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Is there a simple linux command that will tell me what my display manager is?

I'm using Xfce. Are different desktop environments usually affiliated with different display managers?

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Wikipedia has a pretty good article on this: en.wikipedia.org/wiki/X_display_manager_%28program_type%29 –  Let_Me_Be Sep 9 '11 at 14:39
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4 Answers

There isn't. The display manager is not necessarily related to anything else that's running on the same X server. The display manager runs before you log in; it's chosen by the system administrator. Everything else (window manager, session manager, desktop environment, …) is chosen by the user. There doesn't even have to be a display manager: if you log in in text mode and start the GUI with startx, no display manager is involved.

You can check which display manager is the default one on your system. This will only give the right answer under some common but not universal assumptions. If you manually ran a different manager for whatever reason, this method won't tell you.

A good bet is to find out the process ID of the X server: its parent process is probably a display manager, if there is one. This requires that your clients are running on the same machine as the X server. lsof /tmp/.X11-unix/X${DISPLAY#:} will show the X server process (assuming the X sockets live in /tmp/.X11-unix).

x=$(lsof -F '' /tmp/.X11-unix/X0); x=${x#p}
ps -p $(ps -o ppid -p $x)
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Can you provide a walkthrough of that code? –  ptrcao Sep 13 '11 at 1:10
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@ptrcao lsof -F '' prints output like p1234. The -F option means a machine-parseable output format, and '' means to only print the PID, with the letter p before it. x=${x#p} strips off the initial letter p. The last line obtains the PID of the parent of the X server (ps -o ppid -p $x), and calls ps to show information about that parent process. –  Gilles Sep 13 '11 at 23:38
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Unfortunately the configuration differs for each distribution:

Debian (should also apply to Ubuntu)

/etc/X11/default-display-manager

RedHat (should also apply to Fedora)

/etc/sysconfig/desktop

OpenSuSe

/etc/sysconfig/displaymanager
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Huh, why are you telling me about configuration...? –  ptrcao Sep 9 '11 at 15:49
    
@ptrcao Because it specifies what display manager you are using. –  Let_Me_Be Sep 9 '11 at 15:51
    
Suppose I don't know what my display manager is and I want to find out by using terminal. How would I do that? –  ptrcao Sep 9 '11 at 15:55
    
@ptrcao You would look into those files. Either opening them in your favorite editor, or just doing cat. –  Let_Me_Be Sep 9 '11 at 15:56
    
It has only one line in it: /usr/bin/xdm - does that mean xdm is my display manager? –  ptrcao Sep 9 '11 at 15:59
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The display manager name should be in DESKTOP_SESSION

echo $DESKTOP_SESSION

returns "gnome" for me.

EDIT
You're right. They're going back and forth on that on XFCEs bugzilla so it probably isn't very reliable.

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@frabjous: What about gdm and kdm? Are they display managers too or just login managers? –  ptrcao Sep 9 '11 at 14:39
    
I must've accidentally deleted frabjous' comment just above because I remember he remarked that Gnome is a desktop environment, not a display manager. This thread is abound with confusion... –  ptrcao Sep 9 '11 at 15:48
    
This gives information about… the desktop session, which is likely to be the same thing as your desktop environment, but may be something else altogether if you're not using a desktop environment or you have a peculiar configuration. On the machine where I'm writing this, $DESKTOP_SESSION is unknown. –  Gilles Sep 9 '11 at 23:56
    
+1, echo $DESKTOP_SESSION also returns gnome to me... but maybe this only works for gnome? –  Trevor Boyd Smith Sep 17 '11 at 22:37
    
If it only works for gnome, this command will only help you deduce "yes you have Gnome or no you don't have gnome". When the question wants to find out more than that. –  Trevor Boyd Smith Sep 17 '11 at 22:37
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X is highly modular so if you really wanted it, yes, you could mix desktop environments and windows managers. After all a window manager is just a way to paint windows (that's the simple view).

So if you take gnome, metacity used to be the windows manager but now it's been replaced by Mutter.

The issue with mixing is in modern desktop environments, desktop and window manager are highly tied to one another. Too much mixing could end up in fewer functionality.

As mentioned $DESKTOP_SESSION works but if you want to ask your X11 server, you could do this:

xprop -id $(xprop -root _NET_SUPPORTING_WM_CHECK | cut -d\# -f2) WM_NAME

Which decomposes into

xprop -root _NET_SUPPORTING_WM_CHECK | cut -d\# -f2
0x1000052
xprop -id 0x1000052 WM_NAME
WM_NAME(STRING) = "xfwm4"

Which gets the id of the window created by the window manager. The specs says (http://standards.freedesktop.org/wm-spec/1.3/ar01s03.html):

_NET_SUPPORTING_WM_CHECK, WINDOW/32
The Window Manager MUST set this property on the root window to be the ID
of a child window created by himself, to indicate that a compliant window
manager is active.

And then you ask for the WM_NAME property of said window.

WM_NAME being:

The WM_NAME property is an uninterpreted string that the client wants the
window manager to display in association with the window (for example, in
a window headline bar). 
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@frabjous: Xfwm4 is the Xfce window manager. Is that the same as a display manager? –  ptrcao Sep 9 '11 at 14:39
    
@Mathieu You are talking about window managers, not display managers. –  Let_Me_Be Sep 9 '11 at 14:41
    
@Let_Me_Be True but then most of the time, your window manager is tied to your display manager... How often do you use gnome window managerwith kde and vice-versa? And it actually works because gdm/kdm/whatever allow you to select your display manager, meaning reading the configuration file like you mentioned doesn't work if you don't use the provided default... –  Mathieu Sep 9 '11 at 17:23
    
_NET_SUPPORTING_WM_CHECK gives information about the window manager, not about the display manager. It doesn't even always work; on my system, that property is set to a window ID but the window in question doesn't have a name. –  Gilles Sep 9 '11 at 23:54
    
@Mathieu That depends on the distributions. Usually when you install a KDE version of the distro, you get KDM and when a gnome version, you get GDM. But the stuff that you will see more and more is official support for only one display manager (usually a more light-weight variant then GDM/KDM). –  Let_Me_Be Sep 10 '11 at 0:32
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