Function remote_command 

pub fn remote_command(
    cmd: &str,
    cmd_args: &[String],
    cwd: &str,
) -> Option<String>

The command line ssh hands to the remote user’s login shell: the pane’s command, run in cwd when one is set, or for a plain pane with a cwd, the login shell started there. None for a plain pane with no cwd: ssh then starts the login shell itself.

The remote shell parses this, whatever it is (bash, zsh, fish), so only single-quoted words, a leading ~/, ;, exec and "$SHELL" appear, which all of them read the same way.