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LightDM Display Config Script

This is a guide to setup the lightdm display-setup-script option using custom script that uses xrandr or autorandr. This is for people who have multi monitor setup.

Method 1. With xrandr (most easy to use and setup)

Install requirements

sudo pacman -Syu
sudo pacman -S xorg-xrandr

1. Generate .xrandr.conf File Automatically

This script analyzes your current monitor layout using xrandr and creates a .xrandr.conf file in your home directory. The file will contain an xrandr command that can be used to restore your display setup automatically.

You can run this anytime you change your monitor layout to update the config.

#!/bin/bash

OUT="$HOME/.xrandr.conf"
>"$OUT"

XRANDR_CMD="xrandr"

while IFS= read -r line; do
  mon=$(echo "$line" | grep " connected" | awk '{print $1}')
  if [[ -n $mon ]]; then

    # Check if primary
    is_primary=false
    if [[ "$line" == *primary* ]]; then
      is_primary=true
    fi

    # Get resolution and position
    res_pos=$(echo "$line" | grep -o '[0-9]\+x[0-9]\++[0-9]\++[0-9]\+')

    if [[ -n $res_pos ]]; then
      # Read the next line (mode list)
      IFS= read -r next_line
      res=$(echo "$next_line" | awk '{print $1}')

      # Determine refresh rate
      if [[ "$next_line" == *" + "* ]]; then
        rate=$(echo "$next_line" | cut -d'+' -f2 | awk '{print $1}')
      else
        rate=$(echo "$next_line" | awk -v r="$res" '$1 == r {print $2}')
      fi
      rate=${rate//\*/}

      # Extract X and Y
      x_pos=$(echo "$res_pos" | cut -d'+' -f2)
      y_pos=$(echo "$res_pos" | cut -d'+' -f3)

      # Append monitor to xrandr command
      XRANDR_CMD+=" --output $mon --mode $res --rate $rate --pos ${x_pos}x${y_pos}"
      $is_primary && XRANDR_CMD+=" --primary"
    else
      # If disconnected or ignored, turn off
      XRANDR_CMD+=" --output $mon --off"
    fi
  fi
done < <(xrandr -q)

# Save to file
echo "$XRANDR_CMD" >"$OUT"

echo "Saved xrandr command to $OUT"

2. Add following command in your i3wm config or startup scripts

exec_always --no-startup-id "~/.config/i3/scripts/xrandr_command_generator.sh"

You can run the .xrandr.conf file automatically when your window manager (WM) starts. This file is created by the script above and contains xrandr commands to set up your display.

To make sure it runs every time you log in, add the following line to your WM's startup file.

For example, if you're using i3wm, add this line to your ~/.config/i3/config:

exec --no-startup-id bash "$HOME/.xrandr.conf"

This will run the display setup commands at startup.

🛠️ 3. Configure LightDM

To run your display setup script automatically when LightDM starts, follow these steps:

🔍 Step 1: Find the LightDM config file

Use this command to locate lightdm.conf:

sudo ls -al /etc/lightdm | grep 'lightdm.conf'

Step 2: Edit the config file

Open the file in a text editor like nano:

sudo nano /etc/lightdm/lightdm.conf

Find the section that looks like this:

[Seat:*]

In this section, look for the line that starts with display-setup-script=, and replace or add it as follows (remember to replace <your user name> with your actual username):

display-setup-script=/bin/bash -c '[ -f "/home/<your user name>/.xrandr.conf" ] && bash "/home/<your user name>/.xrandr.conf" || true'

✅ This checks if the .xrandr.conf file exists in your home directory and runs it. If the file doesn't exist or there's an error, it still returns true so LightDM won't break.

Having issues?

If the display script isn't working, try adjusting this line in the [LightDM] section of the same file:

logind-check-graphical=true

Try commenting it out (add # at the start of the line) or uncommenting it, depending on its current state. This can help in some setups.

Troubleshooting

If LightDM isn't working as expected, check the logs for errors:

View LightDM logs

sudo cat /var/log/lightdm/lightdm.log

Or check the previous log:

sudo cat /var/log/lightdm/lightdm.log.old

Check X session logs

sudo cat /var/log/lightdm/x-0.log

Or:

sudo cat /var/log/lightdm/x-0.log.old

Method 2: Using autorandr (Easy but needs extra setup)

Step 1: Install autorandr

Run the following commands to install autorandr:

sudo pacman -Syu
sudo pacman -S autorandr

Step 2: Enable and start the autorandr service

To use autorandr, enable and start it as your regular user:

systemctl --user enable autorandr
systemctl --user start autorandr

Or, if needed, enable it as root user (not common):

sudo systemctl enable autorandr
sudo systemctl start autorandr

Step 3: Create an autorandr profile

You can save your current monitor layout into a profile like this:

autorandr -s <profile-name> --force

Replace <profile-name> with something like home, office, etc.

This saves the display setup to:
~/.config/autorandr/<profile-name>/config

Step 4: Create a systemd service to sync the profile

This will auto-copy the saved profile to system space for LightDM to use.

Create a new systemd service file:

sudo nano /etc/systemd/system/autorandrlastUsed@.service

This version is a better then the last one as it works with shutdown, reboot and logout. It will also work if you have separate [/home] and [root] partitions.

Paste the below code:

[Unit]
Description=Copy autorandr lastUsed for %i on logout/shutdown/reboot
DefaultDependencies=no
# Make sure we run before /home unmounts and before shutdown completes
Before=umount.target shutdown.target reboot.target halt.target exit.target

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/bin/true
ExecStop=/usr/local/bin/copy-lastused.sh %i

[Install]
WantedBy=multi-user.target
WantedBy=halt.target
WantedBy=reboot.target
WantedBy=shutdown.target

or below is the first version so I have kept it. only works with shutdown and reboot only if the [/home] and [root] are under one single partition.

[Unit]
Description=Copy autorandr profile from %i user to LightDM dir
DefaultDependencies=no
Before=shutdown.target reboot.target halt.target
ConditionPathExists=/home/%i/.config/autorandr/<profile-name>

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/bin/true
ExecStop=/usr/local/bin/copy-lastused.sh %i
TimeoutStopSec=10

[Install]
WantedBy=multi-user.target

Now enable the service as following

systemctl enable autorandrlastUsed@<your user name>.service

Replace <profile-name> with your saved profile name.

Step 5: Create the copy-lastused.sh script

This script copies your saved profile to /etc/xdg/autorandr/.

Create the script:

sudo nano /usr/local/bin/copy-lastused.sh

Paste this into the file:

#!/bin/bash

USER="$1"
USER_HOME="/home/$USER"
LASTUSED="$USER_HOME/.config/autorandr/<profile-name>"
DEST_DIR="/etc/xdg/autorandr/"
# For better organization of logs
# LOG_FILE="/var/log/custom/monitors-copy.log"
# else will find logs in default place
LOG_FILE="/var/log/monitors-copy.log"
: >"$LOG_FILE"

log() {
  echo "[$(date '+%Y-%m-%d %H:%M:%S')] $1" >>"$LOG_FILE"
  # logger -t copy-lastused "$1"  # Optional system log
}

log "Running copy for: $USER"
log "Checking at: $LASTUSED"

if [[ -d "$LASTUSED" ]]; then
  mkdir -p "$DEST_DIR"
  if cp -r "$LASTUSED" "$DEST_DIR"; then
    log "Copied: $LASTUSED -> $DEST_DIR"
  else
    log "ERROR: Failed to copy"
  fi
else
  log "WARNING: Profile not found at: $LASTUSED"
fi

Make sure the folder /etc/xdg/autorandr/ exists, or make one:

sudo mkdir -r /etc/xdg/autorandr/

Also

Make sure the folder /var/log/custom/ exists if using the organised version:

LOG_FILE="/var/log/custom/monitors-copy.log"

Make the script executable:

chmod +x /usr/local/bin/copy-lastused.sh

Step 6: Add autorandr to LightDM

In your LightDM config file at /etc/lightdm/lightdm.conf,
find the line under [Seat:*] that starts with display-setup-script.

Replace or add this line:

display-setup-script=/bin/bash -c 'autorandr --load <profile-name> || true'

🔁 Replace <profile-name> with the name you saved earlier.

Optional: Link to LightDM setup

See the setup steps for LightDM: Go to LightDM config section️