macOS and the X Window System
If you want to use the X Window System on macOS, you need XQuartz.
Install XQuartz
brew cask install xquartz
After a Reboot you will get the $DISPLAY set correctly. If you want to set it without a reboot see Start XQuartz temprorary
Test with xeyes
/opt/X11/bin/xeyes
SSH X11Forwarding
You can now use ssh -X or ssh -Y
Uninstall
brew cask uninstall xquartz
Start XQuartz temprorary
This is only necessary if you want to start XQuartz without a reboot.
launchctl load /Library/LaunchAgents/org.macosforge.xquartz.startx.plist
launchctl start org.macosforge.xquartz.startx
Set DISPLAY
find the socket
Note the time when you started xquartz
ls -l /private/tmp
total 0
drwx------ 3 harry wheel 96 16 Jan 15:53 com.apple.launchd.Kxdk75I3PA
...
➜ ~ ls -l /private/tmp/com.apple.launchd.Kxdk75I3PA/org.macosforge.xquartz:0
srw-rw-rw- 1 harry wheel 0 16 Jan 15:53 /private/tmp/com.apple.launchd.Kxdk75I3PA/org.macosforge.xquartz:0
Set $DISPLAY
export DISPLAY=/private/tmp/com.apple.launchd.Kxdk75I3PA/org.macosforge.xquartz:0
echo $DISPLAY
/private/tmp/com.apple.launchd.Kxdk75I3PA/org.macosforge.xquartz:0
Test
/opt/X11/bin/xeyes