diff --git a/executable_semantics/BUILD b/executable_semantics/BUILD
index 8b6c24509c73..ccda0b7f98c9 100644
--- a/executable_semantics/BUILD
+++ b/executable_semantics/BUILD
@@ -11,7 +11,10 @@ load("//bazel/testing:golden_test.bzl", "golden_test")
 cc_binary(
     name = "executable_semantics",
     srcs = ["main.cpp"],
-    deps = [":syntax"],
+    deps = [
+        ":syntax",
+        "@llvm-project//llvm:Support",
+    ],
 )
 
 cc_library(
diff --git a/executable_semantics/main.cpp b/executable_semantics/main.cpp
index 4ec90519971b..2f8cbc6b9c3f 100644
--- a/executable_semantics/main.cpp
+++ b/executable_semantics/main.cpp
@@ -7,6 +7,7 @@
 #include <iostream>
 
 #include "executable_semantics/syntax_helpers.h"
+#include "llvm/Support/CommandLine.h"
 
 extern FILE* yyin;
 extern auto yyparse() -> int;  // NOLINT(readability-identifier-naming)
@@ -14,12 +15,19 @@ extern auto yyparse() -> int;  // NOLINT(readability-identifier-naming)
 int main(int argc, char* argv[]) {
   // yydebug = 1;
 
-  if (argc > 1) {
-    Carbon::input_filename = argv[1];
-    yyin = fopen(argv[1], "r");
+  using llvm::cl::desc;
+  using llvm::cl::opt;
+  // TODO: Add with related support.
+  // opt<bool> quiet_option("quiet", desc("Disable tracing"));
+  opt<std::string> input_filename(llvm::cl::Positional, desc("<input file>"));
+  llvm::cl::ParseCommandLineOptions(argc, argv);
+
+  if (input_filename.getNumOccurrences() > 0) {
+    Carbon::input_filename = input_filename.c_str();
+    yyin = fopen(input_filename.c_str(), "r");
     if (yyin == nullptr) {
-      std::cerr << "Error opening '" << argv[1] << "': " << strerror(errno)
-                << std::endl;
+      std::cerr << "Error opening '" << input_filename
+                << "': " << strerror(errno) << std::endl;
       return 1;
     }
   }
diff --git a/executable_semantics/syntax_helpers.cpp b/executable_semantics/syntax_helpers.cpp
index 4b0b4c5fbc6e..fd25da925ff1 100644
--- a/executable_semantics/syntax_helpers.cpp
+++ b/executable_semantics/syntax_helpers.cpp
@@ -11,7 +11,7 @@
 
 namespace Carbon {
 
-char* input_filename = nullptr;
+const char* input_filename = nullptr;
 
 void PrintSyntaxError(char* error, int line_num) {
   std::cerr << input_filename << ":" << line_num << ": " << error << std::endl;
diff --git a/executable_semantics/syntax_helpers.h b/executable_semantics/syntax_helpers.h
index 0efc77449cd9..531b85912181 100644
--- a/executable_semantics/syntax_helpers.h
+++ b/executable_semantics/syntax_helpers.h
@@ -16,7 +16,7 @@
 namespace Carbon {
 
 // Initialized by main(), and used when printing errors.
-extern char* input_filename;
+extern const char* input_filename;
 
 // Prints errors.
 void PrintSyntaxError(char* error, int line_num);