I’m guessing, you mean this then: https://github.com/edc/bass

But well, I was rather thinking of when it’s using Bash-scripting-syntax to combine multiple commands.
Like, maybe there’s a for-loop in there. You just can’t paste that directly into Fish and have it work. Granted, you should probably put that into a script file, even if you’re using Bash, but yeah, just temporarily launching bash is also an option.