mars

LightDM

Display manager.

The display manager is what runs first when we create a desktop environment. It manages the X servers, and provides a login prompt.

After we enter the credentials in the login prompt, it starts the display environment’s session manager. In our case, that’s xfce4-session.

From their home page - https://github.com/canonical/lightdm

LightDM is a lightweight, cross-desktop display manager. A display manager is a daemon that:

Runs display servers (e.g. X) where necessary.

Runs greeters to allow users to pick which user account and session type to use.

Allows greeters to perform authentication using PAM.

Runs session processes once authentication is complete.

Cross-desktop means that it supports different desktop technologies (X, Wayland, Mir, etc).

The core LightDM project does not provide any greeter with it. Popular options are:

LightDM GTK+ greeter

…

Run with no greeter (automatic login only)

Write your own…

cat /etc/os-release

NAME="Alpine Linux"
ID=alpine
VERSION_ID=3.21.3
PRETTY_NAME="Alpine Linux v3.21"
HOME_URL="https://alpinelinux.org/"
BUG_REPORT_URL="https://gitlab.alpinelinux.org/alpine/aports/-/issues"

cat /etc/alpine-release

3.21.3

For Alpine (currently),

The display server is X
The greeter is LightDM GTK+
The session process that LightDM will run once authentication completes is xfce4-session.

Configuration

Per the docs, this is the order in which config is read.

/usr/share/lightdm/lightdm.conf.d/*.conf
/etc/lightdm/lightdm.conf.d/*.conf
/etc/lightdm/lightdm.conf

find /usr/share/lightdm/lightdm.conf.d

find: /usr/share/lightdm/lightdm.conf.d: No such file or directory

find /etc/lightdm/lightdm.conf.d

find: /etc/lightdm/lightdm.conf.d: No such file or directory

wc -l /etc/lightdm/lightdm.conf

163 /etc/lightdm/lightdm.conf

ls /etc/lightdm

keys.conf                 lightdm.conf
lightdm-gtk-greeter.conf  users.conf

cat /etc/lightdm/*.conf | grep -v '^#'

[keyring]
[greeter]
[LightDM]

[Seat:*]

[XDMCPServer]

[VNCServer]
[UserList]
minimum-uid=500
hidden-users=nobody nobody4 noaccess
hidden-shells=/bin/false /usr/sbin/nologin /sbin/nologin

So looks like everything is at the defaults. From the docs

For example, if a sysadmin wanted to override the system configured default session they should make a file /etc/lightdm/lightdm.conf.d/50-myconfig.conf
[Seat:*]
user-session=mysession
Configuration is in keyfile format. For most installations you will want to change the keys in the [Seat:*] section as this applies to all seats on the system (normally just one).

More about seats: tty

How does lightdm know which greeter to use?

cat /etc/lightdm/lightdm.conf | grep greeters-dir

# greeters-directory = Directory to find greeters
#greeters-directory=$XDG_DATA_DIRS/lightdm/greeters:$XDG_DATA_DIRS/xgreeters

env | grep -e 'XDG'

XDG_CONFIG_DIRS=/etc/xdg
XDG_SESSION_PATH=/org/freedesktop/DisplayManager/Session0
XDG_MENU_PREFIX=xfce-
XDG_CONFIG_HOME=/home/m/.config
XDG_SEAT=seat0
XDG_SESSION_DESKTOP=xfce
XDG_SESSION_TYPE=x11
XDG_GREETER_DATA_DIR=/var/lib/lightdm-data/m
XDG_CURRENT_DESKTOP=XFCE
XDG_SEAT_PATH=/org/freedesktop/DisplayManager/Seat0
XDG_CACHE_HOME=/home/m/.cache
XDG_SESSION_CLASS=user
XDG_VTNR=7
XDG_SESSION_ID=c2
XDG_RUNTIME_DIR=/run/user/1000
XDG_DATA_DIRS=/usr/local/share:/usr/share

echo $XDG_DATA_DIRS | tr ':' '\n' | xargs -n 1 -I % ls %/xgreeters

ls: /usr/local/share/xgreeters: No such file or directory
lightdm-gtk-greeter.desktop

apk info --who-owns /usr/share/xgreeters/lightdm-gtk-greeter.desktop

/usr/share/xgreeters/lightdm-gtk-greeter.desktop is owned by lightdm-gtk-greeter-2.0.8-r3

So when I chose Xfce, /usr/sbin/setup-desktop installed both lightdm and lightdm-gtk-greeter, the latter installing this .desktop file which lightdm uses unconfigured (I presume because it’s the only one).

There is also a ~/.dmrc, but from what I gather, lightdm is the one which writes it for us, and going by the creation time, on first login.

cat ~/.dmrc

[Desktop]
Session=xfce

lightdm-gtk-greeter, on success, launches the only xsession it can find

cat /usr/share/xsessions/xfce.desktop | grep -v -e Name -e Comment -e Keywords

[Desktop Entry]
Version=1.0
Exec=startxfce4
Icon=
Type=Application

From Running Xfce^{docs.xfce.org}:

Running Xfce

Display managers: Xfce4-session installs a file that should add an option for display managers to run an Xfce session.

Command line: Use startxfce4 to start an Xfce session.

apk info --who-owns /usr/share/xsessions/xfce.desktop

/usr/share/xsessions/xfce.desktop is owned by xfce4-session-4.20.2-r0

Once startxfce4 is running, the ball is in Xfce’s court.

dm-tool

dm-tool can be used to communicate with lightdm.

dm-tool

Seat0
  CanSwitch=true
  HasGuestAccount=false
  Session2
    UserName='m'

Autologin

If running in a context with prior authentication, e.g. in a VM, it is convenient to autologin into the sole user account.

As described in the lightdm doc section quoted above,

sysadmins … make a file /etc/lightdm/lightdm.conf.d/50-myconfig.conf
[Seat:*]
user-session=mysession

cat /etc/lightdm/lightdm.conf | grep autologin-user

# autologin-user = User to log in with by default (overrides autologin-guest)
# autologin-user-timeout = Number of seconds to wait before loading default user
#autologin-user=
#autologin-user-timeout=0

install is handy to create the config file, creating the directory and also setting the permissions.

echo | install -D -m 644 /dev/stdin /etc/lightdm/lightdm.conf.d/50-autologin.conf <<EOF
[Seat:*]
autologin-user=$(whoami)
EOF

If something is not working, /var/log/lightdm.

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