How can I detect when a monitor is plugged in or unplugged?

NOTE: This was tested on a laptop with a i915 driven graphics card.

---

Background

NOTE: When a new screen is plugged in, no event is sent to the host, this stayed true even after my last edit. So the only way is to use polling. Trying to make them as effiicient as possible...

EDIT #3

Finally there's one better solution (through ACPI):

There's still no event, but ACPI seems more efficient than xrandr to inquire. (Nota: This requires ACPI kernel modules loaded, but doesn't require root privileges).

My final solution (using bash):

isVgaConnected() {
    local crtState
    read -a < /proc/acpi/video/VID/CRT0/state crtState
    test $(( ( ${crtState[1]} >>4 ) ${1:+*-1+1} )) -ne 0
}

Now a test:

$ if isVgaConnected; then echo yes; else echo no; fi 
yes

It's plugged in, so now I unplug it:

$ if isVgaConnected; then echo yes; else echo no; fi 
no

NOTE: ${1:+*-1+1} permit a boolean argument: If something is present, answer would be inverted: ( crtState >> 4 ) * -1 + 1.

and the final script:

#!/bin/bash

export crtProcEntry=/proc/acpi/video/VID/CRT0/state

isVgaConnected() {
    local crtState
    read -a < $crtProcEntry crtState
    test $(( ( ${crtState[1]} >>4 ) ${1:+*-1+1} )) -ne 0
}

delay=.1
unset switch
isVgaConnected || switch=not
while :;do
    while isVgaConnected $switch;do
        sleep $delay
      done
    if [ "$switch" ];then
        unset switch
        echo VGA IS connected
        # doing something while VGA is connected
      else
        switch=not
        echo VGA is NOT connected.
        # doing something else, maybe.
      fi
  done

WARNINGS: lighter than xrandr, but not unimportant with a delay smaller than 0.02 seconds, the Bash script will go to the top of resource eaters process (top)!

While this costs ~0.001 sec:

$ time read -a </proc/stat crtStat

This requires ~0.030 sec:

$ read -a < /proc/acpi/video/VID/CRT0/state crtState

This is big! So depending on what you need, delay could be reasonably set between 0.5 and 2.

EDIT #2

I've finally found something, using this:

Important disclaimer: Playing with /proc and /sys entries could break your system!!! So don't try the following on production systems.

mapfile watchFileList < <(
    find /sys /proc -type f 2>/dev/null |
    grep -i acpi\\\|i91 
)

prompt=("/" "|" '\' '-');

l=0
while :; do
  mapfile watchStat < <(
    grep -H . ${watchFileList[@]} 2>/dev/null
  )

  for ((i=0;i<=${#watchStat[@]};i++)); do
    [ "${watchStat[i]}" == "${oldStat[i]}" ] || echo ${watchStat[i]}
  done

  oldStat=("${watchStat[@]}")
  sleep .5
  printf "\r%s\r" ${prompt[l++]}
  [ $l -eq 4 ]&&l=0
done

... after some cleaning of unwanted entrys:

for ((i=0;i<=${#watchFileList[@]};i++)); do
  [[ "${watchFileList[$i]}" =~ /sys/firmware/acpi/interrupts/sci ]] &&
      unset watchFileList[$i] && echo $i
done

I've been able to read this:

/proc/acpi/video/VID/CRT0/state:state: 0x1d
/proc/acpi/video/VID/CRT0/state:state: 0x0d
/proc/acpi/video/VID/CRT0/state:state: 0x1d

When I plug, unplug, and replug in monitor cable.

Original Answer

When the config is inquired (running system/preferences/monitor or xrandr), graphics cards do a type of scan, so running xrandr -q give you the info, but you have to poll the status.

I've scanned all logs, (kernel, daemon, X and so forth) searching through /proc & /sys, and clearly nothing seems to exist that satisfies your request.

I've tried this too:

export spc50="$(printf "%50s" "")"
watch -n1  '
    find /proc/acpi/video -type f |
        xargs grep -H . |
        sed "s/^\([^:]*):/\1'$spc50'}:/;
             s/^\(.\{50\}\) *:/\1 /"'

After all that, if you run System/Preferences/Monitor while no new screen has just been plugged in, nor unplugged, the tool will appear simply (normally). But if you've plugged or unplugged a screen before, at times you'll run this tool and you'll see your desktop make a type of reset or refresh (same if you run xrandr).

This seems to confirm that this tool asks for xrandr (or works in the same manner) by polling status periodically, starting at the time it's run.

You could try yourself:

$ for ((i=10;i--;)); do xrandr -q | grep ' connected' | wc -l; sleep 1; done
1
1
1
2
2
2
1
1
1
1

This will display how many screens (displays) are connected, for 10 seconds.

While this runs, plug and/or unplug your screen/monitor and look what's happens. So you could create a little Bash test function:

isVgaConnected() {
    local xRandr=$(xrandr -q)
    [ "$xRandr" == "${xRandr#*VGA1 con}" ] || return 0
    return 1
}

which would be useable as in:

$ if isVgaConnected; then echo yes; fi

But be careful, xrandr takes about 0.140 sec to 0.200 sec while no change happens on plugs and up to 0.700 seconds whenever something was plugged or unplugged just before (NOTE: It seems not to be a resource eater).

EDIT #1

To ensure I'm not teaching something incorrect, I've searched around the Web and docs, but didn't find anything about DBus and Screens.

Finally, I've run in two different windows dbus-monitor --system (I've been playing with options too) and the little script I wrote:

$ for ((i=1000;i--;)); do isVgaConnected && echo yes || echo no; sleep .5; done

... and again plugged, than unplugged the monitor, many times. So now I could say:

• In this configuration, using i915 driver, there is no other way than running xrandr -q to know if a monitor is plugged in or not.

But use caution, because there doesn't appear to be other ways. For instance, xrandr seems to share this info, so my GNOME desktop would switch to xinerama automatically... when I ran xrandr.

Some docs

• Script to toggle internal/external display for laptops
• ®Intel HD Graphics OpenSource Programmer's Reference Manual

The following lines appeared in udevadm monitor

KERNEL[46578.184280] change   /devices/pci0000:00/0000:00:02.0/drm/card0 (drm)
UDEV  [46578.195887] change   /devices/pci0000:00/0000:00:02.0/drm/card0 (drm)

when attaching a monitor to the VGA-Connector. So there might be a way to figure this out.

For those who, for whatever reason, don't want to take the hotplug route, it is still possible to not poll within a script using inotifywait:

#!/bin/bash

SCREEN_LEFT=DP2
SCREEN_RIGHT=eDP1
START_DELAY=5

renice +19 $$ >/dev/null

sleep $START_DELAY

OLD_DUAL="dummy"

while [ 1 ]; do
    DUAL=$(cat /sys/class/drm/card0-DP-2/status)

    if [ "$OLD_DUAL" != "$DUAL" ]; then
        if [ "$DUAL" == "connected" ]; then
            echo 'Dual monitor setup'
            xrandr --output $SCREEN_LEFT --auto --rotate normal --pos 0x0 --output $SCREEN_RIGHT --auto --rotate normal --below $SCREEN_LEFT
        else
            echo 'Single monitor setup'
            xrandr --auto
        fi

        OLD_DUAL="$DUAL"
    fi

    inotifywait -q -e close /sys/class/drm/card0-DP-2/status >/dev/null
done

It is best invoked from your .xsessionrc, not forgetting the ending &. Polling with xrandr gave serious usability issues on my brand new laptop (mouse would stall periodically).