Fixed “Failed to start Light Display Manager“ on Ubuntu

Environment:

Ubuntu desktop 20.04.5 LTS

Issue:

Failed to start Light Display Manager, then can't successfully boot desktop system

Logs:

...

[ OK ] Started Hostname Service.

Starting Light Display Manager...

[FAILED] Failed to start Light Display Manager.

See 'systemctl status lightdm.service' for details.

[2022/11/29 20:31:13] (base) zyx@Opt-7080:~$ sudo systemctl status lightdm.service

[2022/11/29 20:31:15] ● lightdm.service - Light Display Manager

[2022/11/29 20:31:15] Loaded: loaded (/lib/systemd/system/lightdm.service; indirect; vendor preset: enabled)

[2022/11/29 20:31:15] Active: failed (Result: exit-code) since Tue 2022-11-29 20:25:40 CST; 3min ago

[2022/11/29 20:31:15] Docs: man:lightdm(1)

[2022/11/29 20:31:15] Process: 1345 ExecStartPre=/bin/sh -c [ "$(basename $(cat /etc/X11/default-display-manager 2>/dev/null))" = "lightdisplay-manager 2>/dev/null))" = "lightdm" ] (code=exited, status=0/SUCCESS)

[2022/11/29 20:31:15] Process: 1349 ExecStart=/usr/sbin/lightdm (code=exited, status=1/FAILURE)

[2022/11/29 20:31:15] Main PID: 1349 (code=exited, status=1/FAILURE)

[2022/11/29 20:31:15]

[2022/11/29 20:31:15] 11月 29 20:25:40 Opt-7080 systemd[1]: lightdm.service: Scheduled restart job, restart counter is at 5.

[2022/11/29 20:31:15] 11月 29 20:25:40 Opt-7080 systemd[1]: Stopped Light Display Manager.

[2022/11/29 20:31:15] 11月 29 20:25:40 Opt-7080 systemd[1]: lightdm.service: Start request repeated too quickly.

[2022/11/29 20:31:15] 11月 29 20:25:40 Opt-7080 systemd[1]: lightdm.service: Failed with result 'exit.

[2022/11/29 20:31:15] 11月 29 20:25:40 Opt-7080 systemd[1]: Failed to start Light Display Manager.

/var/log/lightdm/lightdm.log

[2022/11/29 20:32:55] [+0.00s] DEBUG: Logging to /var/log/lightdm/lightdm.log

[2022/11/29 20:32:55] [+0.00s] DEBUG: Starting Light Display Manager 1.30.0, UID=0 PID=1349

[2022/11/29 20:32:55] [+0.00s] DEBUG: Loading configuration dirs from /usr/share/lightdm/lightdm.conf.d

[2022/11/29 20:32:55] [+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-disable-guest.conf

[2022/11/29 20:32:55] [+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-disable-log-backup.conf

[2022/11/29 20:32:55] [+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-greeter-wrapper.conf

[2022/11/29 20:32:55] [+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-guest-wrapper.conf

[2022/11/29 20:32:55] [+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-xserver-command.conf

[2022/11/29 20:32:55] [+0.00s] DEBUG: Loading configuration dirs from /usr/local/share/lightdm/lightdm.conf.d

[2022/11/29 20:32:55] [+0.00s] DEBUG: Loading configuration dirs from /etc/xdg/lightdm/lightdm.conf.d

[2022/11/29 20:32:55] [+0.00s] DEBUG: Loading configuration from /etc/lightdm/lightdm.conf

[2022/11/29 20:32:55] [+0.00s] DEBUG: Registered seat module local

[2022/11/29 20:32:55] [+0.00s] DEBUG: Registered seat module xremote

[2022/11/29 20:32:55] [+0.00s] DEBUG: Using D-Bus name org.freedesktop.DisplayManager

[2022/11/29 20:32:55] [+0.00s] DEBUG: _g_io_module_get_default: Found default implementation local (GLocalVfs) for ‘gio-vfs’

[2022/11/29 20:32:55] [+0.00s] DEBUG: Monitoring logind for seats

[2022/11/29 20:32:55] [+0.00s] DEBUG: New seat added from logind: seat0

[2022/11/29 20:32:55] [+0.00s] DEBUG: Seat seat0: Loading properties from config section Seat:*

[2022/11/29 20:32:55] [+0.00s] DEBUG: Seat seat0: Starting

[2022/11/29 20:32:55] [+0.00s] DEBUG: Seat seat0: Creating greeter session

[2022/11/29 20:32:55] [+0.00s] DEBUG: Seat seat0: Failed to find session configuration default

[2022/11/29 20:32:55] [+0.00s] DEBUG: Seat seat0: Failed to create greeter session

[2022/11/29 20:32:55] [+0.00s] DEBUG: Failed to start seat: seat0

Resolved:

1. If still can login Ubuntu system via ssh, please skip this step. Or, only can reboot system and make it boot from text mode, please follow below steps:

$ sudo reboot

When see the grub menu, press 'c' to edit command-line, then replace "quiet splash" with "text". Finally press Ctrl-x or F10 to boot.

If don't see the grub menu during restarting, maybe the grub menu is hidden. Please press Ctrl+s to enter the grub menu (on Dell OptiPlex 5090, I used Esc to enter the grub menu) when restarting.

2. Uninstall and reinstall lightdm service, please follow below steps:

1) Check if this file /etc/apt/sources.list is right for Ubuntu 20.04.5 LTS, focal.

$ grep 'focal' /etc/apt/sources.list

2) Run below commands:

$ sudo apt remove --purge lightdm

$ sudo apt update

$ sudo apt install lightdm

During installation, maybe there's going to be a dialogue box, please press enter by default.

Finally system reboot is required, it should successfully start Light Display Manager.

$ sudo reboot