Open the current directory (or '/etc/') in the default file explorer in #Linux using the CLI
xdg-open .
xdg-open /etc/

Open a URL in the default browser:
xdg-open 'https://nixcraft.com

Open an image in the default image viewer:
xdg-open ~/Download/foo.png

Open a PDF in the default PDF viewer:
xdg-open /tmp/bar.pdf

Open a MP3 in the default MUSIC player:
xdg-open /tmp/bar.pdf