diff --git a/llvm/utils/lit/lit/TestRunner.py b/llvm/utils/lit/lit/TestRunner.py
--- a/llvm/utils/lit/lit/TestRunner.py
+++ b/llvm/utils/lit/lit/TestRunner.py
@@ -640,7 +640,7 @@
 
     procs = []
     proc_not_counts = []
-    default_stdin = subprocess.PIPE
+    default_stdin = None
     stderrTempFiles = []
     opened_files = []
     named_temp_files = []
diff --git a/llvm/utils/lit/lit/util.py b/llvm/utils/lit/lit/util.py
--- a/llvm/utils/lit/lit/util.py
+++ b/llvm/utils/lit/lit/util.py
@@ -321,7 +321,7 @@
     * working directory ``cwd`` (str), use None to use the current
       working directory
     * environment ``env`` (dict), use None for none
-    * Input to the command ``input`` (str), use string to pass
+    * Input to the command ``input`` (str), use None to pass
       no input.
     * Max execution time ``timeout`` (int) seconds. Use 0 for no timeout.
     * ``redirect_stderr`` (bool), use True if redirect stderr to stdout
@@ -339,7 +339,7 @@
         input = to_bytes(input)
     err_out = subprocess.STDOUT if redirect_stderr else subprocess.PIPE
     p = subprocess.Popen(command, cwd=cwd,
-                         stdin=subprocess.PIPE,
+                         stdin=subprocess.PIPE if input else None,
                          stdout=subprocess.PIPE,
                          stderr=err_out,
                          env=env, close_fds=kUseCloseFDs)
diff --git a/llvm/utils/lit/tests/Inputs/shtest-stdin/lit.cfg b/llvm/utils/lit/tests/Inputs/shtest-stdin/lit.cfg
new file mode 100644
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/shtest-stdin/lit.cfg
@@ -0,0 +1,26 @@
+# -*- Python -*-
+import os
+import sys
+
+import lit.formats
+
+config.name = 'shtest-stdin'
+
+externalShell = lit_config.params.get('external', '1') != '0'
+config.test_format = lit.formats.ShTest(execute_external=externalShell)
+config.suffixes = ['.py']
+
+config.test_source_root = os.path.dirname(__file__)
+config.test_exec_root = config.test_source_root
+config.target_triple = '(unused)'
+src_root = os.path.join(config.test_source_root, '..')
+
+pythonpath_list = [src_root]
+# Ensure the user's PYTHONPATH is included.
+if 'PYTHONPATH' in os.environ:
+    pythonpath_list.append(os.environ['PYTHONPATH'])
+if 'PYTHONPATH' in config.environment:
+    pythonpath_list.append(config.environment['PYTHONPATH'])
+config.environment['PYTHONPATH'] = os.pathsep.join(pythonpath_list)
+
+config.substitutions.append(('%{python}', '"%s"' % (sys.executable)))
diff --git a/llvm/utils/lit/tests/Inputs/shtest-stdin/print-stdin.py b/llvm/utils/lit/tests/Inputs/shtest-stdin/print-stdin.py
new file mode 100644
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/shtest-stdin/print-stdin.py
@@ -0,0 +1,5 @@
+# Checks that lit provides no input to stdin.
+# RUN: %{python} %s
+import sys
+
+print(sys.stdin.read())
\ No newline at end of file
diff --git a/llvm/utils/lit/tests/shtest-stdin.py b/llvm/utils/lit/tests/shtest-stdin.py
new file mode 100644
--- /dev/null
+++ b/llvm/utils/lit/tests/shtest-stdin.py
@@ -0,0 +1,28 @@
+# REQUIRES: lit-max-individual-test-time
+
+###############################################################################
+# Check tests do not receive spurious stdin input.
+###############################################################################
+# RUN: not %{lit} %{inputs}/shtest-stdin/print-stdin.py \
+# RUN:   --param external=0 --timeout 1 \
+# RUN: | FileCheck --check-prefix=CHECK-STDIN %s
+#
+# RUN: not %{lit} %{inputs}/shtest-stdin/print-stdin.py \
+# RUN:   --param external=1 --timeout 1 \
+# RUN: | FileCheck --check-prefix=CHECK-STDIN %s
+#
+# CHECK-STDIN: TIMEOUT: shtest-stdin :: print-stdin.py
+
+###############################################################################
+# Check tests do receive piped stdin input.
+###############################################################################
+# RUN: echo foobar | %{lit} %{inputs}/shtest-stdin/print-stdin.py \
+# RUN:   --param external=0 -a \
+# RUN: | FileCheck --check-prefix=CHECK-PIPE %s
+#
+# RUN: echo foobar | %{lit} %{inputs}/shtest-stdin/print-stdin.py \
+# RUN:   --param external=1 -a \
+# RUN: | FileCheck --check-prefix=CHECK-PIPE %s
+#
+# CHECK-PIPE: foobar
+
diff --git a/utils/bazel/llvm-project-overlay/llvm/utils/lit/tests/BUILD.bazel b/utils/bazel/llvm-project-overlay/llvm/utils/lit/tests/BUILD.bazel
--- a/utils/bazel/llvm-project-overlay/llvm/utils/lit/tests/BUILD.bazel
+++ b/utils/bazel/llvm-project-overlay/llvm/utils/lit/tests/BUILD.bazel
@@ -33,5 +33,15 @@
             "//llvm:not",
         ] + glob(["Inputs/**"]),
     )
-    for src in glob(["*/*.py"])
+    for src in glob(
+        include = ["**/*.py"],
+        exclude = [
+            "Inputs/**",
+            # FIXME: these tests fail under bazel.
+            "discovery.py",
+            "max-time.py",
+            "selecting.py",
+            "shtest-recursive-substitution.py",
+        ]
+    )
 ]