Read an argument from the commandline.

gongor.rs

@@ -2,7 +2,9 @@ use std::fs::File;
 use std::io::prelude::*;
 
 fn main() ->  std::io::Result<()>{
-    let mut file = File::create("foo.txt")?;
+    let filename = std::env::args().nth(1).expect("no filename given");
+
+    let mut file = File::create(filename)?;
     file.write_all(b"Hello, world!")?;
     Ok(())
 }