Using xrandr is really straight-forward when trying to show a screen externally as a mirror.
To show the list of outputs available just using this command is usually enough. Here is an example of output with a Thinkpad connected:
$ xrandr | grep connected
eDP-1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 276mm x 155mm
DP-1 disconnected (normal left inverted right x axis y axis)
HDMI-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
HDMI-2 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 531mm x 298mm
An important thing is HDMI-2, that indicates the port connected to an external source. Using a mirror screen can then be done like that:
xrandr --output HDMI-2 --mode 1920x1080
Of course the output depends on the size of the monitor used.
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