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To open a file with editor you have to simply pass file path including file protocol prefix in fileUrl parameter to editor url.
Like this:
The url to open a drawing depends on which port and path you selected during server install:
e.g. for port 80 and deploy path /
http://localhost/?fileUrl=file:////home/andrea/Documents/My%20Drawings/test.dwg
or for port 8081 and deploy path /editor
http://localhost:8081/editor?fileUrl=file:////home/andrea/Documents/My%20Drawings/test.dwg
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