diff --git a/SiteGenerator/Sources/SiteGenerator/main.swift b/SiteGenerator/Sources/SiteGenerator/main.swift
index 41bbdf8..a978011 100644
--- a/SiteGenerator/Sources/SiteGenerator/main.swift
+++ b/SiteGenerator/Sources/SiteGenerator/main.swift
@@ -5,6 +5,7 @@
 //  Created by Sami Samhuri on 2019-12-01.
 //
 
+import Darwin
 import Foundation
 
 func main(sourcePath: String, targetPath: String) throws {
@@ -18,9 +19,32 @@ func main(sourcePath: String, targetPath: String) throws {
     try generator.generate(targetURL: targetURL)
 }
 
+guard CommandLine.arguments.count >= 3 else {
+    let name = CommandLine.arguments[0]
+    fputs("Usage: \(name) <site dir> <target dir>", stderr)
+    exit(1)
+}
+
 let sourcePath = CommandLine.arguments[1]
+var isDir: ObjCBool = false
+let sourceExists = FileManager.default.fileExists(atPath: sourcePath, isDirectory: &isDir)
+guard sourceExists, isDir.boolValue else {
+    fputs("error: Site path \(sourcePath) does not exist or is not a directory", stderr)
+    exit(2)
+}
+
 let targetPath = CommandLine.arguments[2]
+let targetExists = FileManager.default.fileExists(atPath: targetPath)
+guard !targetExists else {
+    print("error: Refusing to clobber existing target \(targetPath)")
+    exit(2)
+}
 
-#warning("TODO: validate args")
-
-try! main(sourcePath: sourcePath, targetPath: targetPath)
+do {
+    try main(sourcePath: sourcePath, targetPath: targetPath)
+    exit(0)
+}
+catch {
+    fputs("error: \(error.localizedDescription)", stderr)
+    exit(-1)
+}