How can I get the monitor resolution using the command line?

Taken from this answer:

xdpyinfo | grep dimensions

Or to get just the resolution:

xdpyinfo | awk '/dimensions/{print $2}'

OR

xdpyinfo  | grep -oP 'dimensions:\s+\K\S+'

I would just use xrandr:

$ xrandr 
Screen 0: minimum 320 x 200, current 3520 x 1200, maximum 32767 x 32767
LVDS1 connected 1600x900+1920+0 (normal left inverted right x axis y axis) 310mm x 174mm
   1600x900       60.0*+
   1440x900       59.9  
   1360x768       59.8     60.0  
   1152x864       60.0  
   1024x768       60.0  
   800x600        60.3     56.2  
   640x480        59.9  
VGA1 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
DP1 connected primary 1920x1200+0+0 (normal left inverted right x axis y axis) 518mm x 324mm
   1920x1200      60.0*+
   1920x1080      60.0     50.0     59.9     24.0     24.0  
   1920x1080i     60.1     50.0     60.0  
   1600x1200      60.0  
   1280x1024      75.0     60.0  
   1152x864       75.0  
   1280x720       60.0     50.0     59.9  
   1024x768       75.1     60.0  
   800x600        75.0     60.3  
   720x576        50.0  
   720x480        60.0     59.9  
   640x480        75.0     60.0     59.9  
   720x400        70.1  
HDMI2 disconnected (normal left inverted right x axis y axis)
HDMI3 disconnected (normal left inverted right x axis y axis)
DP2 disconnected (normal left inverted right x axis y axis)
DP3 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)

Here I have two screens, the resolution are:

• 1600x900 (laptop)
• 1920x1200 (monitor)

To get only the resolution of your primary monitor, you can also use this python oneliner:

$ python3 -c 'from gi.repository import Gdk; screen=Gdk.Screen.get_default(); \
geo = screen.get_monitor_geometry(screen.get_primary_monitor()); \
print(geo.width, "x", geo.height)'
1920 x 1200

To get the resolution of your extanded desktop (for a multi monitor setup):

$ python3 -c 'from gi.repository import Gdk; screen=Gdk.Screen.get_default(); \
print(screen.get_width(), "x", screen.get_height())'
3520 x 1200

The request was for the resolution. That is given by

xdpyinfo | grep resolution