I am working on Ubuntu 16.04, I installed modelsim while running I am getting this error.
./vish: error while loading shared libraries: libXext.so.6: cannot open shared object file: No such file or directory
It's possible you are missing this library https://packages.ubuntu.com/search?keywords=libXext
Install withFix with
sudo apt install libxext-dev
sudo apt install libxext-dev
will fix)