First of all download Google App Engine SDK for Linux.
Extract it anywhere within your local folder. It can be anywhere such as in your home directory or Documents directory.
Open terminal (ctrl+alt+t) and navigate to your home directory and open .bashrc to edit with this command:
$ sudo gedit .bashrc
At the end of the file, add these lines by changing the directory to your google_appengine folder. The first line is just a comment and in the second line /usr/local/google_appengine is the path to the google_appengine folder.
### Added for GAE export PATH="/usr/local/google_appengine:$PATH"
Now restart your computer and then you should be able to use $ dev_appserver.py command from anywhere.