Unity launcher does not load. Ubuntu-desktop crashes.

To identify the packages that have file installed in /usr/share/dbus-1/services (shouldn't that read /usr/share/dbus-1/system-services?) you can use dpkg
dpkg-query -S /usr/share/dbus-1/services/
and then reinstall these packages.

If you boot an older kernel is it OK?

@m-hampl
Currently, both the folders - system-services and services exist in the /usr/share/dbus-1/ path. I believe the services folder corresponds to the xfce-desktop. I used the command you provided to list all the packages in the system-services folder and the following were listed:

 systemd, language-selector-common, thermald, policykit-1, software-properties-common, nepomuk-core-data, kde-runtime-data, gnome-applets-data, colord, rtkit, libqapt3-runtime, wpasupplicant, hplip, aptdaemon, python3-aptdaemon.pkcompat, ubuntu-system-service, bluez, upower, apt-xapian-index, usb-creator-common, realmd, k3b-data, udisks2, indicator-cpufreq, consolekit, systemd-shim, network-manager, modemmanager, accountsservice, fwupd, avahi-daemon, screen-resolution-extra, whoopsie-preferences, cups-pk-helper, dbus: /usr/share/dbus-1/system-services

I re-installed them using the command "sudo apt install --reinstall <package-name>". All but one were successfully re-installed. That one package (nepomuk-core-data) refused to be re-installed and returned the following message "Reinstallation of nepomuk-core-data is not possible, it cannot be downloaded."

After this, I rebooted the system. The problems remains unsolved.

@actionparsnip
I booted into 3.19.0-65-generic. No change in the status of the problem.

The following is the situation of the system at present:

Ubuntu/Gnome desktop:
 Can login; No window decorations; No unity launcher in the default desktop; Super key does not launch the dash; Ctrl+Alt+T doesn't launch the terminal; Super+l doesn't lock the screen. The following output was generated for the Ctrl+Alt+T key press with the command "tail -f /var/log/syslog" running in tty1:

Sep 15 09:28:36 Enigma acpid: client connected from 1255[0:0]
Sep 15 09:28:36 Enigma acpid: 1 client rule loaded
Sep 15 09:28:40 Enigma gnome-session[6174]: Error creating terminal: The name org.gnome.Terminal was not provided by any .service files
Sep 15 09:28:42 Enigma console-kit-daemon[10114]: (process:14410): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
Sep 15 09:28:42 Enigma acpid: client 1255[0:0] has disconnected
Sep 15 09:28:44 Enigma gnome-session[6174]: ** (zeitgeist-datahub:7693): WARNING **: zeitgeist-datahub.vala:212: Error during inserting events: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.gnome.zeitgeist.Engine was not provided by any .service files
Sep 15 09:28:44 Enigma gnome-session[6174]: invoking IsSupported() failed for remote volume monitor with dbus name org.gtk.vfs.UDisks2VolumeMonitor:: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.gtk.vfs.UDisks2VolumeMonitor was not provided by any .service files (g-dbus-error-quark, 2)
Sep 15 09:28:44 Enigma gnome-session[6174]: invoking IsSupported() failed for remote volume monitor with dbus name org.gtk.vfs.GPhoto2VolumeMonitor:: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.gtk.vfs.GPhoto2VolumeMonitor was not provided by any .service files (g-dbus-error-quark, 2)
Sep 15 09:28:44 Enigma gnome-session[6174]: invoking IsSupported() failed for remote volume monitor with dbus name org.gtk.vfs.MTPVolumeMonitor:: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.gtk.vfs.MTPVolumeMonitor was not provided by any .service files (g-dbus-error-quark, 2)
Sep 15 09:28:44 Enigma gnome-session[6174]: invoking IsSupported() failed for remote volume monitor with dbus name org.gtk.vfs.AfcVolumeMonitor:: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.gtk.vfs.AfcVolumeMonitor was not provided by any .service files (g-dbus-error-quark, 2)
Sep 15 09:28:44 Enigma gnome-session[6174]: invoking IsSupported() failed for remote volume monitor with dbus name org.gtk.vfs.GoaVolumeMonitor:: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.gtk.vfs.GoaVolumeMonitor was not provided by any .service files (g-dbus-error-quark, 2)
Sep 15 09:28:46 Enigma AptDaemon.Worker: INFO: Finished transaction /org/debian/apt/transaction/2b6ee7c40aad4a8486e4c6dfc8406765
Sep 15 09:28:46 Enigma org.debian.apt[939]: 09:28:46 AptDaemon.Worker [INFO]: Finished transaction /org/debian/apt/transaction/2b6ee7c40aad4a8486e4c6dfc8406765

At this point I terminated the command with Ctrl+c.

Xfce:
No window decorations; Menu bar is present; Cannot switch between applications (Alt+tab doesn't do anything); Close button for some applications like gedt is present but it is absent for google-chrome; Ctrl+Alt+T doesn't launch the terminal; But can use the Applications menu to launch applications.

Found that id=12 corresponds to AT Translated Set 2 keyboard from the command xinput

Performed "xinput test 12" and pressed Ctrl+Alt+T, Ctrl+T, Ctrl, Alt keys and they were recorded. I goutputs like "key press 36", "key release 36", etc.

1. nepomuk-core-data seems to be a leftover from Ubuntu trusty (that package does not exist in xenial any more). For reinstalling you could use https://launchpad.net/ubuntu/+archive/primary/+files/nepomuk-core-data_4.13.3-0ubuntu0.2_all.deb

2. There were reports that problems with missing windows decorators etc. were caused by wrong ownership settings on configuration files or other problems with user-related configuration. To check for this in your system you could do the following:
a. log in in a guest session - does that work?
b. create a new user and try logging in - does that provide a complete desktop?
c. verify the ownership of all files below your home directory with a command like

sudo find ~ ! -user yourusername -ls

3. Has reinstalling the packages with dbus service files helped with anything (USB mounting, clicking on icons, ...)?

Thank you your reply.

1) Can I safely ignore this package (don't want to mess up the system any more)?

2) a&b Guest account and a new test account both seem to be better than the default account in that some of the window decorations (like borders, title bar, close/minimise/restore buttons) are present when logged in to these accounts. But the super key and combinations do not work. Ctrl+<key> works, Alt+<key> works; Ctrl+Alt+T doesn't work.

  c. "sudo find ~ ! -user <username> -ls" lists the files and marks their owner to be root.

3) No. The behavior of the system is still the same.

Another observation:
In the guest/test accounts as well as the main (xfce) account, whenever I launch a gui based application from the UXterminal, I get an error which goes like "the name org.xxx.yyy was not provided by any .services files"

1) the nepomuk packages were part of KDE, I assume you can safely ignore it. Maybe you could even consider deinstalling those packages (I assume ubuntu-support-status --show-unsupported will list them).

2c) you probably should correct the ownership of the configuration files below your home directory
(e.g. with a command like sudo chown -R yourusername:yourusername ~)
Maybe that helps for some of your problems.

4) what is the org.xxx.yyy name? This might indicate another package that should be reinstalled to correct the deletion of the dbus subdirectory.

Thank you.

1) Will do that once the system is back in shape.

2) The command "sudo find ~ ! -user root -ls" lists all the files owned by the user <username> whereas the command "sudo find ~ ! -user <username> -ls" lists the files owned by root. Is that expected?
  I assumed that there is a space each between "~", "!", and "-user" in the above commands (not in a position to copy&paste the commands)

4) the "xxx" and "yyy" part depends on the application in question. Eg: xxx=gnome and yyy=Terminal when I try to launch gnome-terminal from the UXterminal (yes, I tried to do that!). For more such examples check post #4 above.

A package management search shows that the file /usr/share/dbus-1/services/org.gnome.Terminal.service is installed with the gnome-terminal package. Does such file exist? If not, try reinstalling the gnome-terminal package.
Then restart dbus ("sudo service dbus restart" should do that) and re-try gnome-terminal. Do you still get the same error message?

The exclamation mark in the find command acts for negation.
sudo find ~ ! -user <username> -ls
lists all file below the home directory NOT owned by <username> (usually those are owned by root)

Thank you! That trick of re-installing gnome-terminal worked, but restarting dbus throug the command "sudo service dbus restart" through up an error: "Failed to restart dbus.service: Operation refused, unit dbus.service may be requested by dependency only. See system logs and 'systemctl status dbus.service' for details."

Even without restarting dbus I can open gnome-terminal but only through either application menu or through the UXterminal; Ctrl+Alt+T doesn't work.

Also: Such errors (i.e., org.xxx.yyy not found) are being thrown at almost every application launched from the terminal. How do I make sure that all necessary .service files are present in the .../dbus-1/services or the .../dbus-1/system-services paths?

The service files that should be there for the packages installed on your system are supposed to be listed by the package management command

dpkg-query -S /usr/share/dbus-1/services/
and/or
dpkg-query -S /usr/share/dbus-1/system-services/

Try matching the available files with those listed by dpkg to identify the missing ones.
(For the error messages listed above, I assume the package to be reinstalled is gvfs-daemons)

Thank you. I compared the lists generated by the commands "dpkg-query -S /usr/share/dbus-1/services/" and "dpkg -l" The former list (including .../services and system-services) is too small compared to the later. I guess I have a long road ahead unless there is a short cut to this re-installing procedure.

I 'll try to re-install as many packages as possible and post any significant change.

Just as an additional remark:
Make sure that in both lists you only take into account packages that you want to have installed.

The dpkg-query lists the packages that store files there.
dpkg-l lists all files (even those that do not provide any service files). There is no need to reinstall all packages.

Maybe my last comment was a bit misleading:
You better compare the output of dpkg-seach -S ... with the output of
ls .../dbus-1/services and ls .../dbus-1/system-services instead of dpkg -l

Thank you. That will save me a lot of trouble

However, how do I check which service file corresponds to which package?

Re: How do I check which service file corresponds to which package?

With the
dpkg -S filename
command.

Am I supposed to run "dpkg -S filename " for all the files installed in my system? If not I am unable to comprehend the answer.

The commands

dpkg-query -S /usr/share/dbus-1/services/
dpkg-query -S /usr/share/dbus-1/system-services/

will show all packages that have related files in these dbus subdirectories.
Take the lists, drop all packages that you do not need and reinstall these packages.

This will hopefully repair your action of deleting the dbus directories.

Scripting might help, something like this might work (not tested myself!):
dpkg-query -S /usr/share/dbus-1/services/ > packageslist.txt
dpkg-query -S /usr/share/dbus-1/system-services/ >> packageslist.txt
gedit packagelist.txt # eliminate undesired packages and leave only the package names separated by blanks or newline
cat packageslist.txt | xargs sudo apt-get install --reinstall

I ran the dpkg-query command on the .../dbus-1/services and .../dbus-1/system-services paths, re-installed the packages listed there using sudo apt install --reinstall <packagename>, and rebooted the system. While re-installing the system showed for all the packages:

gpgv: Can't check signature: public key not found
dpkg-source: warning: failed to verify signature on /var/cache/apt/sources/<packagename.dsc>
gpgv: Signature made on <date> using RSA key ID ####

(The <date> and key ID #### varied with the package.)

After reboot the system remains the same:

In the main account: No window decorations; No unity launcher; super key and such dash/desktop shortcuts and right click do not work.

In the test account: No unity launcher; no response to super key.

You seem to have another problem with the gpg keys for verifying the integrity of the Ubuntu packages.

Apparently your system is already broken to a degree that a new installation seems better than further repair actions.

Is there any option other than a fresh install? May be using the live usb to repair the system or such?

Thank you very much. I re-installed xenial using a DVD*. Now the (new) system works fine.

*System wouldn't boot into USB even with appropriate BIOS settings; wouldn't burn a DVD, had to burn the .iso file using another system.

Thanks Manfred Hampl, that solved my question.