I installed apache2 on ubuntu 13.10. If I try to restart it using

sudo /etc/init.d/apache2 restart

I get this message:

AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1. Set the 'ServerName' directive globally to suppress this message

So I read that I should edit my httpd.conf file. But, since I can't find it in /etc/apache2/ folder, I tried to locate it using this command:

/usr/sbin/apache2 -V

But the output I get is this:

[Fri Nov 29 17:35:43.942472 2013] [core:warn] [pid 14655] AH00111: Config variable ${APACHE_LOCK_DIR} is not defined
[Fri Nov 29 17:35:43.942560 2013] [core:warn] [pid 14655] AH00111: Config variable ${APACHE_PID_FILE} is not defined
[Fri Nov 29 17:35:43.942602 2013] [core:warn] [pid 14655] AH00111: Config variable ${APACHE_RUN_USER} is not defined
[Fri Nov 29 17:35:43.942613 2013] [core:warn] [pid 14655] AH00111: Config variable ${APACHE_RUN_GROUP} is not defined
[Fri Nov 29 17:35:43.942627 2013] [core:warn] [pid 14655] AH00111: Config variable ${APACHE_LOG_DIR} is not defined
[Fri Nov 29 17:35:43.947913 2013] [core:warn] [pid 14655] AH00111: Config variable ${APACHE_LOG_DIR} is not defined
[Fri Nov 29 17:35:43.948051 2013] [core:warn] [pid 14655] AH00111: Config variable ${APACHE_LOG_DIR} is not defined
[Fri Nov 29 17:35:43.948075 2013] [core:warn] [pid 14655] AH00111: Config variable ${APACHE_LOG_DIR} is not defined

AH00526: Syntax error on line 74 of /etc/apache2/apache2.conf:
Invalid Mutex directory in argument file:${APACHE_LOCK_DIR}

Line 74 of /etc/apache2/apache2.conf is this:

Mutex file:${APACHE_LOCK_DIR} default

I gave a look at my /etc/apache2/envvar file, but I don't know what to do with it.

What should I do?