What are standard tools and shell features? Bash + GNU? Those might be consistently installed on common Linux distros but that doesn't work across *nix systems

What are standard tools and shell features? Bash + GNU? Those might be consistently installed on common Linux distros but that doesn't work across *nix systems (Linux, macOS, BSD). That probably won't work against less-common Linux setups like anything embedded (busybox or container images) either