This is really a note to self because every now and then, the external monitor at my desk, a Dell 27″ monitor, gets confused.

My laptop – a Lenovo T450s running either Fedora 31 or RHEL 8.1 – gets plugged into the dock which is in turn has the monitor plugged into the HDMI slot. Most of the time, the resolution of the monitor is correctly triggered (1920×1080), but every now and then, it fails offering a 720×576 or something like that. I will have to unplug the laptop from the dock and plug it back in (sometimes two or three times), before the full resolution of the monitor is detected.

I’ve decided that instead of that, I should just script this out via a one-liner using the xrandr command and here it is:

xrandr --output DP-2-1 --mode 1920x1200

What are the options above?

Running xrandr on its own, gives this (for my system):

$ xrandr
Screen 0: minimum 320 x 200, current 3840 x 1080, maximum 16384 x 16384
eDP-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 309mm x 174mm
1920x1080 60.02*+ 60.01 59.97 59.96 59.93 
1680x1050 59.95 59.88 
1400x1050 59.98 
1600x900 59.99 59.94 59.95 59.82 
1280x1024 60.02 
1400x900 59.96 59.88 
1280x960 60.00 
1440x810 60.00 59.97 
1368x768 59.88 59.85 
1280x800 59.99 59.97 59.81 59.91 
1280x720 60.00 59.99 59.86 59.74 
1024x768 60.04 60.00 
960x720 60.00 
928x696 60.05 
896x672 60.01 
1024x576 59.95 59.96 59.90 59.82 
960x600 59.93 60.00 
960x540 59.96 59.99 59.63 59.82 
800x600 60.00 60.32 56.25 
840x525 60.01 59.88 
864x486 59.92 59.57 
700x525 59.98 
800x450 59.95 59.82 
640x512 60.02 
700x450 59.96 59.88 
640x480 60.00 59.94 
720x405 59.51 58.99 
684x384 59.88 59.85 
640x400 59.88 59.98 
640x360 59.86 59.83 59.84 59.32 
512x384 60.00 
512x288 60.00 59.92 
480x270 59.63 59.82 
400x300 60.32 56.34 
432x243 59.92 59.57 
320x240 60.05 
360x202 59.51 59.13 
320x180 59.84 59.32 
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 disconnected (normal left inverted right x axis y axis)
DP-2-1 connected 1920x1080+1920+0 (normal left inverted right x axis y axis) 597mm x 336mm
1920x1080 60.00 + 60.00 50.00 59.94 24.00 23.98* 
1600x1200 60.00 
1680x1050 59.88 
1280x1024 75.02 60.02 
1280x800 59.91 
1152x864 75.00 
1280x720 60.00 50.00 59.94 
1024x768 75.03 60.00 
800x600 75.00 60.32 
720x576 50.00 
720x480 60.00 59.94 
640x480 75.00 60.00 59.94 
720x400 70.08 
DP-2-2 disconnected (normal left inverted right x axis y axis)
DP-2-3 disconnected (normal left inverted right x axis y ax

eDP is the laptop’s own screen, DP-1 is the display port on the laptop itself and DP-2 is the display port on the dock. The dock has 3 display options (VGA, DP and HDMI) with the HDMI port connected to the monitor. I guess that is what is labelled as DP-2-1.

So, the xrandr options I am using; –output DP-2-1 and –mode 1600×1200 is based on the information made available from the capabilities listed above. xrandr has lots of useful options and please do explore them for your needs.

[Added on 5 December 2019]

https://www.commandlinefu.com/commands/matching/xrandr/eHJhbmRy