This isn't really a SQL Developer question, you should mark it answered and ask again in the database general forum)
Nowadays it it most likely to be ksh or bash. They are very similar.
PL/SQL and shell scripting are quite distinct - you may embed SQL and PL/SQL within a shell script, but they don't interact specifically.
Shell script in a database environment is very useful as a wrapper round sql scripts. It is better and collecting and validating parameters before calling sqlplus, and for interpretting or reformatting output after the call. Not to mention the many, many more general uses.
Scripting (whether unix shell, windows powershell, or more advanced scripting like perl or python ) is an invaluable skill for either a developer or a dba.