how to check oracle client version on linux

You can try by which is not the correct way..

strings $ORACLE_HOME/bin/oracle|grep "XML Schema Processor version"
but thats not the correct way because some of the client installs don't have an Oracle binary, just a client
library shared object.  A similar method to the above for client-only installs would be:
The command for it is locate correct file that you are using.
#strings $ORACLE_HOME/lib/ | grep '^Version [0-9]'
One challenge is that even if you find all copies of on a
host, there is no guarantee that they are each part of a complete
install.  The only real way to make sure is to try using each one (set

Each install that has been completed to installer standard should have
a line in the file /etc/oraInst.loc that says where the
oraInventory(s) are located on the host.  IIRC, has some
code you could probably crib to query any given oraInventory.

Of course, the existence of an oraInventory that says there is a
client install somewhere does not guarantee that one exists and works.