Add a script to automate the host creation via templates

The script scripts/mkHost.sh automates the generation of a new host,
based on a template file. This template looks like a .nix file with
placeholder \$KEY fields that will be populated dynamically such as
name, site, user, etc.

Author: inverted-tree

scripts/mkHost.sh

@@ -0,0 +1,235 @@
+#! /usr/bin/env bash
+
+set -euo pipefail
+
+usage() {
+	cat <<'EOF'
+Usage:
+    nix run .#mkHost -- <name> <sitename> <username> [--system <x86_64-linux|aarch64-linux>] [--help] [--template <path>] [KEY=VALUE ...]
+
+Arguments:
+    <name>      Hostname / machine name (e.g. alfa, bravo, ...)
+    <sitename>  Logical site / location name (e.g. theta, ...)
+    <username>  User name for the admin user of the new machine
+
+Options:
+    -s, --system     Override detected system (default: based on uname -m)
+                     Allowed: x86_64-linux, aarch64-linux
+    -t, --template   Path to a template file to use instead of /etc/nixos/templates/hosts/default.tmpl
+    -h, --help       Show this help
+
+Extra template variables:
+    Any additional arguments of the form KEY=VALUE after the options will be
+    exported to the environment before rendering the template, so they can be
+    used as \$KEY placeholders inside the template.
+EOF
+}
+
+info() {
+	printf "\e[1;32m[INFO]\e[0m %s\n" "$1"
+}
+
+error() {
+	printf "\e[1;31m[ERROR]\e[0m %s\n\n" "$1" >&2
+	usage >&2
+	exit 1
+}
+
+interaction() {
+	printf "\e[1;33m[ACTION]\e[0m %s" "$1"
+}
+
+prompt() {
+	local msg="$1"
+	interaction "$msg [Y/n] "
+	read -r answer
+
+	case "${answer:-Y}" in
+	[Yy]*)
+		return 0
+		;;
+	*)
+		return 1
+		;;
+	esac
+}
+
+if [[ $# -eq 0 ]]; then
+	usage
+	exit 0
+fi
+
+if [[ "${1:-}" == "-h" || "${1:-}" == "--help" ]]; then
+	usage
+	exit 0
+fi
+
+ARCH="$(uname -m)"
+case "$ARCH" in
+x86_64)
+	SYSTEM="x86_64-linux"
+	;;
+aarch64 | arm64)
+	SYSTEM="aarch64-linux"
+	;;
+*)
+	SYSTEM=""
+	;;
+esac
+
+if [[ $# -lt 3 ]]; then
+	error "missing required arguments <name> <sitename> <username>."
+fi
+
+NAME="$1"
+SITE="$2"
+USER="$3"
+shift 3
+
+if [[ ! "$NAME" =~ ^[a-zA-Z0-9._-]+$ ]]; then
+	error "invalid host name '$NAME'.
+        Allowed characters: letters, numbers, dot, underscore, dash."
+fi
+
+if [[ ! "$SITE" =~ ^[a-zA-Z0-9._-]+$ ]]; then
+	error "invalid site name '$SITE'.
+        Allowed characters: letters, numbers, dot, underscore, dash."
+fi
+
+if [[ ! "$USER" =~ ^[a-z_][a-z0-9_-]*$ ]]; then
+	error "invalid username '$USER'.
+Allowed pattern: ^[a-z_][a-z0-9_-]*$ (like normal Unix usernames)."
+fi
+
+BASE_DIR="/etc/nixos"
+HOSTS_DIR="$BASE_DIR/hosts"
+TARGET_DIR="${HOSTS_DIR}/${NAME}"
+TEMPLATE_FILE="$BASE_DIR/templates/hosts/default.tmpl"
+EXTRA_VARS=()
+
+while [[ $# -gt 0 ]]; do
+	case "$1" in
+	-h | --help)
+		usage
+		exit 0
+		;;
+	-s | --system)
+		if [[ $# -lt 2 ]]; then
+			error "--system requires an argument."
+		fi
+		SYSTEM="$2"
+		case "$SYSTEM" in
+		x86_64-linux | aarch64-linux) ;;
+		*)
+			error "invalid --system value '$SYSTEM'.
+        Allowed: x86_64-linux, aarch64-linux"
+			;;
+		esac
+		shift 2
+		;;
+	-t | --template)
+		if [[ $# -lt 2 ]]; then
+			error "--template requires a path argument."
+		fi
+		TEMPLATE_FILE="$2"
+		shift 2
+		;;
+	-* | --*)
+		error "unknown option '$1'."
+		;;
+	*)
+		if [[ "$1" == *=* ]]; then
+			key="${1%%=*}"
+
+			case "$key" in
+			NAME | SITE | USER | ARCH | SYSTEM | STATE_VERSION | BASE_DIR | HOSTS_DIR | TARGET_DIR | TEMPLATE_FILE | TARGET_FILE | EXTRA_VARS)
+				error "extra variable '$key' is reserved and cannot be overridden."
+				;;
+			esac
+
+			EXTRA_VARS+=("$1")
+			shift
+		else
+			error "unexpected positional argument '$1' after <name> <sitename> <username>."
+		fi
+		;;
+	esac
+done
+
+if [[ -z "${SYSTEM:-}" ]]; then
+	error "unsupported architecture '$ARCH' from uname -m.
+        Please specify --system x86_64-linux or aarch64-linux explicitly."
+fi
+
+if [[ -d "$BASE_DIR" ]]; then
+	if [[ ! -d "$HOSTS_DIR" ]]; then
+		info "creating host directory '$HOSTS_DIR'..."
+		mkdir -p "$HOSTS_DIR"
+	fi
+else
+	error "this script must be run on a NixOS system (with '$BASE_DIR' available)."
+fi
+
+if [[ -e "$TARGET_DIR" ]]; then
+	error "host directory '$TARGET_DIR' already exists."
+fi
+
+mkdir -p "$TARGET_DIR"
+
+if ! command -v nixos-generate-config >/dev/null 2>&1; then
+	error "nixos-generate-config not found in PATH.
+        This script must be run on a NixOS system (or with nixos-generate-config available)."
+fi
+
+info "generating hardware configuration for $NAME..."
+nixos-generate-config --show-hardware-config >"${TARGET_DIR}/hardware.nix"
+
+TARGET_FILE="$TARGET_DIR/default.nix"
+
+if [[ ! -f "$TEMPLATE_FILE" ]]; then
+	error "template file '$TEMPLATE_FILE' not found."
+fi
+
+STATE_VERSION=
+if [[ -z "$STATE_VERSION" ]]; then
+	if command -v nixos-option >/dev/null 2>&1; then
+		v="$(nixos-option system.stateVersion 2>/dev/null |
+			sed -nE 's/^[[:space:]]*"([^"]+)".*/\1/p' |
+			head -n1)"
+		if [[ -n "$v" && "$v" != "null" ]]; then
+			STATE_VERSION="$v"
+		fi
+	fi
+fi
+
+if [[ -z "$STATE_VERSION" ]]; then
+	if command -v nixos-version >/dev/null 2>&1; then
+		v="$(nixos-version | sed -E 's/^([0-9]+\.[0-9]+).*/\1/')"
+		if [[ -n "$v" ]]; then
+			STATE_VERSION="$v"
+		fi
+	fi
+fi
+
+if [[ -z "$STATE_VERSION" ]]; then
+	error "Could not determine system.stateVersion automatically.
+        Please set it manually or ensure nixos-option / nixos-version are available."
+fi
+
+for kv in "${EXTRA_VARS[@]}"; do
+	export "$kv"
+done
+export NAME SITE USER STATE_VERSION
+
+if [[ "$TEMPLATE_FILE" == "$BASE_DIR/templates/hosts/default.tmpl" ]]; then
+	info "generating configuration for $NAME from default template..."
+else
+	info "generating configuration for $NAME from template '$TEMPLATE_FILE'..."
+fi
+envsubst <"$TEMPLATE_FILE" >"$TARGET_FILE"
+
+if prompt "successfully generated a new config for $NAME. Do you want to edit it now?"; then
+	"${EDITOR:-vi}" "$TARGET_FILE"
+else
+	info "skipping edit."
+fi