enh(bash) default value is another variable

CHANGES.md

@@ -17,6 +17,7 @@ Core Changes:
 
 Language Improvements:
 
+- enh(bash) default value is another variable (#2439) [Josh Goebel][]
 - enh(bash) string nested within string (#2439) [Josh Goebel][]
 - enh(bash) Add arithmetic expression support (#2439) [Josh Goebel][]
 - enh(clojure) Add support for global definitions name (#2347) [Alexandre Grison][]

src/languages/bash.js

@@ -7,13 +7,21 @@ Category: common
 */
 
 export default function(hljs) {
-  const VAR = {
+  const VAR = {};
+  const BRACKET_VAR = {
+    begin: /\$\{/, end:/\}/,
+    contains: [
+      { begin: /:-/, contains: [VAR] } // default values
+    ]
+  };
+  Object.assign(VAR,{
     className: 'variable',
     variants: [
       {begin: /\$[\w\d#@][\w\d_]*/},
-      {begin: /\$\{(.*?)}/}
+      BRACKET_VAR
     ]
-  };
+  });
+
   const SUBST = {
     className: 'subst',
     begin: /\$\(/, end: /\)/,

test/markup/bash/variables.expect.txt

@@ -0,0 +1,3 @@
+<span class="hljs-variable">$A</span>
+<span class="hljs-variable">${B}</span>
+<span class="hljs-variable">${WURST:-<span class="hljs-variable">${CARNE}</span>}</span>

test/markup/bash/variables.txt

@@ -0,0 +1,3 @@
+$A
+${B}
+${WURST:-${CARNE}}