Oracle Listener

Listener is a separate process that runs on the database server computer. It receives incoming client connection requests and manages the traffic of these requests to the database server.

listener directory - $ORACLE_HOME/112_64/network/admin

eg: listener is a tool to connect a DB with SAP.

Useful commands:-

show listener status - lsnrctl status
start listener - lsnrctl start
stop listener - lsnrctl stop
check listener process - ps -ef | grep lsnr
test listener connection - tnsping SID
list all option for listener as below - lsnrctl help

The following lsnrctl operations are available
An asterisk (*) denotes a modifier or extended command:
start                             stop                        status
services                       version                   reload
save_config                 trace                      spawn
dbsnmp_start              dbsnmp_stop         dbsnmp_status
change_password       quit                        exit
set*                             show*

By default, listener will point to your local hostname and port 1521.
However there is always circumstances that error happens when someone customize the listener configuration such doing oracle patching and messed up the settings.
You can always check the listener configuration in listener file and confirm everything was right.

Try to check listener status to make sure your listener point to your listener configuration file in %ORACLE_HOME/112_64/network/admin.

If not, you can change it in listener cmd line.
lsnrctl > help > show > show current_version > LISTENER_A (different from listener config file)

change it by using > set current_listener LISTENER > save_config