diff --git a/flake.nix b/flake.nix
index 92eddaa..2804a77 100644
--- a/flake.nix
+++ b/flake.nix
@@ -24,6 +24,7 @@
   };
 
   outputs = flakes: let
+    version = "1.3.0";
     self = flakes.self.packages.x86_64-linux;
     nixpkgs = flakes.nixpkgs.legacyPackages.x86_64-linux.pkgsStatic;
     nixpkgs-dyn = flakes.nixpkgs.legacyPackages.x86_64-linux;
@@ -120,6 +121,7 @@
         ${cxx} \
           -o $out/bin/knock \
           ${./src/knock.cpp} \
+          -D KNOCK_VERSION='"${version}"' \
           -Wl,--as-needed -static \
           ${self.utils-common}/lib/libutils-common.a \
           ${self.gourou}/lib/libgourou.a \
diff --git a/src/knock.cpp b/src/knock.cpp
index 9c5e9a0..014d50d 100644
--- a/src/knock.cpp
+++ b/src/knock.cpp
@@ -3,12 +3,26 @@
 #include "libgourou_common.h"
 #include "libgourou.h"
 
+#ifndef KNOCK_VERSION
+    #error KNOCK_VERSION must be defined
+#endif
+
 std::string get_data_dir();
 void verify_absence(std::string file);
 void verify_presence(std::string file);
 
 int main(int argc, char** argv) try {
 
+    if (argc == 1) {
+        std::cout
+            << "info: knock version " << KNOCK_VERSION << ", libgourou version "
+                << LIBGOUROU_VERSION << "\n"
+            << "usage: " << argv[0] << " [ACSM]" << "\n"
+            << "result: converts file ACSM to a plain EPUB/PDF if present, otherwise prints this"
+            << std::endl;
+        return EXIT_SUCCESS;
+    }
+
     if (argc != 2) {
         throw std::invalid_argument("the ACSM file must be passed as an argument");
     }
diff --git a/tests/tests.py b/tests/tests.py
index fd3e710..ef82f8c 100644
--- a/tests/tests.py
+++ b/tests/tests.py
@@ -19,8 +19,14 @@ if len(sys.argv) != 2:
     sys.exit()
 
 print("Testing " + str(knock))
-workspace = Path(sys.argv[1])
 
+result = subprocess.run(knock)
+if result.returncode != 0:
+    print("Test failed: knock failed to describe itself")
+    sys.exit()
+print("---")
+
+workspace = Path(sys.argv[1])
 if workspace.exists():
     shutil.rmtree(workspace)
 workspace.mkdir()
@@ -50,7 +56,7 @@ for i, link in enumerate(links):
     result = subprocess.run([knock, file])
 
     if result.returncode != 0:
-        print("Failed")
+        print("Test failed: knock failed to convert a file")
         sys.exit()
 
     print("Success\n---")