Expand default values recursively

ruby/shell/word_expander.rb

@@ -3,6 +3,7 @@ require "shellwords"
 module Shell
   class WordExpander
     ENV_VAR_REGEX = /\$(?:\{([^}]+)\}|(\w+)\b)/
+    DEFAULT_VAR_REGEX = /\A(\w+):-([\s\S]*)\z/
     ESCAPED_DOLLAR = "\u0001"
 
     # Splits the given line into multiple words, performing the following transformations:
@@ -15,12 +16,7 @@ module Shell
       protected_line = protect_escaped_dollars(line)
       shellsplit(protected_line)
         .map do |word|
-          word
-            .gsub(ENV_VAR_REGEX) do
-              name = Regexp.last_match(2) || Regexp.last_match(1)
-              ENV.fetch(name)
-            end
-            .tr(ESCAPED_DOLLAR, "$")
+          expand_variables(word).tr(ESCAPED_DOLLAR, "$")
           # TODO: expand globs
         end
     end
@@ -98,6 +94,20 @@ module Shell
       Dir.glob(word)
     end
 
+    def expand_variables(value)
+      value.gsub(ENV_VAR_REGEX) do
+        raw = Regexp.last_match(2) || Regexp.last_match(1)
+        if (m = DEFAULT_VAR_REGEX.match(raw))
+          name = m[1]
+          fallback = m[2]
+          env_value = ENV[name]
+          env_value.nil? || env_value.empty? ? expand_variables(fallback) : env_value
+        else
+          ENV.fetch(raw)
+        end
+      end
+    end
+
     def protect_escaped_dollars(line)
       output = +""
       i = 0

ruby/test/shell_test.rb

@@ -88,6 +88,10 @@ class ShellTest < Minitest::Test
     assert_equal "fallback", `#{A1_PATH} -c 'echo ${A1_UNSET_VAR:-fallback}'`.chomp
   end
 
+  def test_expands_parameter_default_value_with_variable_reference
+    assert_equal Dir.home, `#{A1_PATH} -c 'echo ${A1_UNSET_VAR:-$HOME}'`.chomp
+  end
+
   #################################
   ### Execution and job control ###
   #################################