Enable service at boot for runlevels 2, 3, 4, and 5

Manage the runlevel of services on CentOS 6.
More information: <https://manned.org/chkconfig>.
Trick copyright: tl;dr; <https://github.com/tldr-pages/tldr>

chkconfig --level {{2345}} {{sshd}} on

click the source code to copy install chkconfig on any operating system with command-not-found.com