bash: make the lk yaml templating via envsubst a bit less tedious

- introduce `hook_template_vars` dict (KEY=value)
  - with `HOOK_VERSION`, `HOOK_KERNEL_IMAGE` et al directly
- remove the nameref from `build_hook_linuxkit_container`, instead set directly in the `hook_template_vars` dict (`HOOK_CONTAINER_BOOTKIT_IMAGE` et al)
- derive the envsubst dollar-prefixed list and the actual vars from the dict
- before: refer to the variable 3 times (settage, export, dollar)
- after: add variable to dict (once)

Signed-off-by: Ricardo Pardini <[email protected]>

Author: rpardini

bash/hook-lk-containers.sh

@@ -7,17 +7,17 @@ function build_all_hook_linuxkit_containers() {
 	# when adding new container builds here you'll also want to add them to the
 	# `linuxkit_build` function in the linuxkit.sh file.
 	# # NOTE: linuxkit containers must be in the images/ directory
-	build_hook_linuxkit_container hook-bootkit HOOK_CONTAINER_BOOTKIT_IMAGE
-	build_hook_linuxkit_container hook-docker HOOK_CONTAINER_DOCKER_IMAGE
-	build_hook_linuxkit_container hook-mdev HOOK_CONTAINER_MDEV_IMAGE
-	build_hook_linuxkit_container hook-containerd HOOK_CONTAINER_CONTAINERD_IMAGE
-	build_hook_linuxkit_container hook-runc HOOK_CONTAINER_RUNC_IMAGE
-	build_hook_linuxkit_container hook-embedded HOOK_CONTAINER_EMBEDDED_IMAGE
+	build_hook_linuxkit_container hook-bootkit "HOOK_CONTAINER_BOOTKIT_IMAGE"
+	build_hook_linuxkit_container hook-docker "HOOK_CONTAINER_DOCKER_IMAGE"
+	build_hook_linuxkit_container hook-mdev "HOOK_CONTAINER_MDEV_IMAGE"
+	build_hook_linuxkit_container hook-containerd "HOOK_CONTAINER_CONTAINERD_IMAGE"
+	build_hook_linuxkit_container hook-runc "HOOK_CONTAINER_RUNC_IMAGE"
+	build_hook_linuxkit_container hook-embedded "HOOK_CONTAINER_EMBEDDED_IMAGE"
 }
 
 function build_hook_linuxkit_container() {
 	declare container_dir="${1}"
-	declare -n output_var="${2}" # bash name reference, kind of an output var but weird
+	declare template_var="${2}" # bash name reference, kind of an output var but weird
 	declare container_base_dir="images"
 
 	# Lets hash the contents of the directory and use that as a tag
@@ -28,8 +28,7 @@ function build_hook_linuxkit_container() {
 
 	declare container_oci_ref="${HOOK_LK_CONTAINERS_OCI_BASE}${container_dir}:${container_files_hash_short}-${DOCKER_ARCH}"
 	log info "Consider building LK container ${container_oci_ref} from ${container_base_dir}/${container_dir} for platform ${DOCKER_ARCH}"
-	output_var="${container_oci_ref}" # the the name reference
-	echo "${output_var}" > /dev/null  # no-op; just to avoid shellcheck SC2034 (unused var; but it is actually a bash nameref)
+	hook_template_vars["${template_var}"]="${container_oci_ref}" # set the template var for envsubst
 
 	# If the image is in the local docker cache, skip building
 	log debug "Checking if image ${container_oci_ref} exists in local registry"

bash/linuxkit.sh

@@ -49,29 +49,37 @@ function linuxkit_build() {
 		fi
 	fi
 
+	# A dictionary (bash associative array) with variables and their values, for templating using envsubst.
+	declare -A -g hook_template_vars=(
+		["HOOK_VERSION"]="${HOOK_VERSION}"
+		["HOOK_KERNEL_IMAGE"]="${kernel_oci_image}"
+		["HOOK_KERNEL_ID"]="${inventory_id}"
+		["HOOK_KERNEL_VERSION"]="${kernel_oci_version}"
+	)
+
 	# Build the containers in this repo used in the LinuxKit YAML;
-	build_all_hook_linuxkit_containers # sets HOOK_CONTAINER_BOOTKIT_IMAGE, HOOK_CONTAINER_DOCKER_IMAGE, HOOK_CONTAINER_MDEV_IMAGE, HOOK_CONTAINER_CONTAINERD_IMAGE
+	build_all_hook_linuxkit_containers # sets HOOK_CONTAINER_BOOTKIT_IMAGE, HOOK_CONTAINER_DOCKER_IMAGE and others in the hook_template_vars dict
 
 	# Template the linuxkit configuration file.
 	# - You'd think linuxkit would take --build-args or something by now, but no.
 	# - Linuxkit does have @pkg but that's only useful in its own repo (with pkgs/ dir)
 	# - envsubst doesn't offer a good way to escape $ in the input, so we pass the exact list of vars to consider, so escaping is not needed
-
 	log info "Using Linuxkit template '${kernel_info['TEMPLATE']}'..."
 
-	# HOOK_VERSION is read-only & already exported so is not listed in the env vars here, but is included in the dollar-sign list for envsubst to process
-	# shellcheck disable=SC2002 # Again, no, I love my cat, leave me alone
-	# shellcheck disable=SC2016 # I'm using single quotes to avoid shell expansion, envsubst wants the dollar signs.
-	cat "linuxkit-templates/${kernel_info['TEMPLATE']}.template.yaml" |
-		HOOK_KERNEL_IMAGE="${kernel_oci_image}" HOOK_KERNEL_ID="${inventory_id}" HOOK_KERNEL_VERSION="${kernel_oci_version}" \
-			HOOK_CONTAINER_BOOTKIT_IMAGE="${HOOK_CONTAINER_BOOTKIT_IMAGE}" \
-			HOOK_CONTAINER_DOCKER_IMAGE="${HOOK_CONTAINER_DOCKER_IMAGE}" \
-			HOOK_CONTAINER_MDEV_IMAGE="${HOOK_CONTAINER_MDEV_IMAGE}" \
-			HOOK_CONTAINER_CONTAINERD_IMAGE="${HOOK_CONTAINER_CONTAINERD_IMAGE}" \
-			HOOK_CONTAINER_RUNC_IMAGE="${HOOK_CONTAINER_RUNC_IMAGE}" \
-			HOOK_CONTAINER_EMBEDDED_IMAGE="${HOOK_CONTAINER_EMBEDDED_IMAGE}" \
-			envsubst '$HOOK_VERSION $HOOK_KERNEL_IMAGE $HOOK_KERNEL_ID $HOOK_KERNEL_VERSION $HOOK_CONTAINER_BOOTKIT_IMAGE $HOOK_CONTAINER_DOCKER_IMAGE $HOOK_CONTAINER_MDEV_IMAGE $HOOK_CONTAINER_CONTAINERD_IMAGE $HOOK_CONTAINER_RUNC_IMAGE $HOOK_CONTAINER_EMBEDDED_IMAGE' \
-			> "hook.${inventory_id}.yaml"
+	# Calculate, from hook_template_vars dictionary:
+	# envsubst_arg_string: a space separated list of dollar-prefixed variables name to be substituted
+	# envusbst_env: the environment variables to be passed to envsubst (array of KEY=var) to be used via 'env'
+	declare envsubst_arg_string=""
+	declare -a envsubst_envs=()
+	for key in "${!hook_template_vars[@]}"; do
+		envsubst_arg_string+="\$${key} " # extra space at the end doesn't hurt
+		envsubst_envs+=("${key}=${hook_template_vars["${key}"]}")
+	done
+	log debug "envsubst_arg_string: ${envsubst_arg_string}"
+	log debug "envsubst_envs: ${envsubst_envs[*]}"
+
+	# Run envsubst on the template file, output to a new file; pass the envs and the arg string
+	env "${envsubst_envs[@]}" envsubst "${envsubst_arg_string}" < "linuxkit-templates/${kernel_info['TEMPLATE']}.template.yaml" > "hook.${inventory_id}.yaml"
 
 	declare -g linuxkit_bin=""
 	obtain_linuxkit_binary_cached # sets "${linuxkit_bin}"
@@ -101,7 +109,7 @@ function linuxkit_build() {
 		"${linuxkit_bin}" build "${lk_iso_args[@]}"
 		return 0
 	fi
-	
+
 	declare -a lk_args=(
 		"--docker"
 		"--arch" "${kernel_info['DOCKER_ARCH']}"