Functions

get_hostname(): string { hostname; }
is_active(string service): status { systemctl is-active --quiet "$service"; }
print_status(string service): void { printf '%s\n' "$service"; }
read_logs(string service): stream { journalctl -u "$service"; }

string, status, void, and stream describe stdout, meaningful exit status, no captured result, and streaming stdout respectively. Parameters lower to $1, $2, and defaults:

serve() {
    local host="$1"
    local root="${2:-/opt/server}"
}

Calls never use parentheses: serve "localhost".