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
@@ -1488,13 +1488,17 @@
 
 
 def executeShTest(test, litConfig, useExternalSh,
-                  extra_substitutions=[]):
+                  extra_substitutions=[],
+                  preamble_commands=[]):
     if test.config.unsupported:
         return lit.Test.Result(Test.UNSUPPORTED, 'Test is unsupported')
 
-    script = parseIntegratedTestScript(test)
-    if isinstance(script, lit.Test.Result):
-        return script
+    script = list(preamble_commands)
+    parsed = parseIntegratedTestScript(test, require_script=not script)
+    if isinstance(parsed, lit.Test.Result):
+        return parsed
+    script += parsed
+
     if litConfig.noExecute:
         return lit.Test.Result(Test.PASS)
 
diff --git a/llvm/utils/lit/tests/Inputs/shtest-inject/lit.cfg b/llvm/utils/lit/tests/Inputs/shtest-inject/lit.cfg
new file mode 100644
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/shtest-inject/lit.cfg
@@ -0,0 +1,17 @@
+import lit
+
+class CustomFormat(lit.formats.TestFormat):
+    def execute(self, test, litConfig):
+        commands = [
+            'echo "THIS WAS"',
+            'echo "INJECTED"'
+        ]
+        return lit.TestRunner.executeShTest(test, litConfig,
+                                            useExternalSh=False,
+                                            preamble_commands=commands)
+
+config.name = 'shtest-inject'
+config.suffixes = ['.txt']
+config.test_format = CustomFormat()
+config.test_source_root = None
+config.test_exec_root = None
diff --git a/llvm/utils/lit/tests/Inputs/shtest-inject/test-empty.txt b/llvm/utils/lit/tests/Inputs/shtest-inject/test-empty.txt
new file mode 100644
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/shtest-inject/test-empty.txt
@@ -0,0 +1,3 @@
+
+# This test voluntarily has no RUN lines or anything else. The RUN lines are
+# injected by the test format.
diff --git a/llvm/utils/lit/tests/Inputs/shtest-inject/test-many.txt b/llvm/utils/lit/tests/Inputs/shtest-inject/test-many.txt
new file mode 100644
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/shtest-inject/test-many.txt
@@ -0,0 +1,7 @@
+
+# This test has several RUN lines, but more run lines are prepended to it by
+# the test format in use.
+
+# RUN: echo "IN THE FILE"
+# RUN: echo "IF IT WORKS"
+# RUN: echo "AS EXPECTED"
diff --git a/llvm/utils/lit/tests/Inputs/shtest-inject/test-one.txt b/llvm/utils/lit/tests/Inputs/shtest-inject/test-one.txt
new file mode 100644
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/shtest-inject/test-one.txt
@@ -0,0 +1,5 @@
+
+# This test has one RUN line, but more run lines are prepended to it by the
+# test format in use.
+
+# RUN: echo "IN THE FILE"
diff --git a/llvm/utils/lit/tests/shtest-inject.py b/llvm/utils/lit/tests/shtest-inject.py
new file mode 100644
--- /dev/null
+++ b/llvm/utils/lit/tests/shtest-inject.py
@@ -0,0 +1,49 @@
+# Check that we can inject commands at the beginning of a ShTest using a custom
+# test format.
+
+# RUN: %{lit} -j 1 %{inputs}/shtest-inject/test-empty.txt --show-all | FileCheck --check-prefix=CHECK-TEST1 %s
+#
+# CHECK-TEST1: Script:
+# CHECK-TEST1: --
+# CHECK-TEST1: echo "THIS WAS"
+# CHECK-TEST1: echo "INJECTED"
+# CHECK-TEST1: --
+#
+# CHECK-TEST1: THIS WAS
+# CHECK-TEST1: INJECTED
+#
+# CHECK-TEST1: Expected Passes    : 1
+
+# RUN: %{lit} -j 1 %{inputs}/shtest-inject/test-one.txt --show-all | FileCheck --check-prefix=CHECK-TEST2 %s
+#
+# CHECK-TEST2: Script:
+# CHECK-TEST2: --
+# CHECK-TEST2: echo "THIS WAS"
+# CHECK-TEST2: echo "INJECTED"
+# CHECK-TEST2: echo "IN THE FILE"
+# CHECK-TEST2: --
+#
+# CHECK-TEST2: THIS WAS
+# CHECK-TEST2: INJECTED
+# CHECK-TEST2: IN THE FILE
+#
+# CHECK-TEST2: Expected Passes    : 1
+
+# RUN: %{lit} -j 1 %{inputs}/shtest-inject/test-many.txt --show-all | FileCheck --check-prefix=CHECK-TEST3 %s
+#
+# CHECK-TEST3: Script:
+# CHECK-TEST3: --
+# CHECK-TEST3: echo "THIS WAS"
+# CHECK-TEST3: echo "INJECTED"
+# CHECK-TEST3: echo "IN THE FILE"
+# CHECK-TEST3: echo "IF IT WORKS"
+# CHECK-TEST3: echo "AS EXPECTED"
+# CHECK-TEST3: --
+#
+# CHECK-TEST3: THIS WAS
+# CHECK-TEST3: INJECTED
+# CHECK-TEST3: IN THE FILE
+# CHECK-TEST3: IF IT WORKS
+# CHECK-TEST3: AS EXPECTED
+#
+# CHECK-TEST3: Expected Passes    : 1