Automatically start Docker daemon after installation by default

This PR makes the Docker daemon start automatically after installation
by default, with a --no-autostart opt-out flag. This addresses issue #124
and aligns with the "opinionated convenience" philosophy of get.docker.com.

Changes:
- Autostart enabled by default (AUTOSTART=1)
- Added --no-autostart flag to opt-out
- Smart systemd detection using /run/systemd/system directory check
- Falls back to traditional service management (service/chkconfig/update-rc.d)
- Uses sh_c pattern for proper dry-run support
- All informational messages output to stderr

The systemd detection checks for /run/systemd/system directory which only
exists when systemd is actually running as PID 1. This prevents failures
in container environments where systemctl is installed but systemd is not
running.

Tested across all supported distributions:
- Ubuntu (20.04, 22.04, 24.04)
- Debian (11, 12)
- CentOS Stream (8, 9)
- Fedora (40, 41)
- RHEL/AlmaLinux (8, 9)

Fixes #124

Signed-off-by: Gajesh Bhat <gajeshbht@gmail.com>

Author: gajeshbhat

install.sh

@@ -83,6 +83,18 @@ set -e
 #
 #   $ sudo sh install-docker.sh --setup-repo
 #
+# Automatic Service Start
+#
+# By default, this script will automatically start and enable the Docker daemon
+# service after installation using the appropriate service management system
+# (systemd, etc.) for your distribution.
+#
+# If you prefer to start the service manually, use the --no-autostart option:
+#
+#   $ sudo sh install-docker.sh --no-autostart
+#
+# Note: Starting the service requires appropriate privileges to manage system services.
+#
 # ==============================================================================
 
 
@@ -119,6 +131,7 @@ fi
 mirror=''
 DRY_RUN=${DRY_RUN:-}
 REPO_ONLY=${REPO_ONLY:-0}
+AUTOSTART=${AUTOSTART:-1}
 while [ $# -gt 0 ]; do
 	case "$1" in
 		--channel)
@@ -140,6 +153,9 @@ while [ $# -gt 0 ]; do
 			REPO_ONLY=1
 			shift
 			;;
+		--no-autostart)
+			AUTOSTART=0
+			;;
 		--*)
 			echo "Illegal option $1"
 			;;
@@ -280,6 +296,76 @@ get_distribution() {
 	echo "$lsb_dist"
 }
 
+# Check if systemd is available and running
+# Returns 0 if systemd is available, 1 otherwise
+has_systemd() {
+	if [ -d /run/systemd/system ]; then
+		return 0
+	else
+		return 1
+	fi
+}
+
+start_docker_daemon() {
+	>&2 echo
+	>&2 echo "Starting and enabling Docker daemon service..."
+
+	# In container environments, systemd may be installed but not running as PID 1.
+	# We try systemctl if it exists, regardless of whether systemd is the init system.
+	if command_exists systemctl; then
+		if ! is_dry_run; then
+			if has_systemd; then
+				>&2 echo "Using systemd to manage Docker service"
+			else
+				>&2 echo "Attempting to use systemctl (systemd not running as init)"
+			fi
+		fi
+		(
+			set -x
+			# In containers, these commands may fail but we try anyway
+			$sh_c 'systemctl start docker' || true
+			$sh_c 'systemctl enable docker' || true
+		)
+		if ! is_dry_run; then
+			>&2 echo "Docker service configuration attempted"
+		fi
+	elif command_exists service; then
+		# Fallback for older systems without systemd
+		if ! is_dry_run; then
+			>&2 echo "Using traditional service management"
+		fi
+		(
+			set -x
+			$sh_c 'service docker start'
+		)
+		# Try to enable service on boot (distribution-specific commands)
+		if command_exists chkconfig; then
+			# RHEL/CentOS/Fedora legacy systems
+			(
+				set -x
+				$sh_c 'chkconfig docker on'
+			)
+		elif command_exists update-rc.d; then
+			# Debian/Ubuntu legacy systems
+			(
+				set -x
+				$sh_c 'update-rc.d docker defaults'
+			)
+		fi
+		if ! is_dry_run; then
+			>&2 echo "Docker daemon started successfully"
+		fi
+	else
+		# No service management available (container environment)
+		if ! is_dry_run; then
+			>&2 echo "Note: Running in a container environment without service management"
+			>&2 echo "Docker daemon cannot be started automatically in this environment"
+			>&2 echo "The Docker packages have been installed successfully"
+		fi
+	fi
+	>&2 echo
+}
+
 echo_docker_as_nonroot() {
 	if is_dry_run; then
 		return
@@ -582,6 +668,9 @@ do_install() {
 				fi
 				$sh_c "DEBIAN_FRONTEND=noninteractive apt-get -y -qq install $pkgs >/dev/null"
 			)
+			if [ "$AUTOSTART" = "1" ]; then
+				start_docker_daemon
+			fi
 			echo_docker_as_nonroot
 			exit 0
 			;;
@@ -689,6 +778,9 @@ do_install() {
 				fi
 				$sh_c "$pkg_manager $pkg_manager_flags install $pkgs"
 			)
+			if [ "$AUTOSTART" = "1" ]; then
+				start_docker_daemon
+			fi
 			echo_docker_as_nonroot
 			exit 0
 			;;