ediff-shell is a variable defined in `ediff-diff.el'.
Its value is

The shell used to run diff and patch.
If user's .profile or .cshrc files are set up correctly, any shell
will do. However, some people set $prompt or other things
incorrectly, which leads to undesirable output messages. These may
cause Ediff to fail. In such a case, set `ediff-shell' to a shell that
you are not using or, better, fix your shell's startup file.

You can customize this variable.