Yes the default is /bin/sh not csh nor tcsh. One can rewrite your libc's implementation of the system() function to call another shell if you'd like but it rarely gets invoked as a login shell. Most system() implementations hard wire to <kbd>/bin/sh -c</kbd> or the equivalent for your system (e.g. Linux has a symlink named /bin/sh that points to /bin/bash).

Historically sh was the first shell (circa 1972) and there was no csh (much less a tcsh) until Bill Joy and the BSD gang decided that they wanted a shell with a syntax closer to the C programming language syntax. By most any measure they failed miserably (among other things {t}csh script do not use a semi-colon as a statement terminator nor as statement separator). csh implementations vary quite widely between vendors - since Bill's csh source was never opened various line length limits were allowed to creep into various csh flavors. That csh could not even support shell functions meant that it was nowhere near C's usefulness.

By "default CLI" I think that the previous poster meant /bin/sh, and was not referring to the shell entry in /etc/passwd for any user.