Sometime's you'll notice multiple old kernels taking up disk space on the server, more specifically when you've a seperate /boot partition which is standard practise.
 
 You can remove old kernels using the following commands from the bash shell.
 
 Debian / Ubuntu
 
 # sudo apt-get autoremove
 
 This command will automatically remove any packages, installed by other packages, that are no longer required, this includes kernel modules.  
 
 Firstly, you can check your running kernel by using the following:
 # sudo uname -r
 
 3.16.0-4-amd64 You can also manually remove old kernels, first, you can list the installed kernels with the following: # sudo dpkg --list | grep linux-image
 
 ii linux-image-3.16.0-4-amd64 3.16.7-ckt20-1+deb8u3 amd64 Linux 3.16 for 64-bit PCs
 ii linux-image-amd64 3.16+63 amd64 Linux for 64-bit PCs (meta-package)
 
 Before using apt-get to remove any unrequired kernels on the system.  
 
 # sudo apt-get --remove linux-image-3.16.0-4-amd64

CentOS / RedHat / Fedora
 
 Firstly, you can check your running kernel by using the following:
 
 # sudo uname -r
 
 2.6.18-406.el5
 
 You will need to remove each kernel invidivually using "yum".
 
 The installed kernels can be listed using the following:
 
 # sudo rpm -qa kernel
 
 kernel-2.6.18-404.el5
 kernel-2.6.18-407.el5
 kernel-2.6.18-402.el5
 kernel-2.6.18-408.el5
 kernel-2.6.18-406.el5
 
 Yum can then be used to remove any old kernels. 
 
 # sudo yum remove kernel-2.6.18-404.el5