sudo -u debian-tor nyx
That's it! Thanks a lot!
That's not great for security, now nyx can read your tor private keys. (It probably won't, but still…)
If you add your controller to the debian-tor group, and set the appropriate options to make your cookie file group-readable, then nyx can only read those files.
I'm not sure why that isn't working for you.
Does your torrc-defaults or torrc contain: CookieAuthFileGroupReadable 1
If using a control socket: ControlPort unix:/path/to/file GroupWritable ControlPortFileGroupReadable 1
And maybe: DataDirectoryGroupReadable 1
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