Changing gdm/lightdm user login settings programmatically

Recently, as part of the automated testing efforts in Linaro, I needed to programmatically change the default X session for a user. It used to be the case that editing ~/.dmrc was enough to achieve this. Display managers (DMs), such as gdm and lightdm, would read the contents of the ~/.dmrc at login time to decide which language and X session to use (among other settings). When a user changed a setting through the GUI, the DM would write the new settings to ~/.dmrc (only after successfully logging in, of course).

However, all of this changed with the introduction of AccountsService. As the name suggests, AccountsService provides a service for the management of user accounts, and among other things, some of the login settings that were previously configured in ~/.dmrc. It offers this functionality through the org.freedesktop.Accounts DBus service on the system bus.

Modern Display Managers use AccountsService to manipulate user  login settings, falling back to ~/.dmrc only when the service is unavailable (at least in the case of lightdm). What this means for the end-user is that editing ~/.dmrc manually has no effect. For example, you can try changing the Session field in ~/.dmrc all you want, but next time you log in you will end up in the same session and your ~/.dmrc changes will have been overwritten.

In order to actually change any settings we need to communicate with AccountsService through DBus. The first step is to find out the object that corresponds to the user we want to change the settings for. The path of this object is of the form /org/freedesktop/Accounts/. <USER> is usually of the form User<UID> but there is no guarantee of that.  Thankfully, the /org/freedesktop/Accounts object provides the org.freedesktop.Accounts.FindUserByName and org.freedesktop.Accounts.FindUserById methods, which we can use to get the object path for a user.

Having the user object path, we can use the org.freedesktop.Accounts.User.* methods on the user object to change the required settings.

We can use the dbus-send program to perform the operations mentioned above. For example:

$ USER_PATH=$(dbus-send --print-reply=literal --system --dest=org.freedesktop.Accounts /org/freedesktop/Accounts org.freedesktop.Accounts.FindUserById int64:1000)
$ dbus-send --print-reply --system --dest=org.freedesktop.Accounts $USER_PATH org.freedesktop.Accounts.User.SetXSession string:’xterm’

As I needed to get and set the X session for a user in a user-friendly manner,  I decided to create a small python program instead. You can find the program here: user_xsession.py

You can use user_xsession.py like:

$ ./user_xsession.py [--user-id <ID>|--user-name <NAME>] set <SESSION>
$ ./user_xsession.py [--user-id <ID>|--user-name <NAME>] get

where <SESSION> is one of the sessions available in /usr/share/xsessions/ . Note that you may need to run as root, depending on the account you want to change the settings for.

For example:

$ ./user_xsession.py --user-id 1000 set xterm

You can easily tweak the program to change another setting instead of the X session.

Update 2012-08-10: Fixed problem with wordpress converting -- (double-dash) to – (en-dash) in code snippets.

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