Don't parse square bracket commands as special syntax

corpus/control_flow.txt

@@ -75,10 +75,12 @@ fi
 
 (program
   (if_statement
-    (bracket_command
+    (command
+      (command_name (word))
       (string (command_substitution (command (command_name (word)))))
       (word)
-      (raw_string))
+      (raw_string)
+      (word))
     (command (command_name (word)) (word))))
 
 ====================================

corpus/expressions.txt

@@ -25,7 +25,7 @@ echo ]]] ===
   (command (command_name (word)) (word))
   (command (command_name (word)) (word) (word))
   (list
-    (bracket_command (string) (word) (word))
+    (command (command_name (word)) (string) (word) (word) (word))
     (command (command_name (word)) (word))))
 
 =============================

grammar.js

@@ -23,7 +23,6 @@ module.exports = grammar({
     $.variable_name, // Variable name followed by an operator like '=' or '+='
     '\n',
     ']',
-    ']]',
     '}',
   ],
 
@@ -46,7 +45,6 @@ module.exports = grammar({
       $.variable_assignment,
       $.command,
       $.declaration_command,
-      $.bracket_command,
       $.for_statement,
       $.while_statement,
       $.if_statement,
@@ -153,11 +151,6 @@ module.exports = grammar({
       $._statement
     )),
 
-    bracket_command: $ => choice(
-      seq('[', repeat1($._expression), ']'),
-      seq('[[', repeat1($._expression), ']]')
-    ),
-
     // Commands
 
     command: $ => prec.left(seq(

src/grammar.json

@@ -36,10 +36,6 @@
           "type": "SYMBOL",
           "name": "declaration_command"
         },
-        {
-          "type": "SYMBOL",
-          "name": "bracket_command"
-        },
         {
           "type": "SYMBOL",
           "name": "for_statement"
@@ -507,51 +503,6 @@
         ]
       }
     },
-    "bracket_command": {
-      "type": "CHOICE",
-      "members": [
-        {
-          "type": "SEQ",
-          "members": [
-            {
-              "type": "STRING",
-              "value": "["
-            },
-            {
-              "type": "REPEAT1",
-              "content": {
-                "type": "SYMBOL",
-                "name": "_expression"
-              }
-            },
-            {
-              "type": "STRING",
-              "value": "]"
-            }
-          ]
-        },
-        {
-          "type": "SEQ",
-          "members": [
-            {
-              "type": "STRING",
-              "value": "[["
-            },
-            {
-              "type": "REPEAT1",
-              "content": {
-                "type": "SYMBOL",
-                "name": "_expression"
-              }
-            },
-            {
-              "type": "STRING",
-              "value": "]]"
-            }
-          ]
-        }
-      ]
-    },
     "command": {
       "type": "PREC_LEFT",
       "value": 0,
@@ -1409,10 +1360,6 @@
       "type": "STRING",
       "value": "]"
     },
-    {
-      "type": "STRING",
-      "value": "]]"
-    },
     {
       "type": "STRING",
       "value": "}"

src/scanner.cc

@@ -18,7 +18,6 @@ enum TokenType {
   VARIABLE_NAME,
   NEWLINE,
   CLOSING_BRACKET,
-  CLOSING_DOUBLE_BRACKET,
   CLOSING_BRACE,
 };
 
@@ -158,66 +157,83 @@ struct Scanner {
         }
       }
 
-      bool is_number = true;
+      bool is_numeric = iswdigit(lexer->lookahead);
-      bool is_alphanumeric = true;
+      bool is_alphanumeric = iswalpha(lexer->lookahead);
+
       for (;;) {
-        if (iswdigit(lexer->lookahead)) {
+        // These characters are not allowed in unquoted arguments
-        } else if (iswalpha(lexer->lookahead) || lexer->lookahead == '_') {
+        // or environment variable names
-          is_number = false;
+        if (
-        } else if (
+          lexer->lookahead == 0 ||
-          !iswspace(lexer->lookahead) &&
+          lexer->lookahead == ';' ||
-          lexer->lookahead != 0 &&
+          lexer->lookahead == '"' ||
-          lexer->lookahead != '"' &&
+          lexer->lookahead == '(' ||
-          lexer->lookahead != '\'' &&
+          lexer->lookahead == ')' ||
-          lexer->lookahead != '`' &&
+          lexer->lookahead == '\'' ||
-          lexer->lookahead != '>' &&
+          lexer->lookahead == '&' ||
-          lexer->lookahead != '<' &&
+          lexer->lookahead == '#' ||
-          lexer->lookahead != '#' &&
+          lexer->lookahead == '`' ||
-          lexer->lookahead != '|' &&
+          lexer->lookahead == '|' ||
-          lexer->lookahead != '(' &&
+          lexer->lookahead == '$' ||
-          lexer->lookahead != ')' &&
+          iswspace(lexer->lookahead)
-          lexer->lookahead != ';' &&
+        ) break;
-          lexer->lookahead != '&' &&
+
-          lexer->lookahead != '$'
+        // Curly braces are not allowed in unquoted arguments within curly braces
-        ) {
+        // (e.g. inside of a variable expansion like `${key:arg}`).
-          if (lexer->lookahead == '}' && valid_symbols[CLOSING_BRACE]) break;
+        if (
-          if (lexer->lookahead == ']' && length == 0 && (valid_symbols[CLOSING_BRACKET] || valid_symbols[CLOSING_DOUBLE_BRACKET])) break;
+          lexer->lookahead == '}' &&
-          if (is_alphanumeric && valid_symbols[VARIABLE_NAME] && (lexer->lookahead == '=' || lexer->lookahead == '[' || lexer->lookahead == '+')) break;
+          valid_symbols[CLOSING_BRACE]
-          is_alphanumeric = false;
+        ) break;
-        } else {
+
+        // Square brackets are not allowed in unquoted arguments within square brackets
+        // (e.g. inside of an array subscript like `a[arg]`).
+        if (
+          lexer->lookahead == ']' &&
+          valid_symbols[CLOSING_BRACKET]
+        ) break;
+
+        // Numbers followed by '<' and '>' at the beginning of commands
+        // are parsed as file descriptors.
+        if (lexer->lookahead == '<' || lexer->lookahead == '>') {
+          if (is_numeric && valid_symbols[FILE_DESCRIPTOR]) {
+            lexer->result_symbol = FILE_DESCRIPTOR;
+            return true;
+          }
           break;
         }
 
+        if (!iswdigit(lexer->lookahead)) is_numeric = false;
+
+        if (!iswalnum(lexer->lookahead) && lexer->lookahead != '_') {
+
+          // Alphanumeric strings followed by '=', '[', or '+=' are treated
+          // as environment variable names.
+          if (is_alphanumeric && valid_symbols[VARIABLE_NAME] && length > 0) {
+            if (lexer->lookahead == '+') {
+              lexer->mark_end(lexer);
+              advance(lexer);
+              if (lexer->lookahead == '=') {
+                lexer->result_symbol = VARIABLE_NAME;
+                return true;
+              } else {
+                return false;
+              }
+            } else if (lexer->lookahead == '=' || lexer->lookahead == '[') {
+              lexer->result_symbol = VARIABLE_NAME;
+              return true;
+            }
+          }
+
+          is_alphanumeric = false;
+        }
+
         advance(lexer);
         length++;
       }
 
-      if (length == 0) return false;
+      // Do not handle strings containing only letters, because those
-
+      // might be keywords. Let the normal lexer handle those.
-      if (is_number &&
+      if (length > 0 && valid_symbols[WORD] && !is_alphanumeric) {
-          valid_symbols[FILE_DESCRIPTOR] &&
-          (lexer->lookahead == '>' || lexer->lookahead == '<')) {
-        lexer->result_symbol = FILE_DESCRIPTOR;
-        return true;
-      }
-
-      if (valid_symbols[VARIABLE_NAME]) {
-        if (lexer->lookahead == '+') {
-          lexer->mark_end(lexer);
-          advance(lexer);
-          if (lexer->lookahead == '=') {
-            lexer->result_symbol = VARIABLE_NAME;
-            return true;
-          } else {
-            return false;
-          }
-        } else if (lexer->lookahead == '=' || lexer->lookahead == '[') {
-          lexer->result_symbol = VARIABLE_NAME;
-          return true;
-        }
-      }
-
-      if (valid_symbols[WORD] && !is_alphanumeric) {
         lexer->result_symbol = WORD;
         return true;
       }