I tend to run sqlplus from the command line where it is really easy to set the working directory before each session. You just navigate to where you want the working directory to be and start sqplus from there. When you run “sqlplus” it uses the current directory as the working directory, easy!
This is how I do it in Windows, I assume it is the same in every other OS..
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sqlplusw does not have a CD (change dir) button, but if you set a spool file with the menus, it is easy to navigate to your directory, and this also changes the working directory. No need to use this window application throught the command-line.