List or Check Installed Linux Kernels

The command varies from one Linux distribution to another. In other words, you need to type the command as per your Linux distro.

RedHat / CentOS / RHEL / Fedora Linux user

You need to use standard rpm command or yum command to list installed software. Type the following command at shell prompt:
$ rpm -qa kernel
Sample Outputs:

kernel-2.6.18-4.1.15.el5
kernel-2.6.18-8.1.14.el5
kernel-2.6.18-8.1.10.el5

Here is another outputs from RHEL 8 server:

kernel-4.18.0-80.7.2.el8_0.x86_64
kernel-4.18.0-80.4.2.el8_0.x86_64

One can run the yum command/dnf command as follows:
yum list installed kernel
OR
dnf list installed kernel

To list / display current running kernel version

Type the following uname command:
$ uname -r
$ uname -mrs
Sample outputs:

Linux 2.6.18-8.1.14.el5 x86_64

Another outputs from my Ubuntu Linux 18.04 LTS desktop

uname -r
4.18.0-25-generic

Find installed kernel version for Debian / Ubuntu Linux

Use the dpkg command along with the grep command to list all installed kernel on your Debian or Ubuntu Linux, enter:
$ dpkg --list | grep linux-image
Sample outputs:

ii  linux-image-2.6.20-15-generic        2.6.20-15.27                           Linux kernel image for version 2.6.20 on x86/
ii  linux-image-2.6.20-16-generic        2.6.20-16.32                           Linux kernel image for version 2.6.20 on x86/
ii  linux-image-generic                  2.6.20.16.28.1                         Generic Linux kernel image

Let us see outputs from my Ubuntu Linux 18.04 LTS desktop:

Arch Linux user

pacman -Q | grep linux

SUSE Enterprise Linux or openSUSE Linux user

rpm -qa | grep -i kernel

How do I find manually compiled and installed kernels that aren’t in the package manager?

Try to locate them in /lib/modules/ directory using the ls command:
ls -l /lib/modules/

total 12
drwxr-xr-x 5 root root 4096 Jul 29 17:53 4.15.0-55-generic
drwxr-xr-x 5 root root 4096 Jul 29 22:05 4.18.0-25-generic
drwxr-xr-x 5 root root 4096 Aug  1 17:19 5.0.0-23-generic

Conclusion

You learned how to list installed Kernels version using a bash shell prompt. See how to compile Linux kernel for more information or visit official Linux kernel website here.