samples: shell: introduce threadless shell sample

Introduce threadless shell sample which shows how to build and use
shells in threadless (single threaded) applications.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>

Author: bjarki-andreasen

samples/subsys/shell/threadless/CMakeLists.txt

@@ -0,0 +1,9 @@
+# Copyright 2025 Nordic Semiconductor ASA
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(shell_threadless)
+
+target_sources(app PRIVATE src/main.c)

samples/subsys/shell/threadless/README.rst

@@ -0,0 +1,43 @@
+.. zephyr:code-sample:: threadless_shell
+   :name: Threadless shell
+
+   Run shell in single threaded build
+
+Overview
+********
+
+Demonstrate how to configure and use a shell in a single threaded build
+(:kconfig:option:`CONFIG_MULTITHREADING` disabled). The sample uses the
+UART shell backend using either the async
+(:kconfig:option:`CONFIG_UART_ASYNC_API`) or interrupt driven
+(:kconfig:option:`CONFIG_UART_INTERRUPT_DRIVEN`) APIs. The sample
+implements a single command, which simply echoes back the single
+argument passed to it, along with the built-in "help" command.
+
+.. code-block:: console
+
+   uart:~$ sample foobar
+   foobar
+   uart:~$
+
+Building and Running
+********************
+
+This application can be built and executed as follows:
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/subsys/shell/threadless
+   :host-os: unix
+   :board: nrf54l15dk/nrf54l15/cpuapp
+   :goals: run
+   :compact:
+
+To build for another board, change "nrf54l15dk/nrf54l15/cpuapp" above
+to that board's name.
+
+Sample Output
+*************
+
+.. code-block:: console
+
+   uart:~$

samples/subsys/shell/threadless/prj.conf

@@ -0,0 +1,3 @@
+CONFIG_MULTITHREADING=n
+CONFIG_SHELL=y
+CONFIG_SHELL_MINIMAL=y

samples/subsys/shell/threadless/sample.yaml

@@ -0,0 +1,18 @@
+sample:
+  description: Threadless shell using uart backend
+  name: Threadless shell
+tests:
+  sample.subsys.shell.threadless:
+    tags: threadless
+    min_ram: 2
+    min_flash: 32
+    filter: CONFIG_ARCH_HAS_SINGLE_THREAD_SUPPORT and
+            dt_chosen_enabled("zephyr,shell-uart")
+    harness: shell
+    harness_config:
+      shell_commands:
+        - command: "sample foobar"
+          expected: "foobar"
+    platform_exclude:
+      - qemu_cortex_a53
+      - qemu_cortex_a53/qemu_cortex_a53/smp

samples/subsys/shell/threadless/src/main.c

@@ -0,0 +1,32 @@
+/*
+ * Copyright 2025 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/shell/shell.h>
+#include <zephyr/shell/shell_uart.h>
+
+static int sample_sh_echo(const struct shell *sh, size_t argc, char **argv)
+{
+	shell_print(sh, "%s", argv[1]);
+	return 0;
+}
+
+SHELL_CMD_ARG_REGISTER(sample, NULL, "Echo", sample_sh_echo, 2, 2);
+
+int main(void)
+{
+	/*
+	 * Start the shell. This prints the shell prompt and enables
+	 * input reception.
+	 */
+	shell_start(shell_backend_uart_get_ptr());
+
+	while (1) {
+		/* Check for new input and process it if present */
+		shell_process(shell_backend_uart_get_ptr());
+	}
+
+	return 0;
+}