support appending when saving file (#3992)

This patch implements `>>` operation in bash.

Signed-off-by: Tw <tw19881113@gmail.com>

crates/nu-command/src/commands/filesystem/save.rs

@@ -142,6 +142,7 @@ impl WholeStreamCommand for Save {
                 "treat values as-is rather than auto-converting based on file extension",
                 Some('r'),
             )
+            .switch("append", "append values rather than overriding", Some('a'))
     }
 
     fn usage(&self) -> &str {
@@ -165,6 +166,7 @@ fn save(args: CommandArgs) -> Result<OutputStream, ShellError> {
 
     let path: Option<Tagged<PathBuf>> = args.opt(0)?;
     let save_raw = args.has_flag("raw");
+    let append = args.has_flag("append");
 
     let input: Vec<Value> = args.input.collect();
     if path.is_none() {
@@ -231,7 +233,7 @@ fn save(args: CommandArgs) -> Result<OutputStream, ShellError> {
         };
     };
 
-    shell_manager.save(&full_path, &content?, name.span)
+    shell_manager.save(&full_path, &content?, name.span, append)
 }
 
 fn string_from(input: &[Value]) -> String {

crates/nu-engine/src/filesystem/filesystem_shell.rs

@@ -14,7 +14,8 @@ use nu_protocol::{CommandAction, ConfigPath, TaggedDictBuilder, Value};
 use nu_source::{Span, Tag};
 use nu_stream::{ActionStream, Interruptible, IntoActionStream, OutputStream};
 use std::collections::VecDeque;
-use std::io::ErrorKind;
+use std::fs::OpenOptions;
+use std::io::{ErrorKind, Write};
 use std::path::{Path, PathBuf};
 use std::sync::atomic::AtomicBool;
 use std::sync::Arc;
@@ -873,8 +874,19 @@ impl Shell for FilesystemShell {
         full_path: &Path,
         save_data: &[u8],
         name: Span,
+        append: bool,
     ) -> Result<OutputStream, ShellError> {
-        match std::fs::write(full_path, save_data) {
+        let mut options = OpenOptions::new();
+        if append {
+            options.append(true)
+        } else {
+            options.write(true).create(true).truncate(true)
+        };
+
+        match options
+            .open(full_path)
+            .and_then(|ref mut file| file.write_all(save_data))
+        {
             Ok(_) => Ok(OutputStream::empty()),
             Err(e) => Err(ShellError::labeled_error(
                 e.to_string(),

crates/nu-engine/src/shell/mod.rs

@@ -49,5 +49,6 @@ pub trait Shell: std::fmt::Debug {
         path: &Path,
         contents: &[u8],
         name: Span,
+        append: bool,
     ) -> Result<OutputStream, ShellError>;
 }

crates/nu-engine/src/shell/shell_manager.rs

@@ -105,8 +105,9 @@ impl ShellManager {
         full_path: &Path,
         save_data: &[u8],
         name: Span,
+        append: bool,
     ) -> Result<OutputStream, ShellError> {
-        self.shells.lock()[self.current_shell()].save(full_path, save_data, name)
+        self.shells.lock()[self.current_shell()].save(full_path, save_data, name, append)
     }
 
     pub fn next(&self) {

crates/nu-engine/src/shell/value_shell.rs

@@ -248,6 +248,7 @@ impl Shell for ValueShell {
         _path: &Path,
         _contents: &[u8],
         _name: Span,
+        _append: bool,
     ) -> Result<OutputStream, ShellError> {
         Err(ShellError::unimplemented(
             "save on help shell is not supported",