ive fully installed unity on my Linux machine and everything basic works (besides that it gives a constant stream of errors that don’t actually mean or do anything) however in order to actualy edit my code for any scripts i have to open them through Files and open the script in a text editor (Which doesn’t have the unity command descriptions, so it wont highlight text or variables and it wont auto complete anything) i checked my preferences and its set to open scripts in the “built in” mono develop however upon clicking scripts they do not open. i hear my hard drive spin up and then it gives up. im using Linux Ubuntu.
For me this worked out:
I installed Unity3D with their (official) installer normally (in my case: http://beta.unity3d.com/download/3c89f8d277f5/public_download.html (https://forum.unity.com/threads/unity-on-linux-release-notes-and-known-issues.350256/))
Then I installed monodevelop:
sudo apt install monodevelop
After that in Unity3D I selceted Edit → Preferences → External Tools → External Scripteditor → Browse → and navigated to /usr/bin/monodevelop.
Done 
Hi. I’m on Linux Beta 2018.1.0b08, I’ve tried setting /usr/bin/monodevelop. Unity opens MonoDevelop OK but not the .cs file I tried to open. Any idea’s on how to fix this?
@rodrcle
I had that issue and trying different things I found a solution:
-on mono
“Edit > Preferences > Projects > Load/Save > Default Solution Location” and set default location to where your unity folder is. The problem is that mono only loads script if its under the default path.
-on unity
as requested above you should check “add .unityproj’s to sln”