Q:

Does sqlplus uses TNS_ADMIN variable?

Please tell me does sqlplus uses TNS_ADMIN variable on UNIX and AIX? Can we able to locate sqlplus if this variable could not be located?

A:

Sqlplus is just an executable file after all. Unix searches the directories listed in $PATH for it. You can use which sqlplus to find out full path to sqlplus.
Sqlplus itself can use $TNS_ADMIN, but absense of this variable cannot prevent OS from locating sqlplus.

