diff --git a/src/command.rs b/src/command.rs
index ac5479e..770b989 100644
--- a/src/command.rs
+++ b/src/command.rs
@@ -19,7 +19,7 @@ impl ExitCode {
 }
 
 pub trait Command {
-    fn spawn(&self) -> ExitCode;
+    fn spawn(&mut self) -> ExitCode;
 }
 
 pub struct CommandSequence {
@@ -46,11 +46,11 @@ impl CommandSequence {
 }
 
 impl Command for CommandSequence {
-    fn spawn(&self) -> ExitCode {
+    fn spawn(&mut self) -> ExitCode {
         let mut exit_code = self.command.spawn();
 
         if self.next_command.is_some() {
-            exit_code = self.next_command.as_ref().unwrap().spawn();
+            exit_code = self.next_command.as_mut().unwrap().spawn();
         }
 
         exit_code
@@ -75,7 +75,13 @@ impl Command for UnixProgram {
         let mut command = std::process::Command::new(&program);
         command.args(argv);
 
-        let handle = command.spawn();
+        Self { command }
+    }
+}
+
+impl Command for UnixProgram {
+    fn spawn(&mut self) -> ExitCode {
+        let handle = self.command.spawn();
 
         if handle.is_ok() {
             let exit_code = handle