Tengo instalado ubuntu 9.04, con gnome y kde. Tenía problemas con kde que no conseguí solucionar (me desaparecieron elementos gráficos que no pude recuperar, como el panel de aplicaciones del escritorio) y decidí desinstalar todos los paquetes synaptic de kde. Luego en consola metí "sudo aptitude install gnome-core", como me aconsejaron y reinicié. Ahora al inicio me sale este mensaje:
"No hay ninguna línea exec en el archivo de sesión: kde. Se ejecutará una sesión gnome a prueba de fallos en su lugar"
Soy bastante profano en la materia, apenas me he pasado a linux. Estoy muy contento con el cambio, aunque supongo que me he colado al experimentar.
¿Alguien me puede echar una mano?
Gracias.

Puedes intentar lo que viene aquí:
https://lists.ubuntu.com/archives/ubuntu-es/2008-August/031250.html

Básicamente, lo que creo que tienes que hacer es modificar el fichero /etc/X11/default-display-manager y sustituir el kdm por el gdm.
Antes de nada, yo realizaría una copia de los archivos, por si los errores y explosiones; además deberás tener en cuenta la localización de los ejecutables "kdm" y "gdm" (puedes utilizar el comando "locate" para localizar el gdm)
Lo más seguro es que el gdm esté en /usr/bin/gdm

Si no lo tienes muy claro, puedes poner el archivo y ya te lo intentamos modificar (si es que es eso)

introducí "locate gdm" y me da esto:
guimo@guimo-desktop:~$ locate gdm
/etc/gdm
/etc/apparmor.d/gdm-guest-session
/etc/gdm/Init
/etc/gdm/PostLogin
/etc/gdm/PostSession
/etc/gdm/PreSession
/etc/gdm/XKeepsCrashing
/etc/gdm/Xsession
/etc/gdm/failsafeBlacklist
/etc/gdm/failsafeDexconf
/etc/gdm/failsafeXServer
/etc/gdm/failsafeXinit
/etc/gdm/gdm.conf
/etc/gdm/gdm.conf-custom
/etc/gdm/gdmprefetchlist
/etc/gdm/locale.conf
/etc/gdm/modules
/etc/gdm/Init/Default
/etc/gdm/PostLogin/Default.sample
/etc/gdm/PostSession/Default
/etc/gdm/PreSession/Default
/etc/gdm/modules/AccessDwellMouseEvents
/etc/gdm/modules/AccessKeyMouseEvents
/etc/gdm/modules/factory-AccessDwellMouseEvents
/etc/gdm/modules/factory-AccessKeyMouseEvents
/etc/init.d/gdm
/etc/pam.d/gdm
/etc/pam.d/gdm-autologin
/etc/rc0.d/K01gdm
/etc/rc1.d/K01gdm
/etc/rc2.d/S30gdm
/etc/rc3.d/S30gdm
/etc/rc4.d/S30gdm
/etc/rc5.d/S30gdm
/etc/rc6.d/K01gdm
/usr/bin/gdm-dmx-reconnect-proxy
/usr/bin/gdmXnest
/usr/bin/gdmXnestchooser
/usr/bin/gdmdynamic
/usr/bin/gdmflexiserver
/usr/bin/gdmphotosetup
/usr/bin/gdmthemetester
/usr/lib/gdm
/usr/lib/gdmplay
/usr/lib/gdm/gdm-ssh-session
/usr/lib/gdm/gdmaskpass
/usr/lib/gdm/gdmchooser
/usr/lib/gdm/gdmgreeter
/usr/lib/gdm/gdmlogin
/usr/lib/gdm/gdmopen
/usr/lib/gdm/gdmprefetch
/usr/lib/gdm/gdmtranslate
/usr/lib/python2.6/dist-packages/ubuntu_gdm_themes-0.32.egg-info
/usr/sbin/gdm
/usr/sbin/gdmsetup
/usr/share/gdm
/usr/share/app-install/desktop/gdmap.desktop
/usr/share/app-install/icons/gdmap_icon.png
/usr/share/applications/gdmflexiserver-xnest.desktop
/usr/share/applications/gdmflexiserver.desktop
/usr/share/applications/gdmphotosetup.desktop
/usr/share/applications/gdmsetup.desktop
/usr/share/doc/gdm
/usr/share/doc/gdm-guest-session
/usr/share/doc/ubuntu-gdm-themes
/usr/share/doc/gdm/AUTHORS.gz
/usr/share/doc/gdm/NEWS.gz
/usr/share/doc/gdm/README.Debian
/usr/share/doc/gdm/README.gz
/usr/share/doc/gdm/README.install.gz
/usr/share/doc/gdm/TODO
/usr/share/doc/gdm/changelog.Debian.gz
/usr/share/doc/gdm/changelog.gz
/usr/share/doc/gdm/copyright
/usr/share/doc/gdm-guest-session/README.Debian
/usr/share/doc/gdm-guest-session/changelog.gz
/usr/share/doc/gdm-guest-session/copyright
/usr/share/doc/ubuntu-gdm-themes/AUTHORS
/usr/share/doc/ubuntu-gdm-themes/changelog.gz
/usr/share/doc/ubuntu-gdm-themes/copyright
/usr/share/gdm/BuiltInSessions
/usr/share/gdm/gdmXnestWrapper
/usr/share/gdm/gdmchooser.glade
/usr/share/gdm/gdmphotosetup.glade
/usr/share/gdm/gdmsetup.glade
/usr/share/gdm/guest-session
/usr/share/gdm/themes
/usr/share/gdm/BuiltInSessions/default.desktop
/usr/share/gdm/guest-session/Xsession
/usr/share/gdm/guest-session/guest-session-cleanup.sh
/usr/share/gdm/guest-session/guest-session-launch
/usr/share/gdm/guest-session/guest-session-setup.sh
/usr/share/gdm/themes/Human
/usr/share/gdm/themes/HumanCircle
/usr/share/gdm/themes/HumanList
/usr/share/gdm/themes/circles
/usr/share/gdm/themes/happygnome
/usr/share/gdm/themes/happygnome-list
/usr/share/gdm/themes/Human/GdmGreeterTheme.desktop
/usr/share/gdm/themes/Human/Human.xml
/usr/share/gdm/themes/Human/background.png
/usr/share/gdm/themes/Human/bottom_bar.svg
/usr/share/gdm/themes/Human/boundingbox.png
/usr/share/gdm/themes/Human/boundingbox.svg
/usr/share/gdm/themes/Human/gtk-2.0
/usr/share/gdm/themes/Human/icon-language-active.png
/usr/share/gdm/themes/Human/icon-language-prelight.png
/usr/share/gdm/themes/Human/icon-language.png
/usr/share/gdm/themes/Human/icon-reboot-active.png
/usr/share/gdm/themes/Human/icon-reboot-prelight.png
/usr/share/gdm/themes/Human/icon-reboot.png
/usr/share/gdm/themes/Human/icon-session-active.png
/usr/share/gdm/themes/Human/icon-session-prelight.png
/usr/share/gdm/themes/Human/icon-session.png
/usr/share/gdm/themes/Human/icon-shutdown-active.png
/usr/share/gdm/themes/Human/icon-shutdown-prelight.png
/usr/share/gdm/themes/Human/icon-shutdown.png
/usr/share/gdm/themes/Human/screenshot.png
/usr/share/gdm/themes/Human/userentry.png
/usr/share/gdm/themes/Human/userentry.svg
/usr/share/gdm/themes/Human/gtk-2.0/gtkrc
/usr/share/gdm/themes/HumanCircle/GdmGreeterTheme.desktop
/usr/share/gdm/themes/HumanCircle/HumanCircle.xml
/usr/share/gdm/themes/HumanCircle/circle.png
/usr/share/gdm/themes/HumanCircle/icon-language-active.png
/usr/share/gdm/themes/HumanCircle/icon-language-prelight.png
/usr/share/gdm/themes/HumanCircle/icon-language.png
/usr/share/gdm/themes/HumanCircle/icon-reboot-active.png
/usr/share/gdm/themes/HumanCircle/icon-reboot-prelight.png
/usr/share/gdm/themes/HumanCircle/icon-reboot.png
/usr/share/gdm/themes/HumanCircle/icon-session-active.png
/usr/share/gdm/themes/HumanCircle/icon-session-prelight.png
/usr/share/gdm/themes/HumanCircle/icon-session.png
/usr/share/gdm/themes/HumanCircle/icon-shutdown-active.png
/usr/share/gdm/themes/HumanCircle/icon-shutdown-prelight.png
/usr/share/gdm/themes/HumanCircle/icon-shutdown.png
/usr/share/gdm/themes/HumanCircle/screenshot.png
/usr/share/gdm/themes/HumanList/GdmGreeterTheme.desktop
/usr/share/gdm/themes/HumanList/HumanList.xml
/usr/share/gdm/themes/HumanList/background.png
/usr/share/gdm/themes/HumanList/bottom_bar.svg
/usr/share/gdm/themes/HumanList/boundingbox-0-0.png
/usr/share/gdm/themes/HumanList/boundingbox-0-1.png
/usr/share/gdm/themes/HumanList/boundingbox-0-2.png
/usr/share/gdm/themes/HumanList/boundingbox-bottom.png
/usr/share/gdm/themes/HumanList/boundingbox-middle.png
/usr/share/gdm/themes/HumanList/boundingbox-top.png
/usr/share/gdm/themes/HumanList/boundingbox.png
/usr/share/gdm/themes/HumanList/boundingbox.svg
/usr/share/gdm/themes/HumanList/entry-0-0.png
/usr/share/gdm/themes/HumanList/entry-0-1.png
/usr/share/gdm/themes/HumanList/entry-0-2.png
/usr/share/gdm/themes/HumanList/entry-1-0.png
/usr/share/gdm/themes/HumanList/entry-1-1.png
/usr/share/gdm/themes/HumanList/entry-1-2.png
/usr/share/gdm/themes/HumanList/entry-2-0.png
/usr/share/gdm/themes/HumanList/entry-2-1.png
/usr/share/gdm/themes/HumanList/entry-2-2.png
/usr/share/gdm/themes/HumanList/gtk-2.0
/usr/share/gdm/themes/HumanList/icon-language-active.png
/usr/share/gdm/themes/HumanList/icon-language-prelight.png
/usr/share/gdm/themes/HumanList/icon-language.png
/usr/share/gdm/themes/HumanList/icon-reboot-active.png
/usr/share/gdm/themes/HumanList/icon-reboot-prelight.png
/usr/share/gdm/themes/HumanList/icon-reboot.png
/usr/share/gdm/themes/HumanList/icon-session-active.png
/usr/share/gdm/themes/HumanList/icon-session-prelight.png
/usr/share/gdm/themes/HumanList/icon-session.png
/usr/share/gdm/themes/HumanList/icon-shutdown-active.png
/usr/share/gdm/themes/HumanList/icon-shutdown-prelight.png
/usr/share/gdm/themes/HumanList/icon-shutdown.png
/usr/share/gdm/themes/HumanList/screenshot.png
/usr/share/gdm/themes/HumanList/ubuntu.png
/usr/share/gdm/themes/HumanList/userentry.svg
/usr/share/gdm/themes/HumanList/userlist-bottom.png
/usr/share/gdm/themes/HumanList/userlist-middle.png
/usr/share/gdm/themes/HumanList/userlist-top.png
/usr/share/gdm/themes/HumanList/gtk-2.0/gtkrc
/usr/share/gdm/themes/circles/GdmGreeterTheme.desktop
/usr/share/gdm/themes/circles/background.svg
/usr/share/gdm/themes/circles/circles.xml
/usr/share/gdm/themes/circles/flower.png
/usr/share/gdm/themes/circles/screenshot.png
/usr/share/gdm/themes/happygnome/GdmGreeterTheme.desktop
/usr/share/gdm/themes/happygnome/background.svg
/usr/share/gdm/themes/happygnome/gnome-logo.svg
/usr/share/gdm/themes/happygnome/happygnome.xml
/usr/share/gdm/themes/happygnome/screenshot.png
/usr/share/gdm/themes/happygnome-list/GdmGreeterTheme.desktop
/usr/share/gdm/themes/happygnome-list/background.svg
/usr/share/gdm/themes/happygnome-list/gnome-logo.svg
/usr/share/gdm/themes/happygnome-list/happygnome-list.xml
/usr/share/gdm/themes/happygnome-list/screenshot.png
/usr/share/gnome/help/gdm
/usr/share/gnome/help/gdm/C
/usr/share/gnome/help/gdm/de
/usr/share/gnome/help/gdm/en_GB
/usr/share/gnome/help/gdm/es
/usr/share/gnome/help/gdm/fr
/usr/share/gnome/help/gdm/ko
/usr/share/gnome/help/gdm/oc
/usr/share/gnome/help/gdm/sv
/usr/share/gnome/help/gdm/uk
/usr/share/gnome/help/gdm/C/gdm.xml
/usr/share/gnome/help/gdm/C/legal.xml
/usr/share/gnome/help/gdm/de/gdm.xml
/usr/share/gnome/help/gdm/en_GB/gdm.xml
/usr/share/gnome/help/gdm/es/gdm.xml
/usr/share/gnome/help/gdm/fr/gdm.xml
/usr/share/gnome/help/gdm/ko/gdm.xml
/usr/share/gnome/help/gdm/oc/gdm.xml
/usr/share/gnome/help/gdm/sv/gdm.xml
/usr/share/gnome/help/gdm/uk/gdm.xml
/usr/share/icons/Tangerine/16x16/apps/gdm-setup.png
/usr/share/icons/Tangerine/22x22/apps/gdm-setup.png
/usr/share/icons/Tangerine/24x24/apps/gdm-setup.png
/usr/share/icons/Tangerine/32x32/apps/gdm-setup.png
/usr/share/icons/hicolor/16x16/apps/gdm-xnest.png
/usr/share/icons/hicolor/16x16/apps/gdmflexiserver.png
/usr/share/icons/hicolor/16x16/apps/gdmsetup.png
/usr/share/icons/hicolor/22x22/apps/gdmflexiserver.png
/usr/share/icons/hicolor/22x22/apps/gdmsetup.png
/usr/share/icons/hicolor/24x24/apps/gdmflexiserver.png
/usr/share/icons/hicolor/24x24/apps/gdmsetup.png
/usr/share/icons/hicolor/32x32/apps/gdm-xnest.png
/usr/share/icons/hicolor/32x32/apps/gdmflexiserver.png
/usr/share/icons/hicolor/32x32/apps/gdmsetup.png
/usr/share/icons/hicolor/48x48/apps/gdm-login-photo.png
/usr/share/icons/hicolor/48x48/apps/gdm-xnest.png
/usr/share/icons/hicolor/48x48/apps/gdm.png
/usr/share/icons/hicolor/48x48/apps/gdmflexiserver.png
/usr/share/icons/hicolor/48x48/apps/gdmsetup.png
/usr/share/icons/hicolor/scalable/apps/gdmflexiserver.svg
/usr/share/icons/hicolor/scalable/apps/gdmsetup.svg
/usr/share/locale-langpack/en_AU/LC_MESSAGES/gdm.mo
/usr/share/locale-langpack/en_CA/LC_MESSAGES/gdm.mo
/usr/share/locale-langpack/en_GB/LC_MESSAGES/gdm.mo
/usr/share/locale-langpack/en_GB/LC_MESSAGES/ubuntu-gdm-themes.mo
/usr/share/locale-langpack/es/LC_MESSAGES/gdm.mo
/usr/share/locale-langpack/es/LC_MESSAGES/ubuntu-gdm-themes.mo
/usr/share/man/man1/gdm.1.gz
/usr/share/man/man1/gdmflexiserver.1.gz
/usr/share/man/man8/gdm.8.gz
/usr/share/man/man8/gdmchooser.8.gz
/usr/share/man/man8/gdmlogin.8.gz
/usr/share/menu/gdm
/usr/share/omf/gdm
/usr/share/omf/gdm/gdm-C.omf
/usr/share/omf/gdm/gdm-de.omf
/usr/share/omf/gdm/gdm-en_GB.omf
/usr/share/omf/gdm/gdm-es.omf
/usr/share/omf/gdm/gdm-fr.omf
/usr/share/omf/gdm/gdm-ko.omf
/usr/share/omf/gdm/gdm-oc.omf
/usr/share/omf/gdm/gdm-sv.omf
/usr/share/omf/gdm/gdm-uk.omf
/usr/share/pixmaps/gdm-foot-logo.png
/usr/share/pixmaps/gdm.xpm
/usr/share/pixmaps/gdmDebianLogo.xpm
/usr/share/python-support/gnome-orca/orca/scripts/apps/gdmlogin.py
/var/lib/gdm
/var/lib/dpkg/info/gdm-guest-session.conffiles
/var/lib/dpkg/info/gdm-guest-session.list
/var/lib/dpkg/info/gdm-guest-session.md5sums
/var/lib/dpkg/info/gdm.conffiles
/var/lib/dpkg/info/gdm.config
/var/lib/dpkg/info/gdm.list
/var/lib/dpkg/info/gdm.md5sums
/var/lib/dpkg/info/gdm.postinst
/var/lib/dpkg/info/gdm.postrm
/var/lib/dpkg/info/gdm.preinst
/var/lib/dpkg/info/gdm.prerm
/var/lib/dpkg/info/gdm.templates
/var/lib/dpkg/info/ubuntu-gdm-themes.list
/var/lib/dpkg/info/ubuntu-gdm-themes.md5sums
/var/lib/python-support/python2.6/orca/scripts/apps/gdmlogin.py
/var/lib/python-support/python2.6/orca/scripts/apps/gdmlogin.pyc
/var/log/gdm
/var/log/gdm/:0.log
/var/log/gdm/:0.log.1
/var/log/gdm/:0.log.2
/var/log/gdm/:0.log.3
/var/log/gdm/:0.log.4
/var/log/gdm/failsafe.log
/var/tmp/kdecache-guimo/knewstuff2-entries.cache/d2FsbHBhcGVyOjpxdWJpcSByb29tIDEyODAgdmVyc2lvbg==.meta

cuando introduzco /etc/X11/default-display-manager, me sale "bash: /etc/X11/default-display-manager: Permiso denegado"

Lo cierto es que tengo muy pocas nociones de manejar al consola, aunque soy todo oídos. Estoy dispuesto a empaparme el manual entero, pero necesito solo un poco de luz para dar los primeros pasos.
Gracias.

Antes de nada, creo que es mejor que hagas una copia del fichero /etc/X11/default-display-manager por precaución. Es posible que necesites permisos de root, aunque si lo copias a otra carpeta puede que no necesites esos permisos.

Edita el fichero /etc/X11/default-display-manager:

Necesitarás permisos de root para poder editarlo.

En alguna de las líneas del fichero debería aparecer alguna ruta hacia algún ejecutable de kdm. Según ponía en el enlace, lo más probable es que sea /usr/lib/kdm4/bin/kdm.
Sustituyes esa ruta por /usr/sbin/gdm. Si no funciona, prueba con /etc/init.d/gdm.

Teóricamente con eso debería saber que debe arrancar el gdm y no el kdm.

En el fichero /etc/X11/default-display-manager me aparece:

GNU nano 2.0.9 Fichero: /etc/X11/default-display-manager

/usr/sbin/gdm


















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Aparece justo la ruta que me aconsejas introducir. ¿Debo cambiarla por "/etc/init.d/gdm" o crees que no se trata de eso? Lo siento por la inseguridad, pero no quiero meter más aún la pata.
Gracias por tu paciencia.

No entiendo por qué estás haciendo todo esto.

Inicia gnome. Aunque sea en modo a prueba de fallos.

Ahora cierra sesión, o presiona ctrl+alt+backspace (me dijeron que habían deshabilitado esto en ubuntu, no sé si es verdad).

Ahora antes de loguearte, vas a session, seleccionas gnome y escribes tu usuario y contraseña.

Cuando te pregunte, optas por "convertir en predeterminada".

El problema ya debería estar resuelto con eso.

Algo más, que podrías querer hacer, es
sudo update-alternatives .
Presiona Enter en las opciones en las que no estés seguro. Cuando te pregunte por el escritorio por defecto, selecciona gnome. Cuando te pregunte por el Xlogin por defecto, selecciona gdm. Eso configura lo que estás intentando a los golpes aquí arriba.

Saludos.

He hecho lo que me has dicho y el problema se ha solucionado. De una simpleza exquisita...
Muchas gracias por todo!