The following is an example of an /etc/shells file with a /bin/true listed for FTP guest users:
/sbin/sh /bin/csh /bin/jsh /bin/ksh /bin/remsh /bin/rksh /bin/rsh /bin/sh /usr/bin/csh /usr/bin/ksh /usr/bin/bash /usr/bin/tcsh /usr/bin/zsh /bin/true