -r to verify the running release even if the reboot hint says no reboot is required.
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If the new kernel is not running
Plan a reboot in line with the machine’s availability requirements. Restarting interrupts workloads, so production servers should follow their maintenance and service-impact procedures rather than rebooting casually. After the reboot, run uname -r again and compare the complete release string.
If the version still does not match, the system may have selected a different installed kernel or the intended package may not be present. Check the installed kernel packages and the bootloader’s selected entry using instructions for your distribution, bootloader, and hardware configuration. There is no single recovery command that applies safely to every Linux system; Ubuntu’s kernel guidance supports rebooting and verifying, not a universal bootloader repair procedure.
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Ubuntu Livepatch is not a substitute for booting a new kernel
Ubuntu Livepatch can apply supported security patches to the kernel version already running in memory, reducing the need for an immediate reboot for those patches. It does not activate a newly installed, on-disk kernel package without rebooting. Continue installing ordinary kernel package updates, then verify the running release with uname -r after the planned reboot. Canonical explains this distinction in its Ubuntu cloud-update guidance.
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