Restoring Screen Backlight Brightness in Ubuntu

One of many details available in Windows but not in Ubuntu is automatic backlight change when system switches from AC to battery. And it’s not just a dumb change to predefined value either. Every switch from AC to battery and vice versa restores the last value system had. Ubuntu on the other hand just keeps the backlight as is resulting in me manually adjusting it every time. Lookup on Internet for applications providing this functionality gave me no warm fuzzy feeling so I decided to roll my own. I mean, how hard can it be.

Well, actually annoyingly hard if you want to support every interface out there. As I wanted to support only my Dell XPS 15, I had quite a bit easier work.

The main part of the story is in two files: /sys/class/power_supply/AC/online and /sys/class/backlight/intel_backlight/brightness. All what’s needed was actually a small script handling tracking and restoring brightness values every once in a while. This is roughly what I ended with:

~/bin/backlight-tracer
#!/bin/bash

STORED_AC=`cat /var/cache/.backlight.ac`
STORED_BAT=`cat /var/cache/.backlight.bat`

while(true); do
BRIGHTNESS=`cat /sys/class/backlight/intel_backlight/brightness`
IS_AC=`cat /sys/class/power_supply/AC/online`
if [[ "$IS_AC" != "$LAST_AC" ]]; then
if [[ "$IS_AC" != "0" ]]; then
if [[ "$STORED_AC" != "" ]]; then
echo -e "Restoring AC backlight to $STORED_AC"
echo $STORED_AC > $FILE_BRIGHTNESS
fi
else
if [[ "$STORED_BAT" != "" ]]; then
echo -e "Restoring battery backlight to $STORED_BAT"
echo $STORED_BAT > $FILE_BRIGHTNESS
fi
fi
LAST_AC=$IS_AC
else
if [[ "$IS_AC" != "0" ]]; then
if [[ "$STORED_AC" != "$BRIGHTNESS" ]]; then
echo $BRIGHTNESS > /var/cache/.backlight.ac
STORED_AC=$BRIGHTNESS
fi
else
if [[ "$STORED_BAT" != "$BRIGHTNESS" ]]; then
echo $BRIGHTNESS > /var/cache/.backlight.bat
STORED_BAT=$BRIGHTNESS
fi
fi
fi

sleep 0.5
done

As you can see, the script is just checking in loop if there was an AC status change. If computer was plugged or unplugged, it simply restores the last saved value for that power state. If power status remained the same, it will track any brightness change so it’s possible to restore it later. Really simple and really working.

And yes, the script above contains no error handling. If you want to see the real stuff, it’s available on GitHub. Even better, it’s available as a Debian package if you want to install it.

Using Ctrl+Shift+Escape for System Monitor in Ubuntu

Moving from Windows to Ubuntu, there is one shortcut I surely miss – Ctrl+Shift+Escape. On Ubuntu this does absolutely nothing.

Fortunately one can always add a custom shortcut to System Monitor:

Terminal
gsettings set org.gnome.settings-daemon.plugins.media-keys custom-keybindings \
"['/org/gnome/settings-daemon/plugins/media-keys/custom-keybindings/custom0/']"
GSCHEMA=org.gnome.settings-daemon.plugins.media-keys.custom-keybinding
GPATH=/org/gnome/settings-daemon/plugins/media-keys/custom-keybindings/custom0/
gsettings set $GSCHEMA:$GPATH name "System Monitor"
gsettings set $GSCHEMA:$GPATH command "gnome-system-monitor"
gsettings set $GSCHEMA:$GPATH binding "<Primary><Shift>Escape"

However, this is not quite “it”. The major issue is that, if System Monitor is already open, it will remain in background. As this is Linux, of course there is a command line solution for this.

First we need to install wmctrl package

Terminal
sudo apt install wmctrl

Then we can setup a script to run System Monitor and activate it’s window. Since application itself is single instance, this does exactly what we need:

~/bin/system-monitor
#!/bin/bash
nohup gnome-system-monitor >/dev/null 2>&1 & >/dev/null
wmctrl -Fa 'System Monitor'

To make it runnable, we shouldn’t forget chmod:

Terminal
chmod +x ~/bin/system-monitor

And now finally we can adjust our key binding to call that newly created script:

Terminal
gsettings set org.gnome.settings-daemon.plugins.media-keys custom-keybindings \
"['/org/gnome/settings-daemon/plugins/media-keys/custom-keybindings/custom0/']"
GSCHEMA=org.gnome.settings-daemon.plugins.media-keys.custom-keybinding
GPATH=/org/gnome/settings-daemon/plugins/media-keys/custom-keybindings/custom0/
gsettings set $GSCHEMA:$GPATH name "System Monitor"
gsettings set $GSCHEMA:$GPATH command "$HOME/bin/system-monitor"
gsettings set $GSCHEMA:$GPATH binding "<Primary><Shift>Escape"

I call this close enough.

Ubuntu 19.10 Fails to Install Deb File

As I upgraded to Ubuntu 19.10, I went to install a few of my Linux applications and immediately got Failed to install file: not supported error. Debian packages that worked for me with 19.04 were suddenly causing the issue.

I traced this to my control file:

control
Package: Bimil
Version: MAJOR.MINOR
Architecture: all
Maintainer: Josip Medved <jmedved@jmedved.com>
Homepage: https://www.medo64.com/bimil/
Description: Password manager.
A small password manager compatible with Password Safe file format.
Section: misc
Priority: optional
Depends: mono-complete (>=4.2), gnupg2

While this worked fine for Ubuntu 19.04, in Ubuntu 19.10, one has to have Build-Depends value too:

control
Package: Bimil
Version: MAJOR.MINOR
Architecture: all
Maintainer: Josip Medved <jmedved@jmedved.com>
Homepage: https://www.medo64.com/bimil/
Description: Password manager.
A small password manager compatible with Password Safe file format.
Section: misc
Priority: optional
Depends: mono-complete (>=4.2), gnupg2
Build-Depends: mono-complete (>=4.2)

Once I rebuilt package with it, my .deb files worked once again.

SIGFAULT in QVector size()

After a long time, I started playing with C++ again – this time in the form of QT. Since I learned C++ a while ago, my code was a bit rustic and probably not following all the newest recommendations, but hey, it worked. Until I decided to change it from using QVector<std::shared_ptr<FolderItem>> to QVector<FolderItem*>. Suddenly I kept getting SIGFAULT in the most innocuous method: size().

As Google is my crutch, I did a quick search for this and found absolutely nothing useful – just other people having the same issue. After finally looking at my code, I figured why – because I was asking the wrong question. Of course there was nothing wrong with size() method in QVector class used by millions. It was me all along.

I had _folder variable in main window that I never initialized and I used it to call its method which in turn used QVector.size(). Forgetting to initialize this variable if you’re using shared_ptr is nothing – it simply ends up being null pointer and I had check for those. Once I changed to raw pointer, I lost that protection and uninitialized variable did what uninitialized variable in C++ does – pointed to “something”. Executing function “somewhere” in memory is what brought SIGFAULT – not the poor size() method.

It was all solved by something I used to do reflexively: initialize any variable declared in C++ to some value – even if that value is nullptr.