htop software package provides interactive processes viewer, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install htop
htop is installed in your system.
Make ensure the htop package were installed using the commands given below,
$ sudo dpkg-query -l | grep htop *
You will get with htop package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Utilities |
| Package | htop |
| Version | 2.0.2-1 |
| Description | interactive processes viewer |
| Sub Section | Utils |