Function shell_var_name 

pub fn shell_var_name(shell: &str) -> String

Keyed by the program rather than the subcommand, because that is what the value names. usage powershell runs pwsh, so its variable is USAGECLI_SHELL_PWSH.

The current spelling, since this is what error messages tell people to set. The legacy USAGE_SHELL_* is still read — see setting.