feat: add no-useless-echo rule

README.md

@@ -75,6 +75,7 @@ Each rule mirrors a well-known ShellCheck check.
 | [`no-backticks`](./docs/rules/no-backticks.md)                                     | SC2006     | Use `$(...)` instead of legacy backticks                | ✅      | error       |
 | [`no-expansions-in-single-quotes`](./docs/rules/no-expansions-in-single-quotes.md) | SC2016     | Expressions don't expand in single quotes               |         | warn        |
 | [`no-unquoted-expansions`](./docs/rules/no-unquoted-expansions.md)                 | SC2086/46  | Quote expansions subject to word splitting and globbing | ✅      | error       |
+| [`no-useless-echo`](./docs/rules/no-useless-echo.md)                               | SC2116     | Disallow `$(echo ...)`                                  |         | error       |
 
 ## Configuration comments
 

docs/rules/no-useless-echo.md

@@ -0,0 +1,53 @@
+# no-useless-echo
+
+Disallow useless `echo` inside command substitutions.
+
+## Background
+
+A command substitution that only runs `echo`, such as `$(echo "$name")`, produces the same text that was passed to `echo`. It starts an extra process (or at least an extra step) and makes the code harder to read. Using the value directly is simpler.
+
+## Rule Details
+
+This rule warns about a command substitution, written as `$(...)` or with backticks, whose entire body is a single `echo` command without redirects.
+
+The rule doesn't inspect `echo`'s options, so `$(echo -n "$value")` is also reported.
+
+Substitutions that do more than echo, such as pipelines (`$(echo "$name" | tr a-z A-Z)`) or multiple commands, are not reported.
+
+Examples of **incorrect** code for this rule:
+
+```bash
+# eslint bash/no-useless-echo: "error"
+
+name=$(echo "$first")
+
+ls $(echo /tmp)
+
+files=`echo "$dir"`
+```
+
+Examples of **correct** code for this rule:
+
+```bash
+# eslint bash/no-useless-echo: "error"
+
+name="$first"
+
+upper=$(echo "$name" | tr a-z A-Z)
+
+both=$(echo hello; echo world)
+
+echo "hello"
+```
+
+## Options
+
+This rule has no options.
+
+## When Not to Use It
+
+`$(echo ...)` is occasionally used on purpose for side effects of `echo`, such as collapsing whitespace in an unquoted value or interpreting escape sequences with `echo -e`. If you rely on those behaviors, use a disable comment for those lines or disable this rule.
+
+## Prior Art
+
+- [SC2116](https://www.shellcheck.net/wiki/SC2116)

src/index.spec.ts

@@ -28,6 +28,7 @@ describe("plugin", () => {
 			"no-backticks",
 			"no-expansions-in-single-quotes",
 			"no-unquoted-expansions",
+			"no-useless-echo",
 		]);
 	});
 

src/index.ts

@@ -7,11 +7,13 @@ import { BashLanguage } from "./languages/bash-language.js";
 import noBackticks from "./rules/no-backticks.js";
 import noExpansionsInSingleQuotes from "./rules/no-expansions-in-single-quotes.js";
 import noUnquotedExpansions from "./rules/no-unquoted-expansions.js";
+import noUselessEcho from "./rules/no-useless-echo.js";
 
 const rules = {
 	"no-backticks": noBackticks,
 	"no-expansions-in-single-quotes": noExpansionsInSingleQuotes,
 	"no-unquoted-expansions": noUnquotedExpansions,
+	"no-useless-echo": noUselessEcho,
 };
 
 const plugin = {
@@ -34,6 +36,7 @@ const plugin = {
 				"bash/no-backticks": "error",
 				"bash/no-expansions-in-single-quotes": "warn",
 				"bash/no-unquoted-expansions": "error",
+				"bash/no-useless-echo": "error",
 			},
 		},
 	},

src/rules/no-useless-echo.spec.ts

@@ -0,0 +1,50 @@
+/**
+ * @fileoverview Tests for the no-useless-echo rule.
+ */
+
+import { RuleTester } from "eslint";
+import { BashLanguage } from "../languages/bash-language.js";
+import rule from "./no-useless-echo.js";
+
+const ruleTester = new RuleTester({
+	plugins: {
+		bash: {
+			meta: { namespace: "shell" },
+			languages: { bash: new BashLanguage() },
+		},
+		// eslint-disable-next-line @typescript-eslint/no-explicit-any -- plugin shape is validated by ESLint at runtime.
+	} as any,
+	language: "bash/bash",
+});
+
+ruleTester.run("no-useless-echo", rule as never, {
+	valid: [
+		"echo hello",
+		"x=$(pwd)",
+		"x=$(echo foo | tr a-z A-Z)",
+		"x=$(echo foo; echo bar)",
+		"x=$(echo foo > file)",
+		"x=$(! echo foo)",
+	],
+	invalid: [
+		{
+			code: "x=$(echo foo)",
+			errors: [
+				{
+					messageId: "uselessEcho",
+					line: 1,
+					column: 3,
+					endColumn: 14,
+				},
+			],
+		},
+		{
+			code: 'cmd "$(echo "$var")"',
+			errors: [{ messageId: "uselessEcho" }],
+		},
+		{
+			code: "x=`echo foo`",
+			errors: [{ messageId: "uselessEcho" }],
+		},
+	],
+});

src/rules/no-useless-echo.ts

@@ -0,0 +1,48 @@
+/**
+ * @fileoverview Rule to disallow `$(echo ...)` command substitutions.
+ * Mirrors ShellCheck SC2116.
+ */
+
+import { isCommandNamed } from "./utils.js";
+import type { BashRuleDefinition } from "../types.js";
+
+const rule: BashRuleDefinition<{ MessageIds: "uselessEcho" }> = {
+	meta: {
+		type: "suggestion",
+		languages: ["shell/bash"],
+		docs: {
+			description: "Disallow useless `echo` inside command substitutions",
+			recommended: true,
+			dialects: ["Bash", "POSIX sh", "mksh"],
+			url: "https://github.com/eslint/bash/blob/main/docs/rules/no-useless-echo.md",
+		},
+		schema: [],
+		messages: {
+			uselessEcho:
+				"Useless echo? Instead of 'cmd $(echo foo)', just use 'cmd foo'. (ShellCheck SC2116)",
+		},
+	},
+
+	create(context) {
+		return {
+			CommandSubstitution(node) {
+				const first = node.body[0];
+
+				if (
+					node.body.length === 1 &&
+					first !== undefined &&
+					isCommandNamed(first, "echo") &&
+					!first.negated &&
+					first.redirects.length === 0
+				) {
+					context.report({
+						node,
+						messageId: "uselessEcho",
+					});
+				}
+			},
+		};
+	},
+};
+
+export default rule;