Bass makes it easy to use utilities written for Bash in fish shell. Regular bash scripts can be used in fish shell just as scripts written in any language with proper shebang or explicitly using the interpreter (i.e. using bash script.sh). However, many utilities, such as virtualenv, modify the shell environment and need to be sourced, and therefore cannot be used in fish. Sometimes, counterparts (such as the excellent virtualfish) are created, but that's often not the case.