diff --git a/Sources/main.swift b/Sources/main.swift
index 420e286..7e719de 100644
--- a/Sources/main.swift
+++ b/Sources/main.swift
@@ -1,27 +1,40 @@
+/*
+  tidcli - Touch ID Command Line
+  by @singe
+
+  tidcli is a simple utility to invoke Touch ID and produce an appropriate exit
+  code for a successful or failed authentication.
+
+  It's primary use is to add TouchID auth to shell scripts.
+*/
+
+import Darwin  // required for simple fputs stderr output
 import Foundation
 import LocalAuthentication
 
 let arguments = CommandLine.arguments
 var promptMessage = "authenticate to proceed"
 if arguments.count > 1 {
-    promptMessage = arguments[1]
+  // Limit potential input to 100 characters
+  promptMessage = String(arguments[1].prefix(100))
 }
 
 let context = LAContext()
 var error: NSError?
 
 if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error) {
-    context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: promptMessage) { success, evaluationError in
-        if success {
-            print("Authentication was successful.")
-        } else {
-            print("Authentication failed.")
-        }
-        exit(success ? 0 : 1)
+  context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: promptMessage) {
+    success, evaluationError in
+    if success {
+      fputs("Authentication was successful.\n", stderr)
+    } else {
+      fputs("Authentication failed.\n", stderr)
     }
+    exit(success ? 0 : 1)
+  }
 } else {
-    print("Touch ID is not available.")
-    exit(1)
+  fputs("Touch ID is not available.\n", stderr)
+  exit(1)
 }
 
 // Keep the run loop running to wait for the async authentication callback.
diff --git a/readme.md b/readme.md
index 5e65604..773e40d 100644
--- a/readme.md
+++ b/readme.md
@@ -1,14 +1,27 @@
 # Outrageously simple touch ID command line prompter
 
-TouchID CLI (tidcli) simply pops a TouchID prompt.
+Touch ID Command Line Interface (tidcli) simply pops a Touch ID prompt.
+
 It returns an exit code of 0 on success and 1 on failure.
 
 You can use this to embed additional authentication steps into your shell script or the like.
 
-Custom prompt information can be passed as the first argument.
-
 <img width="372" alt="image" src="https://github.com/singe/tidcli/assets/1150684/999a9a41-75d6-4366-b97b-ee6b425e1c1e">
 
+# Usage
+
+`tidcli [optional prompt context]`
+
+Optional custom prompt information can be passed as the first argument. This is limited to 100 characters.
+
+An example of using it in a bash shell script to exit if there is an authentication failure is:
+
+```
+tidcli "EXAMPLE SCRIPT"
+if [[ "$?" -ne 0 ]]; then
+  exit 1
+fi
+```
 
 # Building
 
@@ -17,3 +30,13 @@ Build a release binary with swift by running:
 `swift build -c release`
 
 The resulting binary will be in the `.build/release` directory as `tidcli`.
+
+# Notes
+
+## Password fallback not allowed
+
+The Touch ID prompt will contain a "Use Password" button, but only biometric authentication is allowed, so clicking it will result in an authentication failure. If you wanted to change this you could use `.deviceOwnerAuthentication` instead of `.deviceOwnerAuthenticationWithBiometrics`.
+
+## Output on standard error
+
+Success and failure messages are output to standard error.