Function render 

pub fn render(list: &[(Joiner, Pipeline)]) -> String

Write an expanded command list back as one line for a real sh -c, quoted so the shell takes every word, value and file name literally and expands nothing a second time: no variables, no ~, no globs (expand those before rendering). What a guard checks in this line is what runs. The one thing left for the shell is STATUS_MARK, written as "$?".