idlwave-shell-explicit-file-name

idlwave-shell-explicit-file-name is a variable defined in `idlwave.el'.
Its value is

"idl"

Documentation:
If non-nil, this is the command to run IDL.
Should be an absolute file path or path relative to the current environment
execution search path. If you want to specify command line switches
for the IDL program, use `idlwave-shell-command-line-options'.

I know the name of this variable is badly chosen, but I cannot change
it without compromising backwards-compatibility.

You can customize this variable.