fix(update): use absolute npm script shell

src/infra/update-global.test.ts

@@ -150,6 +150,47 @@ describe("update global helpers", () => {
     });
   });
 
+  it("uses an absolute POSIX script shell for npm lifecycle scripts during global installs", async () => {
+    const platformSpy = vi.spyOn(process, "platform", "get").mockReturnValue("linux");
+    try {
+      await expect(
+        createGlobalInstallEnv({
+          COREPACK_ENABLE_DOWNLOAD_PROMPT: "1",
+          PATH: "/home/peter/.npm-global/bin",
+        }),
+      ).resolves.toMatchObject({
+        COREPACK_ENABLE_DOWNLOAD_PROMPT: "1",
+        NPM_CONFIG_SCRIPT_SHELL: "/bin/sh",
+      });
+    } finally {
+      platformSpy.mockRestore();
+    }
+  });
+
+  it("preserves explicit npm script shell config for global installs", async () => {
+    const platformSpy = vi.spyOn(process, "platform", "get").mockReturnValue("linux");
+    try {
+      await expect(
+        createGlobalInstallEnv({
+          COREPACK_ENABLE_DOWNLOAD_PROMPT: "1",
+          NPM_CONFIG_SCRIPT_SHELL: "/custom/sh",
+        }),
+      ).resolves.toMatchObject({
+        NPM_CONFIG_SCRIPT_SHELL: "/custom/sh",
+      });
+      await expect(
+        createGlobalInstallEnv({
+          COREPACK_ENABLE_DOWNLOAD_PROMPT: "1",
+          npm_config_script_shell: "/custom/lower-sh",
+        }),
+      ).resolves.toMatchObject({
+        npm_config_script_shell: "/custom/lower-sh",
+      });
+    } finally {
+      platformSpy.mockRestore();
+    }
+  });
+
   it("resolves portable Git paths from process-local app data only", async () => {
     const platformSpy = vi.spyOn(process, "platform", "get").mockReturnValue("win32");
     try {

src/infra/update-global.ts

@@ -41,6 +41,7 @@ const NPM_GLOBAL_INSTALL_OMIT_OPTIONAL_FLAGS = [
   "--omit=optional",
   ...NPM_GLOBAL_INSTALL_QUIET_FLAGS,
 ] as const;
+const NPM_CONFIG_SCRIPT_SHELL_KEYS = ["NPM_CONFIG_SCRIPT_SHELL", "npm_config_script_shell"];
 const FIRST_PACKAGED_DIST_INVENTORY_VERSION = { major: 2026, minor: 4, patch: 15 };
 const OMITTED_PRIVATE_QA_BUNDLED_PLUGIN_ROOTS = new Set([
   "dist/extensions/qa-channel",
@@ -315,6 +316,31 @@ function applyCorepackDownloadPromptEnv(env: Record<string, string>) {
   }
 }
 
+function hasNpmScriptShellSetting(env: NodeJS.ProcessEnv): boolean {
+  return NPM_CONFIG_SCRIPT_SHELL_KEYS.some((key) => Boolean(env[key]?.trim()));
+}
+
+function resolvePosixNpmScriptShell(env: NodeJS.ProcessEnv): string | null {
+  if (process.platform === "win32") {
+    return null;
+  }
+  if (fsSync.existsSync("/bin/sh")) {
+    return "/bin/sh";
+  }
+  const shell = env.SHELL?.trim();
+  return shell && path.isAbsolute(shell) && fsSync.existsSync(shell) ? shell : null;
+}
+
+function applyPosixNpmScriptShellEnv(env: Record<string, string>) {
+  if (hasNpmScriptShellSetting(env)) {
+    return;
+  }
+  const scriptShell = resolvePosixNpmScriptShell(env);
+  if (scriptShell) {
+    env.NPM_CONFIG_SCRIPT_SHELL = scriptShell;
+  }
+}
+
 export function resolveGlobalInstallSpec(params: {
   packageName: string;
   tag: string;
@@ -344,8 +370,13 @@ export async function createGlobalInstallEnv(
   const hasCorepackDownloadPromptSetting = Boolean(
     sourceEnv.COREPACK_ENABLE_DOWNLOAD_PROMPT?.trim(),
   );
+  const missingPosixScriptShell =
+    Boolean(resolvePosixNpmScriptShell(sourceEnv)) && !hasNpmScriptShellSetting(sourceEnv);
   const requiresMergedEnv =
-    pathPrepend.length > 0 || process.platform === "win32" || !hasCorepackDownloadPromptSetting;
+    pathPrepend.length > 0 ||
+    process.platform === "win32" ||
+    !hasCorepackDownloadPromptSetting ||
+    missingPosixScriptShell;
   if (!requiresMergedEnv) {
     return env;
   }
@@ -357,6 +388,7 @@ export async function createGlobalInstallEnv(
   applyPathPrepend(merged, pathPrepend);
   applyWindowsPackageInstallEnv(merged);
   applyCorepackDownloadPromptEnv(merged);
+  applyPosixNpmScriptShellEnv(merged);
   return merged;
 }