fix: parse named file descriptor redirects

.changeset/named-fd-redirects.md

@@ -0,0 +1,5 @@
+---
+"@aliou/sh": patch
+---
+
+Parse `{varname}` file descriptor redirects in Bash and Zsh, and reject them in POSIX and mksh.

README.md

@@ -69,7 +69,7 @@ extractCommandNames(ast); // ["grep", "head"]
 - Command substitution (`$(cmd)`, `` `cmd` ``), arithmetic expansion (`$((expr))`)
 - Process substitution (`<(cmd)`, `>(cmd)`)
 - Heredocs (`<<`, `<<-`), herestrings (`<<<`)
-- All redirect operators (`>`, `>>`, `<`, `>&`, `<&`, `<>`, `>|`, `&>`, `&>>`)
+- All redirect operators (`>`, `>>`, `<`, `>&`, `<&`, `<>`, `>|`, `&>`, `&>>`), including `{varname}` file-descriptor redirects (`foo {fd}<file`, Bash/Zsh only)
 - Assignments (`FOO=bar cmd`), append assignments (`FOO+=bar`)
 - Array expressions (`arr=(a b c)`, `arr=([0]=x [1]=y)`)
 - Declaration builtins as special forms (`declare`, `local`, `export`, `readonly`, `typeset`, `nameref`)

skills/sh-ast/SKILL.md

@@ -257,6 +257,17 @@ extractRedirects(cmd);
 // [{ op: ">", target: "file.txt" }, { op: ">&", fd: "2", target: "1" }]
 ```
 
+`fd` is either a numeric string (`"2"`) or, in Bash/Zsh, a `{varname}` redirect such as `"{fd}"`:
+
+```typescript
+const { ast } = parse("foo {fd}<file");
+const cmd = ast.body[0].command as SimpleCommand;
+extractRedirects(cmd);
+// [{ op: "<", fd: "{fd}", target: "file" }]
+```
+
+`{varname}` redirects are rejected in POSIX and mksh dialects.
+
 ### Count AST Nodes (complexity metric)
 
 ```typescript

src/parser/dialect.test.ts

@@ -53,6 +53,10 @@ describe("dialect enforcement: POSIX", () => {
     expectErr("diff <(foo) <(bar)", "posix", /process subst/);
   });
 
+  it("rejects named file descriptor redirects", () => {
+    expectErr("foo {fd}<f", "posix", /\{varname\}.*bash\/zsh feature/);
+  });
+
   it("rejects extended glob", () => {
     expectErr("ls @(foo)", "posix", /extended glob/);
   });
@@ -98,6 +102,10 @@ describe("dialect enforcement: mksh", () => {
   it("rejects ${!foo@}", () => {
     expectErr("echo ${!foo@}", "mksh", /\$\{!/);
   });
+
+  it("rejects named file descriptor redirects", () => {
+    expectErr("foo {fd}<f", "mksh", /\{varname\}.*bash\/zsh feature/);
+  });
 });
 
 describe("dialect enforcement: bash (default) accepts everything", () => {

src/parser/redirects.test.ts

@@ -66,6 +66,50 @@ describe("parse (phase 3: assignments and redirects)", () => {
       ),
     });
   });
+
+  it.each(["bash", "zsh"] as const)(
+    "parses named file descriptor redirects in %s",
+    (dialect) => {
+      expect(parse("foo {fd}<f", { dialect })).toMatchAst({
+        ast: program(
+          stmt({
+            type: "SimpleCommand",
+            words: [word("foo")],
+            redirects: [redirect("<", "f", "{fd}")],
+          }),
+        ),
+      });
+    },
+  );
+
+  it("parses named file descriptor redirects before the command", () => {
+    expect(parse("{fd}>>out foo")).toMatchAst({
+      ast: program(
+        stmt({
+          type: "SimpleCommand",
+          words: [word("foo")],
+          redirects: [redirect(">>", "out", "{fd}")],
+        }),
+      ),
+    });
+  });
+
+  it.each(["{1fd}>out", "{fd-x}>out", "{}>out"])(
+    "treats %s as a word, not a named redirect",
+    (source) => {
+      const { ast } = parse(`foo ${source}`);
+      const command = ast.body[0]?.command as {
+        words?: { parts: { value?: string }[] }[];
+        redirects?: { fd?: string }[];
+      };
+      expect(command.redirects?.[0]?.fd).toBeUndefined();
+      expect(command.words?.[1]?.parts[0]?.value).toBe(source.split(">")[0]);
+    },
+  );
+
+  it("does not treat brace groups as named redirects", () => {
+    expect(parse("{ foo; }").ast.body[0]?.command.type).toBe("Block");
+  });
 });
 
 describe("parse (phase 12: extended redirects)", () => {

src/tokenizer/tokenize.ts

@@ -1,6 +1,13 @@
 import type { ParseOptions } from "../ast";
 import { checkLang } from "../dialect";
-import { isDigit, operatorChars, redirChars, symbolChars } from "./charsets";
+import {
+  isDigit,
+  isNameChar,
+  isNameStart,
+  operatorChars,
+  redirChars,
+  symbolChars,
+} from "./charsets";
 import { SourceMap } from "./cursor";
 import { scanBacktick } from "./scan-backtick";
 import { scanExpansion } from "./scan-expansion";
@@ -174,6 +181,35 @@ export function tokenize(source: string, options: ParseOptions = {}): Token[] {
       }
     }
 
+    // `{varname}` file descriptor redirects, e.g. `foo {fd}<file`. Only a
+    // valid name wrapped in braces and immediately followed by a redirect
+    // operator counts; anything else falls through to word parsing.
+    if (ch === "{" && atBoundary && isNameStart(source.charAt(i + 1))) {
+      let j = i + 2;
+      while (j < source.length && isNameChar(source.charAt(j))) {
+        j += 1;
+      }
+      if (source.charAt(j) === "}") {
+        const redir = tryRedirOp(source, j + 1);
+        if (redir) {
+          checkLang(options.dialect, map.posAt(i), "`{varname}` redirects", [
+            "bash",
+            "zsh",
+          ]);
+          tokens.push({
+            type: "redir",
+            op: redir.op,
+            fd: source.slice(i, j + 1),
+            pos: map.posAt(i),
+            end: map.posAt(j + 1 + redir.len),
+          });
+          i = j + 1 + redir.len;
+          atBoundary = true;
+          continue;
+        }
+      }
+    }
+
     if (isDigit(ch)) {
       let j = i;
       while (j < source.length && isDigit(source.charAt(j))) {