The ACPID way

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Overview

There are numerous ways of fixing this issue. If you look through the web you will find suggestions on installing power managers used in other desktop environments (DEs) like xfce4-power-manager or gnome-settings-daemon. However, as an LXDE user you are probably striving for a lightweight solution. One lightweight way to display a logout menu on power button press is to utilize an ACPID script.

What is ACPID?

From the ArchWiki:

acpid is a flexible and extensible daemon for delivering ACPI events.
It listens on /proc/acpi/event and when an event occurs, executes
programs to handle the event. These events are triggered by certain
actions, such as:

• Pressing special keys, including the Power/Sleep/Suspend button




• Closing a notebook lid (Un)Plugging an AC power adapter from a notebook




• (Un)Plugging phone jack etc.

Installation

sudo apt-get install acpi acpid

Configuration

Open /etc/acpi/powerbtn.sh in a text editor of your choice, e.g. leafpad:

gksudo leafpad /etc/acpi/powerbtn.sh

At the end of the file you will find the following passage:

# If all else failed, just initiate a plain shutdown.

/sbin/shutdown -h now "Power button pressed"

Just before this passage add the following snippet:

# If LXDE is running execute lxsession-logout

if [ -n $(pidof lxsession) ]; then

    for x in /tmp/.X11-unix/*; do

       displaynum=`echo $x | sed s#/tmp/.X11-unix/X##`

       getXuser;

       if [ x"$XAUTHORITY" != x"" ]; then

           export DISPLAY=":$displaynum"

           export _LXSESSION_PID=`pidof lxsession`

           lxsession-logout

           exit

       fi

    done

fi

Most of the snippet consists of functions and commands that make sure that a graphical application like the logout menu can be run by ACPID (setting environment variables like XUSER and DISPLAY).

Note: If you have GNOME components running in your LXDE session (e.g. nautilus, various daemons) the session detection might go wrong. In these cases I would advise you to insert the code in the following way:

  if pidof x $PMS > /dev/null; then

     if [ -n $(pidof lxsession) ]; then

        for x in /tmp/.X11-unix/*; do

           displaynum=`echo $x | sed s#/tmp/.X11-unix/X##`

           getXuser;

           if [ x"$XAUTHORITY" != x"" ]; then

           export DISPLAY=":$displaynum"

           export _LXSESSION_PID=`pidof lxsession`

           killall lxsession-logout

           lxsession-logout

           fi

        done

        echo "if correct"

        exit

             else

                 exit

     fi

<...>

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Troubleshooting

For some unknown reason these functions don't always work. In that case you will have to replace the snippet above with the following one:

if [ -n $(pidof lxsession) ]; then

   export DISPLAY=:0

   export _LXSESSION_PID=$(pidof lxsession)

   export XUSER=user

   export XAUTHORITY=/home/user/.Xauthority

   lxsession-logout

   exit

fi

The environment variables are hard-coded in this one. Make sure to substitute user for your own user name. It should work for most single-user system configurations.

Here's another workaround, just in case none of the variants above work. Replace the getXuser function with the following passage:

getXuser() {

        user=`pinky -fw | awk '{ if ($2 == ":'$displaynum'" || $(NF) == ":'$displaynum'" ) { print $1; exit; } }'`

        if [ x"$user" = x"" ]; then

                startx=`pgrep -n startx`

                if [ x"$startx" != x"" ]; then

                        user=`ps -o user --no-headers $startx`

                fi

        fi

        if [ x"$user" = x"" ]; then                           # lines added

               user=$(pinky -fw | awk '{ print $1; exit; }')  # lines added

        fi                                                    # lines added

        if [ x"$user" != x"" ]; then

                userhome=`getent passwd $user | cut -d: -f6`

                export XAUTHORITY=$userhome/.Xauthority

        else

                export XAUTHORITY=""

        fi

        export XUSER=$user

}

What does it do?

The powerbtn.sh script is triggered by ACPID each time the power button is pressed. As you can see it already includes a number of checks for common DEs like Unity, GNOME and KDE. These checks are there to make sure that the shutdown event is handled by the respective power manager. Alas, LXDE doesn't have one of those built in.

That's why we added a snippet of our own that does the checking for ACPID. If it detects an LXDE session it makes sure that your PC isn't just shut down right away and instead launches the logout menu you should be familiar with.

With these changes you should now be able to launch your logout menu with your hardware power button - without needing to install a bulky power manager. And even better, this does not interfere with other DEs in any way.

Sources

http://www.ollie-reardon.co.uk/lxde-tip-power-button-not-working/

https://wiki.archlinux.org/index.php/Acpid

http://forum.lxde.org/viewtopic.php?f=8&t=205

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Further reading

Lubuntu - power button doesn't turn off computer,

Laptop power button not working

https://bugs.launchpad.net/xfce4-power-manager/+bug/1008650

https://unix.stackexchange.com/questions/87259/why-wont-this-shutdown-script-work-when-executed-by-acpid