Use your language's command-line option processing libraries.
OptionParser in ruby and argparse in python. There is no reason to eschew these libraries: they're part of the standard library, they require zero coupling to your app logic, and they handle all of the edge cases for free.
"But I can just shift the arguments", you say! Yeah? Great! What if the user pipes input through STDOUT? What if the user passes a flag, a required argument, and then another flag? Will your script read the last flag correctly, or has the naive logic already entered "required argument processing mode"? Just use the library. It's a solved problem!
Use your language's command-line option processing libraries.
OptionParser in ruby and argparse in python. There is no reason to eschew these libraries: they're part of the standard library, they require zero coupling to your app logic, and they handle all of the edge cases for free.
"But I can just shift the arguments", you say! Yeah? Great! What if the user pipes input through STDOUT? What if the user passes a flag, a required argument, and then another flag? Will your script read the last flag correctly, or has the naive logic already entered "required argument processing mode"? Just use the library. It's a solved problem!