Find out Oracle home directory of running Oracle instance on Linux

If /etc/oratab is deleted or corrupted, then
ps aux | grep ora_pmon
oracle   10937  0.0  0.0 2445960 6732 ?        Ss   May25  11:01 ora_pmon_CDRUAT
strings /proc/10937/environ | grep ORACLE_HOME
ORACLE_HOME=/opt/oracle/product/12.1.0.2/ee_3