fonts-powerline software package provides powerline symbols font, 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 fonts-powerline
fonts-powerline is installed in your system.
Make ensure the fonts-powerline package were installed using the commands given below,
$ sudo dpkg-query -l | grep fonts-powerline *
You will get with fonts-powerline package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Fonts |
| Package | fonts-powerline |
| Version | 2.5-1 |
| Description | powerline symbols font |
| Sub Section | Fonts |