Fix default shell when /bin/bash is not present (#58)

Resolves #56.

crates/sshx/examples/stdin_client.rs

@@ -13,7 +13,7 @@ use tracing::{error, info, trace};
 async fn main() -> Result<()> {
     tracing_subscriber::fmt::init();
 
-    let shell = get_default_shell();
+    let shell = get_default_shell().await;
     info!(%shell, "using default shell");
 
     let mut terminal = Terminal::new(&shell).await?;

crates/sshx/src/main.rs

@@ -47,7 +47,10 @@ fn print_greeting(shell: &str, controller: &Controller) {
 
 #[tokio::main]
 async fn start(args: Args) -> Result<()> {
-    let shell = args.shell.unwrap_or_else(get_default_shell);
+    let shell = match args.shell {
+        Some(shell) => shell,
+        None => get_default_shell().await,
+    };
 
     let runner = Runner::Shell(shell.clone());
     let mut controller = Controller::new(&args.server, runner).await?;

crates/sshx/src/terminal.rs

@@ -18,13 +18,23 @@ use nix::sys::signal::{kill, Signal::SIGKILL};
 use nix::sys::wait::waitpid;
 use nix::unistd::{execvp, fork, ForkResult, Pid};
 use pin_project::{pin_project, pinned_drop};
-use tokio::fs::File;
+use tokio::fs::{self, File};
 use tokio::io::{self, AsyncRead, AsyncWrite};
 use tracing::{instrument, trace};
 
 /// Returns the default shell on this system.
-pub fn get_default_shell() -> String {
-    env::var("SHELL").unwrap_or_else(|_| String::from("/bin/bash"))
+pub async fn get_default_shell() -> String {
+    if let Ok(shell) = env::var("SHELL") {
+        if !shell.is_empty() {
+            return shell;
+        }
+    }
+    for shell in ["/bin/bash", "/bin/sh"] {
+        if fs::metadata(shell).await.is_ok() {
+            return shell.to_string();
+        }
+    }
+    String::from("sh")
 }
 
 /// An object that stores the state for a terminal session.