POSIX multi-value option_argument syntax

Question

I have read https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap12.html.

I understand that a short option_argument can look like -kvalue or -k value.

If we want to specify multiple values for the option_argument, we can write -kvalue1 -kvalue2, -k value1 -k value2, or a mix of both syntaxes.

My question:

Is -k value1 value2 valid and equivalent to the above?

Further, what about for long option_arguments? Is --key value1 value2 the same?

Answer 1

-k value1 value2 is usually equivalent to -k value1 -- value2, where value1 is the option_argument to -k, and value2 is an operand (a program argument like in echo value2 or ls value2). Were you to implement this by manipulating optind/OPTIND, you would have the following problems:

• In -k value1 value2 ... valueN input-file, how would you know that input-file is an input file and not intended as -k input-file?
• Does -k -z mean -k had 0 arguments, followed by the -z option, or is -z treated as an option_argument? What about -k value1 value2 -z value4?

Also, -k value1 -k value2 would typically replace value1 with value2, so it would be a good idea to document the additive behavior of -k if it were implemented. You should avoid -( and -) options. On Solaris, per its CLIP guidelines (scroll down to the Command Syntax Standard: Guidelines section), the ( and ) characters delimit long option names, like ?(help)V(version) would recognize --help as -? and --version as -V.

As for long option arguments, POSIX does not support them, whether that is -old-style arg, --gnu-style=arg, or --gnu-style arg. An optstring "-:k:" is not portable according to POSIX either because - is not alphanumeric; even if it is supported, you would also need to deal with changing optind/OPTIND in the case of --key value1 to set the key value to value1 since key is the option_argument for the - option.

If you are merely writing a utility rather than implementing getopt rules in your own option parser, an alternative would be to rely on getopt_long, keeping the implementation differences in mind; it is widely available, even if it is not specified by POSIX. You could also use a separate option parsing library (or shell function or whatever you would use in the language you're dealing with).