What's a service?

A Linux service is a process or a set of processes running in the background usually in charge of executing essential system tasks or running certain kinds of server applications like databases, schedule applications, or even sound system applications.


Most Linux distributions have adopted SystemV (SysV)-like init scripts to manage their services.
sysv-rc-conf gives an easy-to-use interface for managing /etc/rc<runlevel>.d/ symlinks. 

# service --status-all # service ssh status

update-rc.d httpd defaults

redhat :

chkconfig --list 
chkconfig ssh on
chkconfig ssh off


# systemctl start httpd.service
# systemctl stop httpd.service
# systemctl restart httpd.service
# systemctl show httpd.service

# systemctl enable crond.service
# systemctl disable crond.service
# systemctl status httpd.service