Add support for exit codes

- New configurations to CommandRegistration
- Re-using exit code concepts from boot
- Handling exit codes only in non-interactive mode
- Adding e2e commands and tests for better coverage
- Fixes #431

spring-shell-docs/src/main/asciidoc/using-shell-commands-exitcode.adoc

@@ -0,0 +1,39 @@
+[[dynamic-command-exitcode]]
+==== Exit Code
+ifndef::snippets[:snippets: ../../test/java/org/springframework/shell/docs]
+
+Many command line applications when applicable return an _exit code_ which running environment
+can use to differentiate if command has been executed successfully or not. In a `spring-shell`
+this mostly relates when a command is run on a non-interactive mode meaning one command
+is always executed once with an instance of a `spring-shell`.
+
+Default behaviour of an exit codes is as:
+
+- Errors from a command option parsing will result code of `2`
+- Any generic error will result result code of `1`
+- Obviously in any other case result code is `0`
+
+Every `CommandRegistration` can define its own mappings between _Exception_ and _exit code_.
+Essentially we're bound to functionality in `Spring Boot` regarding _exit code_ and simply
+integrate into that.
+
+Assuming there is an exception show below which would be thrown from a command:
+
+====
+[source, java, indent=0]
+----
+include::{snippets}/ExitCodeSnippets.java[tag=my-exception-class]
+----
+====
+
+It is possible to define a mapping function between `Throwable` and exit code. You can also
+just configure a _class_ to _exit code_ which is just a syntactic sugar within configurations.
+
+====
+[source, java, indent=0]
+----
+include::{snippets}/ExitCodeSnippets.java[tag=example1]
+----
+====
+
+NOTE: Exit codes cannot be customized with annotation based configuration

spring-shell-docs/src/main/asciidoc/using-shell-commands.adoc

@@ -18,3 +18,5 @@ include::using-shell-commands-programmaticmodel.adoc[]
 include::using-shell-commands-organize.adoc[]
 
 include::using-shell-commands-availability.adoc[]
+
+include::using-shell-commands-exitcode.adoc[]

spring-shell-docs/src/test/java/org/springframework/shell/docs/ExitCodeSnippets.java

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2022 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.shell.docs;
+
+import org.springframework.shell.command.CommandRegistration;
+
+public class ExitCodeSnippets {
+
+	// tag::my-exception-class[]
+	static class MyException extends RuntimeException {
+
+		private final int code;
+
+		MyException(String msg, int code) {
+			super(msg);
+			this.code = code;
+		}
+
+		public int getCode() {
+			return code;
+		}
+	}
+	// end::my-exception-class[]
+
+	void dump1() {
+		// tag::example1[]
+		CommandRegistration.builder()
+			.withExitCode()
+				.map(MyException.class, 3)
+				.map(t -> {
+					if (t instanceof MyException) {
+						return ((MyException) t).getCode();
+					}
+					return 0;
+				})
+				.and()
+			.build();
+		// end::example1[]
+	}
+}