The correct format for the DISPLAY environment variable is hostname:displaynumber.screennumber, where hostname is the name of the computer where the X server runs, displaynumber is a sequence number (usually
0) that identifies a display, and screennumber is the number of the screen within that display (usually 0). The screennumber can be omitted if it is 0. For example, localhost:0 or myhost:1.0 are valid values for the DISPLAY variable. The other options are either missing the colon, using the wrong separator, or not specifying the display number. References:
* X11 - DISPLAY (environment variable) - Datacadamia
* x11 - How can I specify a display? - Stack Overflow
* What is the $DISPLAY environment variable? - Ask Ubuntu