Match GNU diff's implementation for exit codes (fixes #17)

src/main.rs

@@ -8,6 +8,7 @@ use std::env;
 
 use std::fs;
 use std::io::{self, Write};
+use std::process::{exit, ExitCode};
 
 mod context_diff;
 mod ed_diff;
@@ -15,25 +16,35 @@ mod normal_diff;
 mod params;
 mod unified_diff;
 
-fn main() -> Result<(), String> {
+// Exit codes are documented at
+// https://www.gnu.org/software/diffutils/manual/html_node/Invoking-diff.html.
+//     An exit status of 0 means no differences were found,
+//     1 means some differences were found,
+//     and 2 means trouble.
+fn main() -> ExitCode {
     let opts = env::args_os();
     let Params {
         from,
         to,
         context_count,
         format,
-    } = parse_params(opts)?;
+    } = parse_params(opts).unwrap_or_else(|error| {
+        eprintln!("{error}");
+        exit(2);
+    });
     // read files
     let from_content = match fs::read(&from) {
         Ok(from_content) => from_content,
         Err(e) => {
-            return Err(format!("Failed to read from-file: {e}"));
+            eprintln!("Failed to read from-file: {e}");
+            return ExitCode::from(2);
         }
     };
     let to_content = match fs::read(&to) {
         Ok(to_content) => to_content,
         Err(e) => {
-            return Err(format!("Failed to read to-file: {e}"));
+            eprintln!("Failed to read to-file: {e}");
+            return ExitCode::from(2);
         }
     };
     // run diff
@@ -53,8 +64,15 @@ fn main() -> Result<(), String> {
             &to.to_string_lossy(),
             context_count,
         ),
-        Format::Ed => ed_diff::diff(&from_content, &to_content)?,
+        Format::Ed => ed_diff::diff(&from_content, &to_content).unwrap_or_else(|error| {
+            eprintln!("{error}");
+            exit(2);
+        }),
     };
     io::stdout().write_all(&result).unwrap();
-    Ok(())
+    if result.is_empty() {
+        ExitCode::SUCCESS
+    } else {
+        ExitCode::from(1)
+    }
 }