fix: forward process.stdin to child when no explicit input is provided

build/core.cjs

@@ -574,9 +574,17 @@ var _ProcessPromise = class _ProcessPromise extends Promise {
         )) || cb();
       },
       on: {
-        start: () => {
+        start: (child) => {
+          var _a2;
           $2.log({ kind: "cmd", cmd: $2.cmd, cwd, verbose: self.isVerbose(), id });
           self.timeout($2.timeout, $2.timeoutSignal);
+          if (!$2.input && !self.sync && (child == null ? void 0 : child.stdin) && !child.stdin.destroyed && ((_a2 = import_node_process2.default.stdin) == null ? void 0 : _a2.readable) && !import_node_process2.default.stdin.readableEnded && _ProcessPromise.bus.sources(self).length === 0) {
+            import_node_process2.default.stdin.pipe(child.stdin);
+            child.stdin.on("error", import_util.noop);
+            child.once("close", () => {
+              if (child.stdin) import_node_process2.default.stdin.unpipe(child.stdin);
+            });
+          }
         },
         stdout: (data) => {
           $2.log({ kind: "stdout", data, verbose: !self._piped && self.isVerbose(), id });

src/core.ts

@@ -348,9 +348,29 @@ export class ProcessPromise extends Promise<ProcessOutput> {
         ) || cb()
       },
       on: {
-        start: () => {
+        start: (child) => {
           $.log({ kind: 'cmd', cmd: $.cmd, cwd, verbose: self.isVerbose(), id })
           self.timeout($.timeout, $.timeoutSignal)
+
+          // Forward process.stdin to the child when no explicit input is
+          // provided and the process isn't receiving piped input from another
+          // process.  This enables interactive commands like `$\`cat\`` to
+          // read from the terminal, matching standard shell behavior.
+          if (
+            !$.input &&
+            !self.sync &&
+            child?.stdin &&
+            !child.stdin.destroyed &&
+            process.stdin?.readable &&
+            !process.stdin.readableEnded &&
+            ProcessPromise.bus.sources(self).length === 0
+          ) {
+            process.stdin.pipe(child.stdin)
+            child.stdin.on('error', noop)
+            child.once('close', () => {
+              if (child.stdin) process.stdin.unpipe(child.stdin)
+            })
+          }
         },
         stdout: (data) => {
           // If the process is piped, don't print its output.

test/core.test.js

@@ -19,7 +19,7 @@ import { basename } from 'node:path'
 import { WriteStream } from 'node:fs'
 import { Readable, Transform, Writable } from 'node:stream'
 import { Socket } from 'node:net'
-import { ChildProcess } from 'node:child_process'
+import { ChildProcess, spawn as cpSpawn } from 'node:child_process'
 import {
   $,
   ProcessPromise,
@@ -642,6 +642,32 @@ describe('core', () => {
       assert.equal((await p).stdout, 'bar')
     })
 
+    test('forwards process.stdin to child when no input is provided', async () => {
+      const buildPath = process.cwd() + '/build/core.js'
+      const script = tempfile(
+        'stdin-forward-test.mjs',
+        `import {$} from '${buildPath}'\n` +
+          'const r = await $`cat`\n' +
+          'process.stdout.write(r.stdout)\n'
+      )
+
+      const child = cpSpawn(process.execPath, [script], {
+        stdio: ['pipe', 'pipe', 'pipe'],
+      })
+      child.stdin.write('hello from stdin\n')
+      child.stdin.end()
+
+      const chunks = []
+      child.stdout.on('data', (d) => chunks.push(d))
+
+      const code = await new Promise((resolve) => child.on('close', resolve))
+      const stdout = Buffer.concat(chunks).toString()
+
+      assert.equal(code, 0)
+      assert.equal(stdout.trim(), 'hello from stdin')
+      await fs.rm(script)
+    })
+
     describe('pipe()', () => {
       test('accepts Writable', async () => {
         let contents = ''