diff --git a/src/Symfony/Component/Process/Process.php b/src/Symfony/Component/Process/Process.php
index 753a880cdbdd..40690c404fdd 100644
--- a/src/Symfony/Component/Process/Process.php
+++ b/src/Symfony/Component/Process/Process.php
@@ -43,6 +43,11 @@ class Process implements \IteratorAggregate
     // Timeout Precision in seconds.
     const TIMEOUT_PRECISION = 0.2;
 
+    const ITER_NON_BLOCKING = 1; // By default, iterating over outputs is a blocking call, use this flag to make it non-blocking
+    const ITER_KEEP_OUTPUT = 2;  // By default, outputs are cleared while iterating, use this flag to keep them in memory
+    const ITER_SKIP_OUT = 4;     // Use this flag to skip STDOUT while iterating
+    const ITER_SKIP_ERR = 8;     // Use this flag to skip STDERR while iterating
+
     private $callback;
     private $hasCallback = false;
     private $commandline;
@@ -503,41 +508,49 @@ public function getIncrementalOutput()
     /**
      * Returns an iterator to the output of the process, with the output type as keys (Process::OUT/ERR).
      *
-     * @param bool $blocking    Whether to use a blocking read call.
-     * @param bool $clearOutput Whether to clear or keep output in memory.
+     * @param int $flags A bit field of Process::ITER_* flags.
      *
      * @throws LogicException in case the output has been disabled
      * @throws LogicException In case the process is not started
      *
      * @return \Generator
      */
-    public function getIterator($blocking = true, $clearOutput = true)
+    public function getIterator($flags = 0)
     {
         $this->readPipesForOutput(__FUNCTION__, false);
 
-        while (null !== $this->callback || !feof($this->stdout) || !feof($this->stderr)) {
-            $out = stream_get_contents($this->stdout, -1, $this->incrementalOutputOffset);
+        $clearOutput = !(self::ITER_KEEP_OUTPUT & $flags);
+        $blocking = !(self::ITER_NON_BLOCKING & $flags);
+        $yieldOut = !(self::ITER_SKIP_OUT & $flags);
+        $yieldErr = !(self::ITER_SKIP_ERR & $flags);
 
-            if (isset($out[0])) {
-                if ($clearOutput) {
-                    $this->clearOutput();
-                } else {
-                    $this->incrementalOutputOffset = ftell($this->stdout);
-                }
+        while (null !== $this->callback || ($yieldOut && !feof($this->stdout)) || ($yieldErr && !feof($this->stderr))) {
+            if ($yieldOut) {
+                $out = stream_get_contents($this->stdout, -1, $this->incrementalOutputOffset);
+
+                if (isset($out[0])) {
+                    if ($clearOutput) {
+                        $this->clearOutput();
+                    } else {
+                        $this->incrementalOutputOffset = ftell($this->stdout);
+                    }
 
-                yield self::OUT => $out;
+                    yield self::OUT => $out;
+                }
             }
 
-            $err = stream_get_contents($this->stderr, -1, $this->incrementalErrorOutputOffset);
+            if ($yieldErr) {
+                $err = stream_get_contents($this->stderr, -1, $this->incrementalErrorOutputOffset);
 
-            if (isset($err[0])) {
-                if ($clearOutput) {
-                    $this->clearErrorOutput();
-                } else {
-                    $this->incrementalErrorOutputOffset = ftell($this->stderr);
-                }
+                if (isset($err[0])) {
+                    if ($clearOutput) {
+                        $this->clearErrorOutput();
+                    } else {
+                        $this->incrementalErrorOutputOffset = ftell($this->stderr);
+                    }
 
-                yield self::ERR => $err;
+                    yield self::ERR => $err;
+                }
             }
 
             if (!$blocking && !isset($out[0]) && !isset($err[0])) {
diff --git a/src/Symfony/Component/Process/ProcessUtils.php b/src/Symfony/Component/Process/ProcessUtils.php
index a7024eb1da5a..500202e5844c 100644
--- a/src/Symfony/Component/Process/ProcessUtils.php
+++ b/src/Symfony/Component/Process/ProcessUtils.php
@@ -96,6 +96,9 @@ public static function validateInput($caller, $input)
             if (is_scalar($input)) {
                 return (string) $input;
             }
+            if ($input instanceof Process) {
+                return $input->getIterator($input::ITER_SKIP_ERR);
+            }
             if ($input instanceof \Iterator) {
                 return $input;
             }
diff --git a/src/Symfony/Component/Process/Tests/ProcessTest.php b/src/Symfony/Component/Process/Tests/ProcessTest.php
index 045088b1b6a8..0ebd307ee28e 100644
--- a/src/Symfony/Component/Process/Tests/ProcessTest.php
+++ b/src/Symfony/Component/Process/Tests/ProcessTest.php
@@ -1154,7 +1154,7 @@ public function pipesCodeProvider()
      * @dataProvider provideVariousIncrementals
      */
     public function testIncrementalOutputDoesNotRequireAnotherCall($stream, $method) {
-        $process = new Process(self::$phpBin.' -r '.escapeshellarg('$n = 0; while ($n < 3) { file_put_contents(\''.$stream.'\', $n, 1); $n++; usleep(1000); }'), null, null, null, null);
+        $process = $this->getProcess(self::$phpBin.' -r '.escapeshellarg('$n = 0; while ($n < 3) { file_put_contents(\''.$stream.'\', $n, 1); $n++; usleep(1000); }'), null, null, null, null);
         $process->start();
         $result = '';
         $limit = microtime(true) + 3;
@@ -1182,7 +1182,7 @@ public function testIteratorInput()
             yield 'pong';
         };
 
-        $process = new Process(self::$phpBin.' -r '.escapeshellarg('stream_copy_to_stream(STDIN, STDOUT);'), null, null, $input());
+        $process = $this->getProcess(self::$phpBin.' -r '.escapeshellarg('stream_copy_to_stream(STDIN, STDOUT);'), null, null, $input());
         $process->run();
         $this->assertSame('pingpong', $process->getOutput());
     }
@@ -1191,7 +1191,7 @@ public function testSimpleInputStream()
     {
         $input = new InputStream();
 
-        $process = new Process(self::$phpBin.' -r '.escapeshellarg('echo \'ping\'; stream_copy_to_stream(STDIN, STDOUT);'));
+        $process = $this->getProcess(self::$phpBin.' -r '.escapeshellarg('echo \'ping\'; stream_copy_to_stream(STDIN, STDOUT);'));
         $process->setInput($input);
 
         $process->start(function ($type, $data) use ($input) {
@@ -1225,7 +1225,7 @@ public function testInputStreamWithCallable()
         $input->onEmpty($stream);
         $input->write($stream());
 
-        $process = new Process(self::$phpBin.' -r '.escapeshellarg('stream_copy_to_stream(STDIN, STDOUT);'));
+        $process = $this->getProcess(self::$phpBin.' -r '.escapeshellarg('echo fread(STDIN, 3);'));
         $process->setInput($input);
         $process->start(function ($type, $data) use ($input) {
             $input->close();
@@ -1243,7 +1243,7 @@ public function testInputStreamWithGenerator()
             $input->close();
         });
 
-        $process = new Process(self::$phpBin.' -r '.escapeshellarg('stream_copy_to_stream(STDIN, STDOUT);'));
+        $process = $this->getProcess(self::$phpBin.' -r '.escapeshellarg('stream_copy_to_stream(STDIN, STDOUT);'));
         $process->setInput($input);
         $process->start();
         $input->write('ping');
@@ -1257,7 +1257,7 @@ public function testInputStreamOnEmpty()
         $input = new InputStream();
         $input->onEmpty(function () use (&$i) {++$i;});
 
-        $process = new Process(self::$phpBin.' -r '.escapeshellarg('echo 123; echo fread(STDIN, 1); echo 456;'));
+        $process = $this->getProcess(self::$phpBin.' -r '.escapeshellarg('echo 123; echo fread(STDIN, 1); echo 456;'));
         $process->setInput($input);
         $process->start(function ($type, $data) use ($input) {
             if ('123' === $data) {
@@ -1274,7 +1274,7 @@ public function testIteratorOutput()
     {
         $input = new InputStream();
 
-        $process = new Process(self::$phpBin.' -r '.escapeshellarg('fwrite(STDOUT, 123); fwrite(STDERR, 234); fwrite(STDOUT, fread(STDIN, 3)); fwrite(STDERR, 456);'));
+        $process = $this->getProcess(self::$phpBin.' -r '.escapeshellarg('fwrite(STDOUT, 123); fwrite(STDERR, 234); flush(); usleep(10000); fwrite(STDOUT, fread(STDIN, 3)); fwrite(STDERR, 456);'));
         $process->setInput($input);
         $process->start();
         $output = array();
@@ -1310,12 +1310,12 @@ public function testNonBlockingNorClearingIteratorOutput()
     {
         $input = new InputStream();
 
-        $process = new Process(self::$phpBin.' -r '.escapeshellarg('fwrite(STDOUT, fread(STDIN, 3));'));
+        $process = $this->getProcess(self::$phpBin.' -r '.escapeshellarg('fwrite(STDOUT, fread(STDIN, 3));'));
         $process->setInput($input);
         $process->start();
         $output = array();
 
-        foreach ($process->getIterator(false, false) as $type => $data) {
+        foreach ($process->getIterator($process::ITER_NON_BLOCKING | $process::ITER_KEEP_OUTPUT) as $type => $data) {
             $output[] = array($type, $data);
             break;
         }
@@ -1326,7 +1326,7 @@ public function testNonBlockingNorClearingIteratorOutput()
 
         $input->write(123);
 
-        foreach ($process->getIterator(false, false) as $type => $data) {
+        foreach ($process->getIterator($process::ITER_NON_BLOCKING | $process::ITER_KEEP_OUTPUT) as $type => $data) {
             if ('' !== $data) {
                 $output[] = array($type, $data);
             }
@@ -1342,6 +1342,21 @@ public function testNonBlockingNorClearingIteratorOutput()
         $this->assertSame($expectedOutput, $output);
     }
 
+    public function testChainedProcesses()
+    {
+        $p1 = new Process(self::$phpBin.' -r '.escapeshellarg('fwrite(STDERR, 123); fwrite(STDOUT, 456);'));
+        $p2 = $this->getProcess(sprintf('%s -r %s', self::$phpBin, escapeshellarg('stream_copy_to_stream(STDIN, STDOUT);')));
+        $p2->setInput($p1);
+
+        $p1->start();
+        $p2->run();
+
+        $this->assertSame('123', $p1->getErrorOutput());
+        $this->assertSame('', $p1->getOutput());
+        $this->assertSame('', $p2->getErrorOutput());
+        $this->assertSame('456', $p2->getOutput());
+    }
+
     /**
      * @param string      $commandline
      * @param null|string $cwd