Adding an option to fail upon unset env variables

.gitignore

@@ -4,3 +4,4 @@ diagnostic.test
 tests/*.diff
 spec/
 spec-runner/
+node_modules/

mo

@@ -13,6 +13,7 @@
 #/
 #/    mo [--false] [--help] [--source=FILE] filenames...
 #/
+#/ --fail-not-set - Fail upon expansion of an unset variable.
 #/ --false        - Treat the string "false" as empty for conditionals.
 #/ --help         - This message.
 #/ --source=FILE  - Load FILE into the environment before processing templates.
@@ -28,6 +29,8 @@
 # Public: Template parser function.  Writes templates to stdout.
 #
 # $0             - Name of the mo file, used for getting the help message.
+# --fail-not-set - Fail upon expansion of an unset variable.  Default behavior
+#                  is to silently ignore and expand into empty string.
 # --false        - Treat "false" as an empty value.  You may set the
 #                  MO_FALSE_IS_EMPTY environment variable instead to a non-empty
 #                  value to enable this behavior.
@@ -40,6 +43,8 @@
 #
 # Mo uses the following environment variables:
 #
+# MO_FAIL_ON_UNSET    - When set to a non-empty value, expansion of an unset
+#                       env variable will be aborted with an error.
 # MO_FALSE_IS_EMPTY   - When set to a non-empty value, the string "false"
 #                       will be treated as an empty value for the purposes
 #                       of conditionals.
@@ -69,6 +74,11 @@ mo() (
                         exit 0
                         ;;
 
+                    --fail-not-set)
+                        # shellcheck disable=SC2030
+                        MO_FAIL_ON_UNSET=true
+                        ;;
+
                     --false)
                         # shellcheck disable=SC2030
                         MO_FALSE_IS_EMPTY=true
@@ -746,7 +756,13 @@ moShow() {
             eval moJoin moJoined "," "\${$1[@]}"
             echo -n "$moJoined"
         else
+            # shellcheck disable=SC2031
+            if [[ -z "$MO_FAIL_ON_UNSET" ]] || moTest "$1"; then
                 echo -n "${!1}"
+            else
+                echo "Env variable not set: $1" >&2
+                exit 1
+            fi
         fi
     else
         # Further subindexes are disallowed

tests/fail-not-set.expected

@@ -0,0 +1 @@
+This will fail: Env variable not set: __NO_SUCH_VAR

tests/fail-not-set.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+
+cd "${0%/*}"
+unset __NO_SUCH_VAR
+echo "This will fail: {{__NO_SUCH_VAR}}" | ../mo --fail-not-set 2>&1
+
+if [[ $? -ne 1 ]]; then
+    echo "Did not return 1"
+fi

tests/help.expected

@@ -11,6 +11,7 @@ Simple usage:
 
    mo [--false] [--help] [--source=FILE] filenames...
 
+--fail-not-set - Fail upon expansion of an unset variable.
 --false        - Treat the string "false" as empty for conditionals.
 --help         - This message.
 --source=FILE  - Load FILE into the environment before processing templates.