SHELL

linux apache start

sudo service apache2 start
sudo service apache2 restart
sudo service apache2 stop

OR 

sudo /etc/init.d/apache2 start
sudo /etc/init.d/apache2 restart
sudo /etc/init.d/apache2 stop

How to start apache2 server

# Start Apache 2 server on Linux/Unix
sudo service apache2 start

start apache service

systemctl status apache2

starting apache2 server

$ sudo systemctl stop apache2
$ sudo systemctl start apache2

attemting to start apache server

// Go to this directory using cmd
C:xamppapachein
Then, type

httpd

It would return a statement indicating why there's an issue. 

Most probably, ServerName in httpd.conf file has two parameters instead of one