feat: detect shell for --kernel

- Move detect_shell() to new src/utils.rs for reuse
- Use detected shell in QEMU VM mode (bash when available)
- Update chroot.rs and qemu_vm.rs to use shared function

src/chroot.rs

@@ -18,22 +18,13 @@ pub fn run_chroot(
         eprintln!("Warning: Failed to set hostname: {}", e);
     }
 
+    // Detect shell before chroot (we're still outside)
+    let shell = crate::utils::detect_shell(rootfs);
+
     // Change to root directory in chroot
     chroot(rootfs).context("Failed to chroot")?;
 
     // Now we're inside the chroot - set up environment based on chroot filesystem
-    // Determine shell path (check inside chroot, not host)
-    let shell = if Path::new("/bin/bash").exists() {
-        "/bin/bash"
-    } else if Path::new("/bin/sh").exists() {
-        "/bin/sh"
-    } else if Path::new("/usr/bin/bash").exists() {
-        "/usr/bin/bash"
-    } else if Path::new("/usr/bin/sh").exists() {
-        "/usr/bin/sh"
-    } else {
-        "/bin/sh" // Will fail with clear error if not present
-    };
 
     // Set up environment variables (after chroot, so paths are correct)
     let env = setup_environment(shell, &host_term);

src/main.rs

@@ -1,4 +1,5 @@
 mod chroot;
+mod utils;
 mod cli;
 mod config;
 mod distro;

src/qemu_vm.rs

@@ -54,6 +54,9 @@ pub fn launch_qemu(config: QemuConfig) -> Result<()> {
         qemu_bin, get_arch_package_suffix(&config.arch), get_arch_package_suffix(&config.arch), get_arch_package_suffix(&config.arch)
     ))?;
 
+    // Detect the best available shell in the rootfs
+    let shell = crate::utils::detect_shell(&config.rootfs_path);
+    
     // Generate a unique hostname like "ecr-vm-a1b2c3"
     let hostname_suffix = format!("{:x}", (std::process::id() as u64)
         .wrapping_mul(std::time::SystemTime::now()
@@ -72,14 +75,14 @@ pub fn launch_qemu(config: QemuConfig) -> Result<()> {
     let kernel_append = if let Some(ref cmd) = config.command {
         let cmd_str = cmd.join(" ");
         format!(
-            "console=ttyS0 quiet rdinit=/bin/sh -- -c \"echo {} >/etc/hostname; hostname {}; setsid sh -c 'exec sh </dev/ttyS0 >/dev/ttyS0 2>&1 -c {}'\"",
+            "console=ttyS0 quiet rdinit=/bin/sh -- -c \"echo {} >/etc/hostname; hostname {}; setsid sh -c 'exec {} </dev/ttyS0 >/dev/ttyS0 2>&1 -c {}'\"",
-            hostname, hostname, cmd_str
+            hostname, hostname, shell, cmd_str
         )
     } else {
         // Default to interactive shell with proper job control
         format!(
-            "console=ttyS0 quiet rdinit=/bin/sh -- -c \"echo {} >/etc/hostname; hostname {}; setsid sh -c 'exec sh </dev/ttyS0 >/dev/ttyS0 2>&1'\"",
+            "console=ttyS0 quiet rdinit=/bin/sh -- -c \"echo {} >/etc/hostname; hostname {}; setsid sh -c 'exec {} </dev/ttyS0 >/dev/ttyS0 2>&1'\"",
-            hostname, hostname
+            hostname, hostname, shell
         )
     };
 

src/utils.rs

@@ -0,0 +1,19 @@
+use std::path::Path;
+
+/// Detect the best available shell in a rootfs
+/// Checks for bash first, falls back to sh
+/// Returns the path relative to the rootfs (e.g., "/bin/bash")
+pub fn detect_shell(rootfs: &Path) -> &'static str {
+    // Check for bash first (preferred)
+    if rootfs.join("bin/bash").exists() {
+        "/bin/bash"
+    } else if rootfs.join("bin/sh").exists() {
+        "/bin/sh"
+    } else if rootfs.join("usr/bin/bash").exists() {
+        "/usr/bin/bash"
+    } else if rootfs.join("usr/bin/sh").exists() {
+        "/usr/bin/sh"
+    } else {
+        "/bin/sh" // Will fail with clear error if not present
+    }
+}