replace-lightdm-with-gdm.sh

#!/usr/bin/env bash
# Script that replaces LightDM display manager with GDM one.

# You need root permissions to run this script.
if [[ "${UID}" != '0' ]]; then
    echo '> You need to become root to run this script.'
    echo '> Aborting.'
    exit 1
fi

# Function that checks if required binary exists and installs it if necessary.
ENSURE_PACKAGE () {
    REQUIRED_BINARY=$(basename "${1}")
    REPO_PACKAGES="${*:2}"

    if [[ "${REQUIRED_BINARY}" != '-' ]]; then
        [[ -n "${REPO_PACKAGES}" ]] || REPO_PACKAGES="${REQUIRED_BINARY}"

        if command -v "${REQUIRED_BINARY}" 1> /dev/null; then
            REPO_PACKAGES=''
        fi
    fi

    [[ -n "${REPO_PACKAGES}" ]] || return

    if [[ "${REPO_REFRESHED}" == '0' ]]; then
        echo '> Refreshing package repository.'
        dnf check-update 1> /dev/null
        REPO_REFRESHED=1
    fi

    for REPO_PACKAGE in ${REPO_PACKAGES}
    do
        dnf install -y "${REPO_PACKAGE}"
    done
}

# Variable that keeps track if repository is already refreshed.
REPO_REFRESHED=0

# Install packages.
ENSURE_PACKAGE 'gdm'

# Disable LightDM.
systemctl disable lightdm

# Enable GDM.

# Block that perform replacing display manager.
if ! systemctl enable gdm; then
    echo '> Replacing LightDM with GDM failed.'
    echo '> Re-enabling LightDM and removing GDM package.'
    
    systemctl disable gdm
    systemctl enable lightdm
    dnf remove -y gdm
fi

# Let user know that script has finished its job.
echo '> Finished.'