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 <[email protected]>

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 $1"
 			;;
@@ -280,6 +296,69 @@ get_distribution() {
 	echo "$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..."
+
+	if has_systemd; then
+		# Use systemd for modern distributions (most current Linux distributions)
+		if ! is_dry_run; then
+			>&2 echo "Using systemd to manage Docker service"
+		fi
+		(
+			set -x
+			$sh_c 'systemctl start docker'
+			$sh_c 'systemctl enable docker'
+		)
+		if ! is_dry_run; then
+			>&2 echo "Docker daemon started and enabled successfully"
+		fi
+	else
+		# Fallback for older systems without systemd (legacy init systems)
+		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'
+			)
+		else
+			# Unknown init system - warn the user
+			if ! is_dry_run; then
+				>&2 echo "Warning: Could not enable Docker service to start on boot"
+				>&2 echo "Please manually configure Docker to start on boot for your system"
+			fi
+		fi
+		if ! is_dry_run; then
+			>&2 echo "Docker daemon started successfully"
+		fi
+	fi
+	>&2 echo
+}
+
 echo_docker_as_nonroot() {
 	if is_dry_run; then
 		return
@@ -582,6 +661,9 @@ 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 +771,9 @@ 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
 			;;