Shell commands (#547)

* Implement shell interaction commands

* Use slice instead of iterator for shell invocation

* Default to `sh` instead of `$SHELL` for shell commands

* Enforce trailing comma in `commands` macro

* Use `|` register for shell commands

* Move shell config to `editor` and use in command

* Update shell command prompts

* Remove clone of shell config

* Change shell function names to match prompts

* Log stderr contents upon external command error

* Remove `unwrap` calls on potential common errors

`shell` will no longer panic if:
  * The user-configured shell cannot be found
  * The shell command does not output UTF-8

* Remove redundant `pipe` parameter

* Rename `ShellBehavior::None` to `Ignore`

* Display error when shell command is used and `shell = []`

* Document shell commands in `keymap.md`

4 files changed, 156 insertions, 2 deletions

diff --git a/book/src/keymap.md b/book/src/keymap.md
index 61378863..d85fb936 100644
--- a/book/src/keymap.md
+++ b/book/src/keymap.md
@@ -122,6 +122,16 @@ in reverse, or searching via smartcase.
 | `[D`  | Go to first diagnostic in document | `goto_first_diag` |
 | `]D`  | Go to last diagnostic in document  | `goto_last_diag`  |
 
+### Shell
+
+| Key    | Description                                                              | Command               |
+| ------ | -----------                                                              | -------               |
+| `\|`   | Pipe each selection through shell command, replacing with output         | `shell_pipe`          |
+| `A-\|` | Pipe each selection into shell command, ignoring output                  | `shell_pipe_to`       |
+| `!`    | Run shell command, inserting output before each selection                | `shell_insert_output` |
+| `A-!`  | Run shell command, appending output after each selection                 | `shell_append_output` |
+| `$`    | Pipe each selection into shell command, removing if the command exits >0 | `shell_keep_pipe`     |
+
 ## Select / extend mode
 
 I'm still pondering whether to keep this mode or not. It changes movement
diff --git a/helix-term/src/commands.rs b/helix-term/src/commands.rs
index d21bbe42..6437bf52 100644
--- a/helix-term/src/commands.rs
+++ b/helix-term/src/commands.rs
@@ -131,7 +131,7 @@ pub struct Command {
 }
 
 macro_rules! commands {
-    ( $($name:ident, $doc:literal),* ) => {
+    ( $($name:ident, $doc:literal,)* ) => {
         $(
             #[allow(non_upper_case_globals)]
             pub const $name: Self = Self {
@@ -302,7 +302,12 @@ impl Command {
         surround_delete, "Surround delete",
         select_textobject_around, "Select around object",
         select_textobject_inner, "Select inside object",
-        suspend, "Suspend"
+        shell_pipe, "Pipe selections through shell command",
+        shell_pipe_to, "Pipe selections into shell command, ignoring command output",
+        shell_insert_output, "Insert output of shell command before each selection",
+        shell_append_output, "Append output of shell command after each selection",
+        shell_keep_pipe, "Filter selections with shell predicate",
+        suspend, "Suspend",
     );
 }
 
@@ -4292,6 +4297,133 @@ fn surround_delete(cx: &mut Context) {
     })
 }
 
+#[derive(Eq, PartialEq)]
+enum ShellBehavior {
+    Replace,
+    Ignore,
+    Insert,
+    Append,
+    Filter,
+}
+
+fn shell_pipe(cx: &mut Context) {
+    shell(cx, "pipe:", ShellBehavior::Replace);
+}
+
+fn shell_pipe_to(cx: &mut Context) {
+    shell(cx, "pipe-to:", ShellBehavior::Ignore);
+}
+
+fn shell_insert_output(cx: &mut Context) {
+    shell(cx, "insert-output:", ShellBehavior::Insert);
+}
+
+fn shell_append_output(cx: &mut Context) {
+    shell(cx, "append-output:", ShellBehavior::Append);
+}
+
+fn shell_keep_pipe(cx: &mut Context) {
+    shell(cx, "keep-pipe:", ShellBehavior::Filter);
+}
+
+fn shell(cx: &mut Context, prompt: &str, behavior: ShellBehavior) {
+    use std::io::Write;
+    use std::process::{Command, Stdio};
+    if cx.editor.config.shell.is_empty() {
+        cx.editor.set_error("No shell set".to_owned());
+        return;
+    }
+    let pipe = match behavior {
+        ShellBehavior::Replace | ShellBehavior::Ignore | ShellBehavior::Filter => true,
+        ShellBehavior::Insert | ShellBehavior::Append => false,
+    };
+    let prompt = Prompt::new(
+        prompt.to_owned(),
+        Some('|'),
+        |_input: &str| Vec::new(),
+        move |cx: &mut compositor::Context, input: &str, event: PromptEvent| {
+            let shell = &cx.editor.config.shell;
+            if event == PromptEvent::Validate {
+                let (view, doc) = current!(cx.editor);
+                let selection = doc.selection(view.id);
+                let mut error: Option<&str> = None;
+                let transaction =
+                    Transaction::change_by_selection(doc.text(), selection, |range| {
+                        let mut process;
+                        match Command::new(&shell[0])
+                            .args(&shell[1..])
+                            .arg(input)
+                            .stdin(Stdio::piped())
+                            .stdout(Stdio::piped())
+                            .stderr(Stdio::piped())
+                            .spawn()
+                        {
+                            Ok(p) => process = p,
+                            Err(e) => {
+                                log::error!("Failed to start shell: {}", e);
+                                error = Some("Failed to start shell");
+                                return (0, 0, None);
+                            }
+                        }
+                        if pipe {
+                            let stdin = process.stdin.as_mut().unwrap();
+                            let fragment = range.fragment(doc.text().slice(..));
+                            stdin.write_all(fragment.as_bytes()).unwrap();
+                        }
+
+                        let output = process.wait_with_output().unwrap();
+                        if behavior != ShellBehavior::Filter {
+                            if !output.status.success() {
+                                let stderr = output.stderr;
+                                if !stderr.is_empty() {
+                                    log::error!(
+                                        "Shell error: {}",
+                                        String::from_utf8_lossy(&stderr)
+                                    );
+                                }
+                                error = Some("Command failed");
+                                return (0, 0, None);
+                            }
+                            let stdout = output.stdout;
+                            let tendril;
+                            match Tendril::try_from_byte_slice(&stdout) {
+                                Ok(t) => tendril = t,
+                                Err(_) => {
+                                    error = Some("Process did not output valid UTF-8");
+                                    return (0, 0, None);
+                                }
+                            }
+                            let (from, to) = match behavior {
+                                ShellBehavior::Replace => (range.from(), range.to()),
+                                ShellBehavior::Insert => (range.from(), range.from()),
+                                ShellBehavior::Append => (range.to(), range.to()),
+                                _ => (range.from(), range.from()),
+                            };
+                            (from, to, Some(tendril))
+                        } else {
+                            // if the process exits successfully, keep the selection, otherwise delete it.
+                            let keep = output.status.success();
+                            (
+                                range.from(),
+                                if keep { range.from() } else { range.to() },
+                                None,
+                            )
+                        }
+                    });
+
+                if let Some(error) = error {
+                    cx.editor.set_error(error.to_owned());
+                } else if behavior != ShellBehavior::Ignore {
+                    doc.apply(&transaction, view.id);
+                    doc.append_changes_to_history(view.id);
+                }
+            }
+        },
+    );
+
+    cx.push_layer(Box::new(prompt));
+}
+
 fn suspend(_cx: &mut Context) {
     #[cfg(not(windows))]
     signal_hook::low_level::raise(signal_hook::consts::signal::SIGTSTP).unwrap();
diff --git a/helix-term/src/keymap.rs b/helix-term/src/keymap.rs
index 492dc292..f3e160b1 100644
--- a/helix-term/src/keymap.rs
+++ b/helix-term/src/keymap.rs
@@ -508,6 +508,11 @@ impl Default for Keymaps {
             },
 
             "\"" => select_register,
+            "|" => shell_pipe,
+            "A-|" => shell_pipe_to,
+            "!" => shell_insert_output,
+            "A-!" => shell_append_output,
+            "$" => shell_keep_pipe,
             "C-z" => suspend,
         });
         let mut select = normal.clone();
diff --git a/helix-view/src/editor.rs b/helix-view/src/editor.rs
index 18cb9106..e5ff93ad 100644
--- a/helix-view/src/editor.rs
+++ b/helix-view/src/editor.rs
@@ -33,6 +33,8 @@ pub struct Config {
     pub scroll_lines: isize,
     /// Mouse support. Defaults to true.
     pub mouse: bool,
+    /// Shell to use for shell commands. Defaults to ["cmd", "/C"] on Windows and ["sh", "-c"] otherwise.
+    pub shell: Vec<String>,
     /// Line number mode.
     pub line_number: LineNumber,
     /// Middle click paste support. Defaults to true
@@ -55,6 +57,11 @@ impl Default for Config {
             scrolloff: 5,
             scroll_lines: 3,
             mouse: true,
+            shell: if cfg!(windows) {
+                vec!["cmd".to_owned(), "/C".to_owned()]
+            } else {
+                vec!["sh".to_owned(), "-c".to_owned()]
+            },
             line_number: LineNumber::Absolute,
             middle_click_paste: true,
         }