feat: make template a configurable command

Instead of taking a template string, delegate the initial content
creation to a configurable command. This allows for more powerful
configuration like carrying over old tasks.

agenda

@@ -25,18 +25,31 @@ AGENDA_EDITOR=${VISUAL:-${EDITOR:-vi}}
 AGENDA_DIR=${AGENDA_DIR:-}
 
 # The default `-e` option.
-AGENDA_EXT=${AGENDA_EXT:-"md"}
+AGENDA_EXTENSION=${AGENDA_EXTENSION:-"md"}
 
 # The default name if the `-t` option is omitted.
 AGENDA_DEFAULT=${AGENDA_DEFAULT:-"agenda"}
 
-# A template that is automatically written to any created agenda file (excluding
-# backlogs). A few magic strings are replaced:
+# Command that provides the initial contents of new agenda files.
 #
-# - `{date}` The agenda date (`%Y-%m-%d`)
-# - `{name}` The agenda name
-# - `{slug}` The agenda name (slugged)
-AGENDA_TMPL="${AGENDA_TMPL:-"# {name} {date}\\n\\n- [ ] "}"
+# The value is executed as a single binary, meaning that it is not possible to
+# define any arguments for the given value. The command has access to the following
+# env vars:
+#
+# - `AGENDA_DATE`
+#   The date of the agenda that will be created
+# - `AGENDA_NAME`
+#   The agenda name of the agenda file that will be created.
+# - `AGENDA_SLUG`
+#   The slugged AGENDA_NAME.
+# - `AGENDA_FILE`
+#   The agenda file that will be created.
+# - `AGENDA_LAST`
+#   The last agenda file dated previous to AGENDA_FILE if one exists.
+# - `AGENDA_NEXT`
+#   The next agenda file dated next to AGENDA_FILE if one exists.
+#
+AGENDA_TEMPLATE=${AGENDA_TEMPLATE:-"__agenda_default_template"}
 
 __usage() {
 	fold -w <<-USAGE
@@ -106,7 +119,7 @@ __edit() {
 	opt_b="no"
 	opt_c="no"
 	opt_E="no"
-	opt_e="$AGENDA_EXT"
+	opt_e="$AGENDA_EXTENSION"
 	opt_t="$AGENDA_DEFAULT"
 	while getopts 'bcEe:t:' opt; do
 		case $opt in
@@ -310,18 +323,67 @@ __agenda_create() {
 		return 1
 	fi
 
-	local content
+	trap 'rm -f "${tmp:-}" "${tmp_err:-}"; trap - RETURN' RETURN
+	local tmp tmp_err
+	tmp=$(mktemp)
+	tmp_err=$(mktemp)
+
 	if [ "$backlog" = "no" ]; then
-		content=$(echo "$AGENDA_TMPL" \
-			| sed "s/{date}/$date/g" \
-			| sed "s/{name}/$name/g" \
-			| sed "s/{slug}/$slug/g")
+		local last
+		while read -r REPLY; do
+			if [ "$REPLY" = "$file" ]; then
+				break
+			else
+				last=$REPLY
+			fi
+		done < <(
+			{
+				echo "$file"
+				find "$AGENDA_DIR/$name" -type f
+			} | sort
+		)
+
+		# Capture exit code of the template command.
+		local code
+		set +e
+		AGENDA_DATE=$date AGENDA_NAME=$name AGENDA_SLUG=$slug \
+		AGENDA_FILE=$file AGENDA_LAST=${last:-} \
+			"$AGENDA_TEMPLATE" > "$tmp" 2> "$tmp_err"
+		code=$?
+		set -e
+
+		if [ $code -eq 127 ]; then
+			echo "${0##*/}: cannot create '$file': template '$AGENDA_TEMPLATE' does not exist" >&2
+			return 127
+		fi
+
+		if [ -s "$tmp_err" ]; then
+			while read -r REPLY; do
+				printf "agenda: $AGENDA_TEMPLATE: %s\n" "${REPLY#"${0}: line"*": "}" >&2
+			done < "$tmp_err"
+		fi
+
+		if [ $code -gt 0 ]; then
+			echo "${0##*/}: cannot create '$file': template error" >&2
+			return $code
+		fi
 	fi
 
 	mkdir -p "${file%/*}"
+	mv "$tmp" "$file"
+}
 
-	# shellcheck disable=SC2059
-	printf "${content:-}" > "$file"
+__agenda_default_template() {
+	case $AGENDA_EXTENSION in
+		"md")
+			printf "# %s %s\n\n- [ ] " "$AGENDA_NAME" "$AGENDA_DATE"
+			;;
+		"txt"|"xit"|"")
+			printf "%s %s\n[ ] " "$AGENDA_NAME" "$AGENDA_DATE"
+			;;
+		*)
+			;;
+	esac
 }
 
 # https://stackoverflow.com/a/49035906