How to add target manually in 12c EM ?

Prepare the 12c EM server
# vi /etc/hosts
10.10.10.10 alperdb.com
-> remote machine (agent) IP and hostname

Prepare the environment (at new host)
# useradd grid12 -g dba
# vi /etc/hosts
10.10.10.2 grid12.com 
-> Current grid12 server machine IP and hostname

# vi /etc/sudoers
grid12 ALL=NOPASSWD: ALL
grid12 ALL=(ALL) ALL

# vi /etc/oraInst.loc
inventory_loc=/home/grid12/oraInventory
inst_group=dba

# passwd grid12
-> password: grid12

# sudo su - grid12
$ mkdir /home/grid12/oraInventory

Adding Host
Login -> Setup from top-right corner -> Add Target -> Add Target Manually -> Add Host -> Installation Base Directory: /home/grid12/agent -> Instance Directory field will be filled automatically -> 

Configure Agent
Login -> Setup from top-right corner -> Add Target -> Add Target Manually -> Select Add Targets Declaratively by Specifying Target Monitoring Properties -> Target Type: Database Instance, Monitoring Agent: 10.10.10.10:3872 -> Target Name: ALPERDB, Database System: ALPERDB, Oracle Home Path: echo $ORACLE_HOME, Port: 1521 -> Click Test Connection -> Submit -> Done! :)

How to configure EM (Enterprose Manager) if has any problem?

On linux terminal;
set ORACLE_SID=<YOURSID>
emca -deconfig dbcontrol db -repos drop;
then;
emca -config dbcontrol db -repos create;
or
emca -config dbcontrol db -repos recreate;

Enterprise Manager and OEM Agent commands in Oracle

Please login to database server with ssh as oracle user and run that commands.

Go to /bin path
su - grid12
cd agent/agent_inst/bin

How to start/stop Enterprise Manager
./emctl start dbconsole;
./emctl stop dbconsole;

for 12c grid EM
/home/grid12/oms/bin/emctl start oms
/home/grid12/oms/bin/emctl stop oms

How to start/stop Oem Agent
Be sure run root.sh as root user and run below commands as oracle user:
./emctl stop agent
./emctl start agent

How to check Oem Agent
./emctl status agent

How to restore Oem Agent
Run as grid12 user:
./emctl clearstate agent; and then
./emctl upload agent;