feat: add args

__enclave/virt_enclave/examples/sign_file.rs

@@ -1,4 +1,4 @@
-use std::io::Read;
+use std::io::{ Read, Write };
 use std::fs::File;
 use ring::digest;
 use virt_enclave::sig::*;
@@ -31,4 +31,15 @@ fn main() {
     println!("File  : {}", f);
     println!("Hex   : {}", digest_hex);
     println!("Signed: {}", serde_json::to_string(&signed_message).unwrap());
+
+    if let Some(of) = args.next() {
+        if File::open(&of).is_ok() {
+            println!("Output file exists: {}", of);
+            return;
+        }
+        if let Ok(mut out_file) = File::create(&of) {
+            out_file.write_all(&serde_json::to_string(&signed_message).unwrap().as_bytes()).ok();
+            println!("Write file successed: {}", of);
+        }
+    }
 }