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Options are used to provide extra information to tune or customize the
execution of a program. In case it wasn't clear, options are usually
optional. A program should be able to run just fine with no options
whatsoever. (Pick a random program from the Unix or GNU toolsets. Can
it run without any options at all and still make sense? The main
exceptions are find, tar, and dd--all of which are mutant
oddballs that have been rightly criticized for their non-standard syntax
and confusing interfaces.)
Lots of people want their programs to have ``required options''. Think about it. If it's required, then it's not optional! If there is a piece of information that your program absolutely requires in order to run successfully, that's what positional arguments are for.
As an example of good command-line interface design, consider the humble
cp utility, for copying files. It doesn't make much sense to try to
copy files without supplying a destination and at least one source.
Hence, cp fails if you run it with no arguments. However, it has a
flexible, useful syntax that does not require any options at all:
cp SOURCE DEST cp SOURCE ... DEST-DIR
You can get pretty far with just that. Most cp implementations
provide a bunch of options to tweak exactly how the files are copied:
you can preserve mode and modification time, avoid following symlinks,
ask before clobbering existing files, etc. But none of this distracts
from the core mission of cp, which is to copy either one file to
another, or several files to another directory.
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