Add Input & Output check to CLI (#17311)

Throw an error if the input and output file for the CLI are identical.

---------

Co-authored-by: Philipp Spiess <hello@philippspiess.com>
Co-authored-by: Robin Malfait <malfait.robin@gmail.com>

CHANGELOG.md

@@ -24,9 +24,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
-- Vite: Ensure that updates to an imported CSS file are properly propagated after updating templates ([#17347](https://github.com/tailwindlabs/tailwindcss/pull/17347))
 - Fix class extraction followed by `(` in Pug ([#17320](https://github.com/tailwindlabs/tailwindcss/pull/17320))
+- Vite: Ensure that updates to an imported CSS file are properly propagated after updating templates ([#17347](https://github.com/tailwindlabs/tailwindcss/pull/17347))
 - Pre process `Slim` templates embedded in Ruby files ([#17336](https://github.com/tailwindlabs/tailwindcss/pull/17336))
+- Error when input and output files resolve to the same file when using the CLI ([#17311](https://github.com/tailwindlabs/tailwindcss/pull/17311))
 
 ### [4.0.15] - 2025-03-20
 

integrations/cli/index.test.ts

@@ -1387,6 +1387,76 @@ test(
   },
 )
 
+test(
+  'fails when input file does not exist',
+  {
+    fs: {
+      'package.json': json`
+        {
+          "dependencies": {
+            "tailwindcss": "workspace:^",
+            "@tailwindcss/cli": "workspace:^"
+          }
+        }
+      `,
+    },
+  },
+  async ({ exec, expect }) => {
+    await expect(exec('pnpm tailwindcss --input index.css --output dist/out.css')).rejects.toThrow(
+      /Specified input file.*does not exist./,
+    )
+  },
+)
+
+test(
+  'fails when input file and output file are the same',
+  {
+    fs: {
+      'package.json': json`
+        {
+          "dependencies": {
+            "tailwindcss": "workspace:^",
+            "@tailwindcss/cli": "workspace:^"
+          }
+        }
+      `,
+      'input.css': '',
+    },
+  },
+  async ({ exec, expect }) => {
+    await expect(exec('pnpm tailwindcss --input input.css --output input.css')).rejects.toThrow(
+      /Specified input file.*and output file.*are identical./,
+    )
+    await expect(
+      exec('pnpm tailwindcss --input input.css --output ./src/../input.css'),
+    ).rejects.toThrow(/Specified input file.*and output file.*are identical./)
+  },
+)
+
+test(
+  'input and output flags can be the same if `-` is used',
+  {
+    fs: {
+      'package.json': json`
+        {
+          "dependencies": {
+            "tailwindcss": "workspace:^",
+            "@tailwindcss/cli": "workspace:^"
+          }
+        }
+      `,
+      'index.html': html`<div class="flex"></div>`,
+    },
+  },
+  async ({ exec, expect }) => {
+    expect(
+      await exec('pnpm tailwindcss --input - --output -', undefined, {
+        stdin: '@tailwind utilities;',
+      }),
+    ).toContain(candidate`flex`)
+  },
+)
+
 function withBOM(text: string): string {
   return '\uFEFF' + text
 }

packages/@tailwindcss-cli/src/commands/build/index.ts

@@ -95,6 +95,17 @@ export async function handle(args: Result<ReturnType<typeof options>>) {
     }
   }
 
+  // Check if the input and output file paths are identical, otherwise return an
+  // error to the user.
+  if (args['--input'] === args['--output'] && args['--input'] !== '-') {
+    eprintln(header())
+    eprintln()
+    eprintln(
+      `Specified input file ${highlight(relative(args['--input']))} and output file ${highlight(relative(args['--output']))} are identical.`,
+    )
+    process.exit(1)
+  }
+
   let start = process.hrtime.bigint()
 
   let input = args['--input']