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I use curl and the Roku external controller API to lauch Spotify. It looks like I should be able to specify additional paramaters when launching the channel to include a playlist.
The following works to launch the app and start playing a playlist.
% curl -X POST -A '' -I http://192.168.0.192:8060/launch/19977
# Then I wait about 5 seconds before pressing the down and play buttons.
% curl -X POST -A '' -I http://192.168.0.192:8060/keypress/Down
% curl -X POST -A '' -I http://192.168.0.192:8060/keypress/Play
According to the Roku docs (http://sdkdocs.roku.com/plugins/viewsource/viewpagesrc.action?pageId=3113168) I should be able to specify additional options. I think these would be specific for the spotify app.
I am hoping that something like the command below would launch the app and start playlist 1:
% curl -X POST -A '' -I http://192.168.0.192:8060/launch/19977?content_ID=playlist1
Piling on. More options when launching Spotify on Roku via curl will help Spotify on Roku become an effective option for home automation projects.