Termux

Termux is an Android terminal emulator and Linux environment app that works directly with no rooting or setup required. A minimal base system is installed automatically - additional packages are available using the APT package manager. The command to install package is the same as Ubuntu. If you are familiar with Ubuntu or Debian you easily navigate and install Linux package on your phone. I have seen some people who used this app for pentesting and security purposes such as installing the Metasploit Framework. 

Click here to visit the official website.