Wrapper for running anonymous code blocks in Salesforce, print() and println() allow you to write messages to a single output stream and have the out rendered to debug log and also in an AlertException. The AlertException is the always thrown at end of execution which ensures that nothing is ever committed to the Database.

AnonApp.cls

public abstract class AnonApp implements MainMethod {
    final String[] OUT = new String[0];
    
    public static void run(Type clazz) {
        AnonApp app = (AnonApp) clazz.newInstance();
        app.run();
    }
    
    public void run() {
    	try {
            ((MainMethod) this).main();
        } finally {
            alertOutput();
    	}
    }

    protected void println(Object obj) {
        print(obj);
        OUT.add('\n');
    }

    protected void print(Object obj) {
        OUT.add(String.valueOf(obj));
    }
    
    void alertOutput() {
        String output = String.join(OUT, '');
        System.debug(output);
        throw new AlertException(output);
    }

    class AlertException extends Exception {}
    
    public interface MainMethod {
        void main();
    }
}

ApexExecAnon

class Main extends AnonApp {
    public void main() {
        println('foo');
    	print('bar');
    }
}
AnonApp.run(Main.class);