Error: Couldn't open display (null) and solution

If you are seeing the error "Error: Couldn't open display (null)" when trying to run a graphical program on a Linux or Unix system, it means that the system is unable to find an X server (the software that provides a graphical interface) to display the program's output.

This can be caused by a variety of factors, such as:

• The DISPLAY environment variable is not set: The DISPLAY variable specifies the location of the X server, and it must be set in order for graphical programs to run. You can check the value of the DISPLAY variable by running the following command:

echo $DISPLAY

If the variable is not set, you can set it by running the following command:

export DISPLAY=:0

This will set the DISPLAY variable to the default value (display number 0).

• The X server is not running: The X server must be running in order for graphical programs to work. You can check if the X server is running by looking for a process named Xorg or X. If it is not running, you can start it by running the following command:

sudo service startx start

• There is a network issue: If you are trying to run a graphical program remotely over a network.