ssh-add could not openm a connection to your authentication agent code example

Example 1: ssh-add could not open a connection to your authentication agent centos

##1
$ cd ~/.ssh
##2
#ubuntu
$ eval $(ssh-agent)
#CentOS
$ exec ssh-agent bash
##3
$ ssh-add my_id_rsa

Example 2: ssh-add could not open a connection to your authentication agent

eval `ssh-agent -s`
ssh-add