hive-bash-mcp: invoke bash via /usr/bin/env (review)

hive-bash-mcp/src/bin/mcp.rs

@@ -151,7 +151,7 @@ fn render_bash_status(id: &str, resp: Result<DaemonResponse>, waited: bool) -> S
 
 #[derive(Debug, Deserialize, JsonSchema)]
 struct BashRunArgs {
-    /// Shell command to run (passed to `sh -c`).
+    /// Shell command to run (passed to `bash -c`).
     cmd: String,
     /// Timeout in seconds. Defaults to `None` (no timeout) — task runs until
     /// natural exit. Pass an explicit value to kill the task after N seconds

hive-bash-mcp/src/runner.rs

@@ -1,4 +1,4 @@
-//! Bash subprocess runner: spawns `sh -c <cmd>` tasks, writes status
+//! Bash subprocess runner: spawns `bash -c <cmd>` tasks, writes status
 //! files under `harness/bash-tasks/`, and fires hyperhive wake signals
 //! on completion. Mirrors the logic previously embedded in `hive-ag3nt`.
 //!
@@ -400,8 +400,13 @@ async fn run_task(mut task: TaskFile, socket: &Path) {
     send_wake(socket, &id, &summary, Some((out_snippet, err_snippet))).await;
 }
 
-/// Run `sh -c cmd`, streaming output to files. Returns `(exit_code, timed_out)`.
+/// Run `bash -c cmd`, streaming output to files. Returns `(exit_code, timed_out)`.
 /// `timeout_secs = None` means no timeout — run until natural exit.
+///
+/// Tasks run under `bash`, not `sh`: on NixOS `/bin/sh` is bash in POSIX
+/// mode, which disables bashisms (arrays, `[[ … ]]`, `local`, process
+/// substitution, …). Agents write bash, so we invoke `bash` (via
+/// `/usr/bin/env bash`).
 async fn exec_cmd(
     cmd: &str,
     out_path: &Path,
@@ -411,8 +416,13 @@ async fn exec_cmd(
     use tokio::process::Command;
     // SAFETY: `nice` is async-signal-safe and modifies only the calling
     // process's scheduling priority before exec. No allocations, no locks.
-    let mut cmd_builder = Command::new("sh");
+    // `/usr/bin/env bash` rather than a bare `bash`: `/usr/bin/env` is at a
+    // fixed absolute path (coreutils, present on NixOS), and it resolves
+    // `bash` via PATH — the same controlled PATH the daemon's systemd unit
+    // sets. Avoids hardcoding a nix store / generation path in the binary.
+    let mut cmd_builder = Command::new("/usr/bin/env");
     cmd_builder
+        .arg("bash")
         .arg("-c")
         .arg(cmd)
         .stdout(std::process::Stdio::piped())

nix/templates/harness-base.nix

@@ -1419,14 +1419,14 @@ in
     systemd.services.hive-bash-daemon = {
       description = "bash task runner daemon for hive-bash-mcp";
       wantedBy = [ "multi-user.target" ];
-      # The daemon runs every bash task via `Command::new("sh")` and the
+      # The daemon runs every bash task via `Command::new("bash")` and the
       # commands themselves (hive-forge, git, jq, …) resolve from PATH.
       # Pre-split this ran inside hive-ag3nt.service and inherited the
-      # agent's PATH; the standalone daemon needs the same or `sh` itself
+      # agent's PATH; the standalone daemon needs the same or `bash` itself
       # isn't found (spawn fails with ENOENT, the task is marked done in
       # 0s with no output / no .out/.err). Mirror the harness unit's PATH:
       # NixOS appends `/bin` to each entry → /run/wrappers/bin (setuid
-      # sudo) + /run/current-system/sw/bin (sh, coreutils, hive-forge, …).
+      # sudo) + /run/current-system/sw/bin (bash, coreutils, hive-forge, …).
       path = [
         "/run/wrappers"
         "/run/current-system/sw"