ezyang’s blog

This post describes how to change which monitor xmobar shows up on in a multi-monitor setup. This had always been an annoyance for me, since on an initial switch to multi-monitor, xmobar would be living on the correct monitor, but if I ever restarted XMonad thereafter, it would migrate to my other monitor, much to my annoyance. Note that a monitor is different from an X screen, which can be directly configured from xmobar using the -x command line.

How does xmobar pick what screen to use? It selects the “primary” monitor, which by default is the first entry in your xrandr list:

Screen 0: minimum 320 x 200, current 2464 x 900, maximum 8192 x 8192
VGA1 connected 1440x900+1024+0 (normal left inverted right x axis y axis) 408mm x 255mm
   1440x900       59.9*+   75.0
   1280x1024      75.0     60.0
   1280x960       60.0
   1152x864       75.0
   1024x768       75.1     70.1     66.0     60.0
   832x624        74.6
   800x600        72.2     75.0     60.3     56.2
   640x480        72.8     75.0     66.7     66.0     60.0
   720x400        70.1
LVDS1 connected 1024x768+0+0 (normal left inverted right x axis y axis) 245mm x 184mm
   1024x768       60.0*+
   800x600        60.3     56.2
   640x480        59.9

We can switch the primary monitor using the xrandr --output $MONITOR --primary command. However, this change is not persistent; you’d have to run this command every time you add a new monitor.

Fortunately, it turns out gnome-settings-daemon records information about monitors it has seen in order to configure them properly. This information is in .config/monitors.xml:

<monitors version="1">
  <configuration>
      <clone>no</clone>
      <output name="VGA1">
          <vendor>HSD</vendor>
          <product>0x8991</product>
          <serial>0x01010101</serial>
          <width>1440</width>
          <height>900</height>
          <rate>60</rate>
          <x>0</x>
          <y>0</y>
          <rotation>normal</rotation>
          <reflect_x>no</reflect_x>
          <reflect_y>no</reflect_y>
          <primary>no</primary>
      </output>
      <output name="LVDS1">
          <vendor>LEN</vendor>
          <product>0x4002</product>
          <serial>0x00000000</serial>
          <width>1024</width>
          <height>768</height>
          <rate>60</rate>
          <x>1440</x>
          <y>0</y>
          <rotation>normal</rotation>
          <reflect_x>no</reflect_x>
          <reflect_y>no</reflect_y>
          <primary>no</primary>
      </output>
  </configuration>
</monitors>

So all we need to do is tweak primary to be yes on the appropriate monitor.

Hat tip to David Benjamin and Evan Broder for letting me know how to do this.