Write to stdout when --output is omitted

CHANGELOG.md

@@ -14,6 +14,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Resolve values in functional utilities based on `@theme` options ([#15623](https://github.com/tailwindlabs/tailwindcss/pull/15623))
 - Discard invalid variants such as `data-checked-[selected=1]:*` ([#15629](https://github.com/tailwindlabs/tailwindcss/pull/15629))
 - Ensure `-outline-offset-*` utilities are suggested in IntelliSense ([#15646](https://github.com/tailwindlabs/tailwindcss/pull/15646))
+- Write to `stdout` when `--output` flag is omitted for `@tailwindcss/cli` ([#15656](https://github.com/tailwindlabs/tailwindcss/pull/15656))
+- Write to `stdout` when `--output -` flag is used for `@tailwindcss/cli` ([#15656](https://github.com/tailwindlabs/tailwindcss/pull/15656))
 - _Upgrade (experimental)_: Pretty print `--spacing(…)` to prevent ambiguity ([#15596](https://github.com/tailwindlabs/tailwindcss/pull/15596))
 
 ## [4.0.0-beta.9] - 2025-01-09

integrations/cli/index.test.ts

@@ -95,6 +95,149 @@ describe.each([
     },
   )
 
+  test(
+    'production build — read input from stdin',
+    {
+      fs: {
+        'package.json': json`{}`,
+        'pnpm-workspace.yaml': yaml`
+          #
+          packages:
+            - project-a
+        `,
+        'project-a/package.json': json`
+          {
+            "dependencies": {
+              "tailwindcss": "workspace:^",
+              "@tailwindcss/cli": "workspace:^"
+            }
+          }
+        `,
+        'project-a/index.html': html`
+          <div
+            class="underline 2xl:font-bold hocus:underline inverted:flex *:flex **:flex"
+          ></div>
+        `,
+        'project-a/plugin.js': js`
+          module.exports = function ({ addVariant }) {
+            addVariant('inverted', '@media (inverted-colors: inverted)')
+            addVariant('hocus', ['&:focus', '&:hover'])
+          }
+        `,
+        'project-a/tailwind.config.js': js`
+          module.exports = {
+            content: ['../project-b/src/**/*.js'],
+          }
+        `,
+        'project-a/src/index.js': js`
+          const className = "content-['project-a/src/index.js']"
+          module.exports = { className }
+        `,
+        'project-b/src/index.html': html`
+          <div class="flex" />
+        `,
+        'project-b/src/index.js': js`
+          const className = "content-['project-b/src/index.js']"
+          module.exports = { className }
+        `,
+      },
+    },
+    async ({ root, fs, exec }) => {
+      await exec(
+        `${command} --input - --output dist/out.css`,
+        { cwd: path.join(root, 'project-a') },
+        {
+          stdin: css`
+            @import 'tailwindcss/utilities';
+            @config './tailwind.config.js';
+            @source '../project-b/src/**/*.html';
+            @plugin './plugin.js';
+          `,
+        },
+      )
+
+      await fs.expectFileToContain('project-a/dist/out.css', [
+        candidate`underline`,
+        candidate`flex`,
+        candidate`content-['project-a/src/index.js']`,
+        candidate`content-['project-b/src/index.js']`,
+        candidate`inverted:flex`,
+        candidate`hocus:underline`,
+        candidate`*:flex`,
+        candidate`**:flex`,
+      ])
+    },
+  )
+
+  test(
+    'production build — (write to stdout)',
+    {
+      fs: {
+        'package.json': json`{}`,
+        'pnpm-workspace.yaml': yaml`
+          #
+          packages:
+            - project-a
+        `,
+        'project-a/package.json': json`
+          {
+            "dependencies": {
+              "tailwindcss": "workspace:^",
+              "@tailwindcss/cli": "workspace:^"
+            }
+          }
+        `,
+        'project-a/index.html': html`
+          <div
+            class="underline 2xl:font-bold hocus:underline inverted:flex *:flex **:flex"
+          ></div>
+        `,
+        'project-a/plugin.js': js`
+          module.exports = function ({ addVariant }) {
+            addVariant('inverted', '@media (inverted-colors: inverted)')
+            addVariant('hocus', ['&:focus', '&:hover'])
+          }
+        `,
+        'project-a/tailwind.config.js': js`
+          module.exports = {
+            content: ['../project-b/src/**/*.js'],
+          }
+        `,
+        'project-a/src/index.css': css`
+          @import 'tailwindcss/utilities';
+          @config '../tailwind.config.js';
+          @source '../../project-b/src/**/*.html';
+          @plugin '../plugin.js';
+        `,
+        'project-a/src/index.js': js`
+          const className = "content-['project-a/src/index.js']"
+          module.exports = { className }
+        `,
+        'project-b/src/index.html': html`
+          <div class="flex" />
+        `,
+        'project-b/src/index.js': js`
+          const className = "content-['project-b/src/index.js']"
+          module.exports = { className }
+        `,
+      },
+    },
+    async ({ root, expect, exec }) => {
+      let stdout = await exec(`${command} --input src/index.css --output -`, {
+        cwd: path.join(root, 'project-a'),
+      })
+
+      expect(stdout).toContain(candidate`underline`)
+      expect(stdout).toContain(candidate`flex`)
+      expect(stdout).toContain(candidate`content-['project-a/src/index.js']`)
+      expect(stdout).toContain(candidate`content-['project-b/src/index.js']`)
+      expect(stdout).toContain(candidate`inverted:flex`)
+      expect(stdout).toContain(candidate`hocus:underline`)
+      expect(stdout).toContain(candidate`*:flex`)
+      expect(stdout).toContain(candidate`**:flex`)
+    },
+  )
+
   test(
     'watch mode',
     {

integrations/utils.ts

@@ -25,6 +25,7 @@ interface ChildProcessOptions {
 
 interface ExecOptions {
   ignoreStdErr?: boolean
+  stdin?: string
 }
 
 interface TestConfig {
@@ -112,7 +113,7 @@ export function test(
           }
           if (debug) console.log(`> ${command}`)
           return new Promise((resolve, reject) => {
-            exec(
+            let child = exec(
               command,
               {
                 cwd,
@@ -134,6 +135,10 @@ export function test(
                 }
               },
             )
+            if (execOptions.stdin) {
+              child.stdin?.write(execOptions.stdin)
+              child.stdin?.end()
+            }
           })
         },
         async spawn(command: string, childProcessOptions: ChildProcessOptions = {}) {

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

@@ -32,6 +32,7 @@ export function options() {
       type: 'string',
       description: 'Output file',
       alias: '-o',
+      default: '-',
     },
     '--watch': {
       type: 'boolean | string',
@@ -72,8 +73,10 @@ export async function handle(args: Result<ReturnType<typeof options>>) {
 
   let base = path.resolve(args['--cwd'])
 
-  // Resolve the output as an absolute path.
-  if (args['--output']) {
+  // Resolve the output as an absolute path. If the output is a `-`, then we
+  // don't need to resolve it because this is a flag to indicate that we want to
+  // use `stdout` instead.
+  if (args['--output'] && args['--output'] !== '-') {
     args['--output'] = path.resolve(base, args['--output'])
   }
 
@@ -129,7 +132,7 @@ export async function handle(args: Result<ReturnType<typeof options>>) {
 
     // Write the output
     DEBUG && I.start('Write output')
-    if (args['--output']) {
+    if (args['--output'] && args['--output'] !== '-') {
       await outputFile(args['--output'], output)
     } else {
       println(output)

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

@@ -36,7 +36,7 @@ if (command) {
 //
 //   - `tailwindcss -o output.css`  // should run the build command, not show the help message
 //   - `tailwindcss > output.css`   // should run the build command, not show the help message
-if ((process.stdout.isTTY && !flags['--output']) || flags['--help']) {
+if ((process.stdout.isTTY && process.argv[2] === undefined) || flags['--help']) {
   help({
     usage: ['tailwindcss [--input input.css] [--output output.css] [--watch] [options…]'],
     options: { ...build.options(), ...sharedOptions },

packages/@tailwindcss-cli/src/utils/args.ts

@@ -67,8 +67,20 @@ export type Result<T extends Arg> = {
 }
 
 export function args<const T extends Arg>(options: T, argv = process.argv.slice(2)): Result<T> {
+  for (let [idx, value] of argv.entries()) {
+    if (value === '-') {
+      argv[idx] = '__IO_DEFAULT_VALUE__'
+    }
+  }
+
   let parsed = parse(argv)
 
+  for (let key in parsed) {
+    if (parsed[key] === '__IO_DEFAULT_VALUE__') {
+      parsed[key] = '-'
+    }
+  }
+
   let result: { _: string[]; [key: string]: unknown } = {
     _: parsed._,
   }