Added runner for testing external executables

Package.swift

@@ -1,23 +1,17 @@
 // swift-tools-version:5.1
-// The swift-tools-version declares the minimum version of Swift required to build this package.
 
 import PackageDescription
 
 let package = Package(
     name: "XCTestExtensions",
     products: [
-        // Products define the executables and libraries produced by a package, and make them visible to other packages.
         .library(
             name: "XCTestExtensions",
             targets: ["XCTestExtensions"]),
     ],
     dependencies: [
-        // Dependencies declare other packages that this package depends on.
-        // .package(url: /* package url */, from: "1.0.0"),
     ],
     targets: [
-        // Targets are the basic building blocks of a package. A target can define a module or a test suite.
-        // Targets can depend on other targets in this package, and on products in packages which this package depends on.
         .target(
             name: "XCTestExtensions",
             dependencies: []),

Sources/XCTestExtensions/XCTestExtensions.swift

@@ -135,6 +135,17 @@ extension XCTestCase {
         return Bundle.main.bundleURL
       #endif
     }
+
+    #if os(macOS) || os(Linux)
+    /// Run an external executable in the same location as the test bundle, and
+    /// return its output.
+    @available(macOS 10.13, *) public func run(_ command: String, arguments: [String] = []) -> XCTestRunner.Result {
+        let runner = XCTestRunner(for: command)
+        let result = runner.run(with: arguments)
+        XCTAssertNotNil(result)
+        return result!
+    }
+    #endif
 }
 
 /// Assert that a result is a success.

Sources/XCTestExtensions/XCTestRunner.swift

@@ -0,0 +1,82 @@
+// -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+//  Created by Sam Deane on 23/03/2020.
+//  All code (c) 2020 - present day, Elegant Chaos Limited.
+// -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+
+import Foundation
+
+#if os(macOS) || os(Linux)
+@available(macOS 10.13, *) public class XCTestRunner {
+    var environment: [String:String]
+    let executable: URL
+    public var cwd: URL?
+
+    public struct Result {
+        public let status: Int32
+        public let stdout: String
+        public let stderr: String
+    }
+
+    /**
+      Initialise with an explicit URL to the executable.
+    */
+
+    public init(for executable: URL, cwd: URL? = nil, environment: [String:String] = ProcessInfo.processInfo.environment) {
+        self.executable = executable
+        self.environment = environment
+        self.cwd = cwd
+    }
+
+    /**
+      Initialise to run a command in the same built products directory as this test bundle.
+    */
+
+    public convenience init(for command: String, cwd: URL? = nil, environment: [String:String] = ProcessInfo.processInfo.environment) {
+        let url = Bundle(for: XCTestRunner.self).bundleURL.deletingLastPathComponent()
+        self.init(for: url.appendingPathComponent(command), cwd: cwd, environment: environment)
+    }
+
+
+    /**
+     Invoke a command and some optional arguments synchronously.
+     Waits for the process to exit and returns the captured output plus the exit status.
+     */
+
+    public func run(with arguments: [String] = []) -> Result? {
+        let stdout = Pipe()
+        let stderr = Pipe()
+        let process = Process()
+        if let cwd = cwd {
+            process.currentDirectoryURL = cwd
+        }
+        process.executableURL = executable
+        process.arguments = arguments
+        process.standardOutput = stdout
+        process.standardError = stderr
+        process.environment = environment
+        process.launch()
+        process.waitUntilExit()
+
+        return Result(status: process.terminationStatus, stdout: stdout.asString, stderr: stderr.asString)
+    }
+
+    /// Extract text from an output pipe
+    /// - Parameter source: the pipe
+    func captureString(from pipe: Any?) -> String {
+        if let pipe = pipe as? Pipe {
+            let data = pipe.fileHandleForReading.readDataToEndOfFile()
+            if let string = String(data: data, encoding: .utf8) {
+                return string
+            }
+        }
+        return ""
+    }
+}
+
+public extension Pipe {
+    var asString: String {
+        let data = fileHandleForReading.readDataToEndOfFile()
+        return  String(data: data, encoding: .utf8) ?? ""
+    }
+}
+#endif