Move exit code function to separate file

src/util/exit_code.rs

@@ -0,0 +1,22 @@
+use std::process::ExitStatus;
+
+#[cfg(unix)]
+pub fn extract_exit_code(status: ExitStatus) -> Option<i32> {
+    use std::os::unix::process::ExitStatusExt;
+
+    // From the ExitStatus::code documentation:
+    //
+    //   "On Unix, this will return None if the process was terminated by a signal."
+    //
+    // In that case, ExitStatusExt::signal should never return None.
+    //
+    // To differentiate between "normal" exit codes and signals, we are using a technique
+    // similar to bash (https://tldp.org/LDP/abs/html/exitcodes.html) and add 128 to the
+    // signal value.
+    status.code().or_else(|| status.signal().map(|s| s + 128))
+}
+
+#[cfg(not(unix))]
+pub fn extract_exit_code(status: ExitStatus) -> Option<i32> {
+    status.code()
+}

src/util/mod.rs

@@ -1,3 +1,4 @@
+pub mod exit_code;
 pub mod min_max;
 pub mod number;
 pub mod units;