Shell: Add support for special parameter that expands to return-code of last program executed

Author: https://github.com/MinusGix
Commit: https://github.com/SerenityOS/serenity/commit/91a609d9454
Pull-request: https://github.com/SerenityOS/serenity/pull/537
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/bugaevc

Shell/GlobalState.h

@@ -14,6 +14,7 @@ struct GlobalState {
     struct termios termios;
     bool was_interrupted { false };
     bool was_resized { false };
+    int last_return_code { 0 };
 };
 
 extern GlobalState g;

Shell/main.cpp

@@ -307,6 +307,8 @@ static Vector<String> expand_parameters(const StringView& param)
         return { param };
 
     String variable_name = String(param.substring_view(1, param.length() - 1));
+    if (variable_name == "?")
+        return { String::number(g.last_return_code) };
 
     char* env_value = getenv(variable_name.characters());
     if (env_value == nullptr)
@@ -546,6 +548,8 @@ static int run_command(const String& cmd)
         }
     }
 
+    g.last_return_code = return_value;
+
     // FIXME: Should I really have to tcsetpgrp() after my child has exited?
     //        Is the terminal controlling pgrp really still the PGID of the dead process?
     tcsetpgrp(0, getpid());