If you are on a Linux shell and want to open a file with whatever default application is configured for that type, this often works:

xdg-open Quote.odt
xdg-open invoice.pdf
xdg-open index.html

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Make an alias so you have a simpler API (like Mac OS): alias open=xdg-open or alias x=xdg-open.

Background

You can choose your "Default applications" via UI in the Ubuntu "Settings" application (gnome-control-center). This is just a very rough setting (e.g. open Photos with Shotwell Viewer).

If a certain file type (or better mime type) should be opened with a different application, this can be set via command line:

# Query the default application:
xdg-mime query default $(xdg-mime query filetype <file>)

# Override the default application for this filetype with:
mimeopen -Dd <file>

Changed mime default are stored in ~/.config/mimeapps.list (and sometimes ~/.config/*-mimeapps.list).

Further reading

• Stop sipgate softphone from opening html files on click