Add back --append flag to save command (#4744)

crates/nu-command/tests/commands/save.rs

@@ -1,6 +1,7 @@
 use nu_test_support::fs::{file_contents, Stub::FileWithContent};
 use nu_test_support::nu;
 use nu_test_support::playground::Playground;
+use std::io::Write;
 
 #[test]
 fn figures_out_intelligently_where_to_write_out_with_metadata() {
@@ -48,8 +49,6 @@ fn writes_out_csv() {
     })
 }
 
-// FIXME: jt: needs more work
-#[ignore]
 #[test]
 fn save_append_will_create_file_if_not_exists() {
     Playground::setup("save_test_3", |dirs, sandbox| {
@@ -64,6 +63,31 @@ fn save_append_will_create_file_if_not_exists() {
 
         let actual = file_contents(expected_file);
         println!("{}", actual);
-        assert!(actual == "hello");
+        assert_eq!(actual, "hello");
+    })
+}
+
+#[test]
+fn save_append_will_not_overwrite_content() {
+    Playground::setup("save_test_4", |dirs, sandbox| {
+        sandbox.with_files(vec![]);
+
+        let expected_file = dirs.test().join("new-file.txt");
+
+        {
+            let mut file =
+                std::fs::File::create(&expected_file).expect("Failed to create test file");
+            file.write_all("hello ".as_bytes())
+                .expect("Failed to write to test file")
+        }
+
+        nu!(
+            cwd: dirs.root(),
+            r#"echo world | save --append save_test_4/new-file.txt"#,
+        );
+
+        let actual = file_contents(expected_file);
+        println!("{}", actual);
+        assert_eq!(actual, "hello world");
     })
 }