Show usage when invoked without argments

* exit with code 1 when no arguments
* add a test for no arguments.
* Update test_sed.rs
* cargo fmt
* pass -n for missing script arguments test

Issue: #274
PR: #409

src/sed/mod.rs

@@ -36,6 +36,15 @@ const USAGE: &str = "sed [OPTION]... [script] [file]...";
 #[uucore::main]
 pub fn uumain(args: impl uucore::Args) -> UResult<()> {
     let matches = uu_app().try_get_matches_from(args)?;
+
+    // Don't use arg_required_else_help when declaring command
+    // as it exits with code 2 and we use it to check
+    // default matches in tests.
+    if !matches.args_present() {
+        let _ = uu_app().print_help();
+        std::process::exit(1);
+    }
+
     let (scripts, files) = get_scripts_files(&matches)?;
     let mut context = build_context(&matches);
 

tests/by-util/test_sed.rs

@@ -54,11 +54,20 @@ fn test_silent_alias() {
 #[test]
 fn test_missing_script_argument() {
     new_ucmd!()
+        .args(&["-n"])
         .fails()
         .code_is(1)
         .stderr_contains("missing script");
 }
 
+#[test]
+fn test_no_arguments() {
+    new_ucmd!()
+        .fails()
+        .code_is(1)
+        .stdout_contains("Stream editor for filtering and transforming text");
+}
+
 #[test]
 fn test_positional_script_ok() {
     new_ucmd!().arg("l").succeeds().code_is(0);