diff --git a/rust/chg/src/lib.rs b/rust/chg/src/lib.rs
--- a/rust/chg/src/lib.rs
+++ b/rust/chg/src/lib.rs
@@ -9,7 +9,11 @@ extern crate futures;
 extern crate libc;
 extern crate tokio;
 extern crate tokio_hglib;
+extern crate tokio_process;
 
 pub mod attachio;
 pub mod message;
 pub mod procutil;
+mod uihandler;
+
+pub use uihandler::{ChgUiHandler, SystemHandler};
diff --git a/rust/chg/src/uihandler.rs b/rust/chg/src/uihandler.rs
new file mode 100644
--- /dev/null
+++ b/rust/chg/src/uihandler.rs
@@ -0,0 +1,83 @@
+// Copyright 2018 Yuya Nishihara <yuya@tcha.org>
+//
+// This software may be used and distributed according to the terms of the
+// GNU General Public License version 2 or any later version.
+
+use futures::Future;
+use futures::future::IntoFuture;
+use std::io;
+use std::os::unix::io::AsRawFd;
+use std::os::unix::process::ExitStatusExt;
+use std::process::{Command, Stdio};
+use tokio;
+use tokio_process::{ChildStdin, CommandExt};
+
+use super::message::CommandSpec;
+use super::procutil;
+
+/// Callback to process shell command requests received from server.
+pub trait SystemHandler: Sized {
+    type PagerStdin: AsRawFd;
+    type SpawnPagerResult: IntoFuture<Item = (Self, Self::PagerStdin), Error = io::Error>;
+    type RunSystemResult: IntoFuture<Item = (Self, i32), Error = io::Error>;
+
+    /// Handles pager command request.
+    ///
+    /// Returns the pipe to be attached to the server if the pager is spawned.
+    fn spawn_pager(self, spec: CommandSpec) -> Self::SpawnPagerResult;
+
+    /// Handles system command request.
+    ///
+    /// Returns command exit code (positive) or signal number (negative).
+    fn run_system(self, spec: CommandSpec) -> Self::RunSystemResult;
+}
+
+/// Default cHg implementation to process requests received from server.
+pub struct ChgUiHandler {
+}
+
+impl ChgUiHandler {
+    pub fn new() -> ChgUiHandler {
+        ChgUiHandler {}
+    }
+}
+
+impl SystemHandler for ChgUiHandler {
+    type PagerStdin = ChildStdin;
+    type SpawnPagerResult = io::Result<(Self, Self::PagerStdin)>;
+    type RunSystemResult = Box<dyn Future<Item = (Self, i32), Error = io::Error> + Send>;
+
+    fn spawn_pager(self, spec: CommandSpec) -> Self::SpawnPagerResult {
+        let mut pager = new_shell_command(&spec)
+            .stdin(Stdio::piped())
+            .spawn_async()?;
+        let pin = pager.stdin().take().unwrap();
+        procutil::set_blocking_fd(pin.as_raw_fd())?;
+        tokio::spawn(pager.map(|_| ()).map_err(|_| ()));  // just ignore errors
+        Ok((self, pin))
+    }
+
+    fn run_system(self, spec: CommandSpec) -> Self::RunSystemResult {
+        let fut = new_shell_command(&spec)
+            .spawn_async()
+            .into_future()
+            .flatten()
+            .map(|status| {
+                let code = status.code().or_else(|| status.signal().map(|n| -n))
+                    .expect("either exit code or signal should be set");
+                (self, code)
+            });
+        Box::new(fut)
+    }
+}
+
+fn new_shell_command(spec: &CommandSpec) -> Command {
+    let mut builder = Command::new("/bin/sh");
+    builder
+        .arg("-c")
+        .arg(&spec.command)
+        .current_dir(&spec.current_dir)
+        .env_clear()
+        .envs(spec.envs.iter().cloned());
+    builder
+ }