Atfile environment variables with default values

nb-apis/nb-api/src/main/java/io/nosqlbench/nb/api/system/NBEnvironment.java

@@ -217,15 +217,20 @@ public class NBEnvironment {
      * @return The interpolated value, after substitutions, or null if any lookup failed
      */
     public Optional<String> interpolate(String word, Map<String,String> supplemental) {
-        Pattern envpattern = Pattern.compile("(\\$(?<env1>[a-zA-Z_][A-Za-z0-9_.]+)|\\$\\{(?<env2>[^}]+)\\})");
+        Pattern envpattern = Pattern.compile("\\$(?<env1>[a-zA-Z_][A-Za-z0-9_.]+)|\\$\\{(?<env2>[a-zA-Z_][A-Za-z0-9_.]*)(:(?<default>[a-zA-Z0-9_.]+))?\\}");
         Matcher matcher = envpattern.matcher(word);
         StringBuilder sb = new StringBuilder();
         while (matcher.find()) {
             String envvar = matcher.group("env1");
+            String defaultValue = null;
             if (envvar == null) {
                 envvar = matcher.group("env2");
+                defaultValue = matcher.group("default");
             }
             String value = peek(envvar,supplemental);
+            if (value == null) {
+                value = defaultValue;
+            }
             if (value == null) {
                 if (logger != null) {
                     logger.debug("no value found for '" + envvar + "', returning Optional.empty() for '" + word + "'");

nb-engine/nb-engine-cli/src/test/java/io/nosqlbench/engine/cli/atfiles/NBAtFileTest.java

@@ -97,6 +97,12 @@ class NBAtFileTest {
         assertThat(strings).containsExactly("My value environment");
     }
 
+    @Test
+    public void testAtfileEnvironmentVariableDefault() {
+        LinkedList<String> strings = NBAtFile.includeAt("@src/test/resources/atfiles/environment_variable_default.yaml");
+        assertThat(strings).containsExactly("My default value environment");
+    }
+
     @Test
     public void testAtfileMissingEnvironmentVariable() {
         assertThrows(RuntimeException.class, () -> NBAtFile.includeAt("@src/test/resources/atfiles/environment_variable_missing.yaml:>:"));

nb-engine/nb-engine-cli/src/test/resources/atfiles/environment_variable_default.yaml

@@ -0,0 +1 @@
+- My ${MISSING_ENV_VAR:default value} environment