Securing Linux systems and applications; understanding Linux security features.
Linux is a free software operating system kernel similar to Unix unix kernels. It provides standard Unix-style discretionary access control, with the ability to use mandatory access control. The name “Linux” more often than not means a Unix-like operating system based on the Linux kernel: a Linux distribution. Although Android is based on a Linux kernel, the rest of the system is very different.
For functional questions (“How do I use this tool which is not specifically a security tool?”), try asking on our sister site Unix & Linux.