Oracle Enterprise Manager 11g Database Control - Agent Unreachable Single instance DB. I can login to Oracle Enterprise Manger (OEM). However,  on the Home Page it says:

Status - Agent Unreachable   


Most tasks such as checking RMAN status, adding datafiles, etc. can still be done through the OEM. I do not know how Agent Unreachable impacts the OEM in this case (DB Control)
I checked status of the agent from $ORACLE_HOME/bin/emctl status agent, it showed:

The Agent is not Running


So, I want to start the agent, but running the command $ORACLE_HOME/bin/emctl start agent is not working. The command is not available.
In Oracle docs, it is mentioned that the command emctl start agent should be executed from AGENT_HOME/bin. However, I do not know the AGENT_HOME path.
How can I start the OEM Agent or figure out what the underlying issue is?
Do you have separate Oracle Homes for your Database and Grid Infrastructure (which was new to 11gR2, IIRC)?
If so, then the Agent probably runs from the Grid Infrastructure's Oracle Home.
Annoyingly, the bin directory in both Oracle Homes contains a program called emctl but only one of them will work as you expect!
If in doubt use the Linux find command to locate anything called emctl ...
find ???  -type f  -name emctl  -ls 

I'd suggest, once you've found it, you add the Agent's &quot;Oracle Home&quot; into /etc/oratab under its own name, to make it easier to find it next time.