STDOUTListener.swift

import Foundation
import XCTest

final class STDOUTListener {
    /// Consume messages sent to STDOUT
    let input = Pipe()

    /// Sends messages back to STDOUT
    let output = Pipe()

    /// Buffers strings written to stdout
    private(set) var buffer = ""
    
    /// Fulfilled when buffer is captured.
    ///
    /// This is needed because writing to STDOUT is asynchronous
    let expectation = XCTestExpectation(
        description: "Fulfilled when buffer is captured"
    )
    
    init() {
        // Set up a read handler which fires when data is written to our inputPipe
        input.fileHandleForReading.readabilityHandler = { [weak self] handle in
            guard let self else {
                assertionFailure("`self` is nil")
                return
            }

            let data = handle.availableData
            if let string = String(data: data, encoding: String.Encoding.utf8) {
                self.buffer += string
            }

            // Write input back to stdout
            self.output.fileHandleForWriting.write(data)
            expectation.fulfill()
        }
    }
    
    /// Intercepts messages written to STDOUT
    func open() {
        // Copy the STDOUT file descriptor to `output`
        dup2(STDOUT_FILENO, output.fileHandleForWriting.fileDescriptor)

        // Intercept STDOUT with `inputPipe`
        dup2(input.fileHandleForWriting.fileDescriptor, STDOUT_FILENO)
    }
    
    /// Removes the interceptor
    func close() {
        // Restore stdout
        freopen("/dev/stdout", "a", stdout)

        [input.fileHandleForReading, output.fileHandleForWriting].forEach {
            $0.closeFile()
        }
    }
}

Description

Captures stdout messages for use in XCTests

final class MyTests: XCTestCase {
    func testSTDOUTListener() async throws {
      //-- Arrange
      let place = "world"
      let expectation = "Hello, world!\n" // print always appends a newline
      let listener = STDOUTListener()
      listener.open()
        
      //-- Act
      print("Hello, \(place)!")

      // Wait until the stdout buffer has captured text
      await fulfillment(of: [listener.expectation], timeout: 0.01)
        
      //-- Assert
      XCTAssertEqual(listener.buffer, expectation)
      listener.close()
    }
}

Credit

Lifted from the blog post Intercepting stdout in Swift