Remove pipe && Fix cat and huge_write.

user/src/bin/cat.rs

@@ -21,14 +21,12 @@ pub fn main(argc: usize, argv: &[&str]) -> i32 {
         panic!("Error occured when opening file");
     }
     let fd = fd as usize;
-    let mut buf = [0u8; 16];
-    let mut s = String::new();
+    let mut buf = [0u8; 256];
     loop {
         let size = read(fd, &mut buf) as usize;
         if size == 0 { break; }
-        s.push_str(core::str::from_utf8(&buf[..size]).unwrap());
+        println!("{}", core::str::from_utf8(&buf[..size]).unwrap());
     }
-    println!("{}", s);
     close(fd);
     0
-}
+}