Add variable expansions

corpus/commands.txt

@@ -100,3 +100,31 @@ cat a b 2> /dev/null
   (command (simple_command
     (file_redirect (file_descriptor) (file_descriptor))
     (command_name))))
+
+===============================
+Variable expansions
+===============================
+
+cat $FOO
+
+---
+
+(program
+  (command (simple_command
+    (command_name)
+    (expansion (variable_name)))))
+
+===============================
+Variable expansion operators
+===============================
+
+cat ${BAR} ${ABC=def} ${GHI:?jkl}
+
+---
+
+(program
+  (command (simple_command
+    (command_name)
+    (operator_expansion (variable_name))
+    (operator_expansion (variable_name) (argument))
+    (operator_expansion (variable_name) (argument)))))

grammar.js

@@ -23,7 +23,11 @@ module.exports = grammar({
       rename($.leading_word, 'command_name'),
       optional(seq(
         /\s+/,
-        repeat(rename($.word, 'argument'))
+        repeat(choice(
+          rename($.word, 'argument'),
+          $.expansion,
+          $.operator_expansion
+        ))
       )),
       repeat(
         $.file_redirect
@@ -55,6 +59,21 @@ module.exports = grammar({
       rename($.word, 'argument')
     ),
 
+    expansion: $ => seq(
+      '$',
+      rename($.word, 'variable_name')
+    ),
+
+    operator_expansion: $ => seq(
+      '${',
+      rename($.leading_word, 'variable_name'),
+      optional(seq(
+        choice(':', ':?', '='),
+        rename($.word, 'argument')
+      )),
+      '}'
+    ),
+
     file_redirect: $ => seq(
       optional($.file_descriptor),
       choice('<', '>', '<&', '>&'),
@@ -66,9 +85,9 @@ module.exports = grammar({
 
     file_descriptor: $ => token(prec(1, /\d+/)),
 
-    leading_word: $ => /[^\s=|;]+/,
+    leading_word: $ => /[^\s=|;:{}]+/,
 
-    word: $ => /[^\s<>&]+/,
+    word: $ => /[^\s$<>{}&]+/,
 
     control_operator: $ => choice(
       '\n',

src/grammar.json

@@ -74,12 +74,25 @@
                 {
                   "type": "REPEAT",
                   "content": {
+                    "type": "CHOICE",
+                    "members": [
+                      {
                         "type": "RENAME",
                         "content": {
                           "type": "SYMBOL",
                           "name": "word"
                         },
                         "value": "argument"
+                      },
+                      {
+                        "type": "SYMBOL",
+                        "name": "expansion"
+                      },
+                      {
+                        "type": "SYMBOL",
+                        "name": "operator_expansion"
+                      }
+                    ]
                   }
                 }
               ]
@@ -205,6 +218,82 @@
         }
       ]
     },
+    "expansion": {
+      "type": "SEQ",
+      "members": [
+        {
+          "type": "STRING",
+          "value": "$"
+        },
+        {
+          "type": "RENAME",
+          "content": {
+            "type": "SYMBOL",
+            "name": "word"
+          },
+          "value": "variable_name"
+        }
+      ]
+    },
+    "operator_expansion": {
+      "type": "SEQ",
+      "members": [
+        {
+          "type": "STRING",
+          "value": "${"
+        },
+        {
+          "type": "RENAME",
+          "content": {
+            "type": "SYMBOL",
+            "name": "leading_word"
+          },
+          "value": "variable_name"
+        },
+        {
+          "type": "CHOICE",
+          "members": [
+            {
+              "type": "SEQ",
+              "members": [
+                {
+                  "type": "CHOICE",
+                  "members": [
+                    {
+                      "type": "STRING",
+                      "value": ":"
+                    },
+                    {
+                      "type": "STRING",
+                      "value": ":?"
+                    },
+                    {
+                      "type": "STRING",
+                      "value": "="
+                    }
+                  ]
+                },
+                {
+                  "type": "RENAME",
+                  "content": {
+                    "type": "SYMBOL",
+                    "name": "word"
+                  },
+                  "value": "argument"
+                }
+              ]
+            },
+            {
+              "type": "BLANK"
+            }
+          ]
+        },
+        {
+          "type": "STRING",
+          "value": "}"
+        }
+      ]
+    },
     "file_redirect": {
       "type": "SEQ",
       "members": [
@@ -273,11 +362,11 @@
     },
     "leading_word": {
       "type": "PATTERN",
-      "value": "[^\\s=|;]+"
+      "value": "[^\\s=|;:{}]+"
     },
     "word": {
       "type": "PATTERN",
-      "value": "[^\\s<>&]+"
+      "value": "[^\\s$<>{}&]+"
     },
     "control_operator": {
       "type": "CHOICE",