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From: Kurt H Maier <khm@sciops.net>
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Subject: Re: [9front] Tar's usage messages
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On Sun, Dec 13, 2020 at 01:47:19PM +0600, kl wrote:
> Hi,
> 
> How should I read this,
> 
> 
> 	tar {crtx}[PRTfgkmpuvz] [archive] file1 file2...
> 
> 
> 
> Is there any document out there which describes the conventions for 
> documenting command line arguments?

There is no central rulebook for usage messages; each program must print
its own.  Generally, elements in square brackets are optional arguments,
and elements without them are required.   In tar(1)'s case the {crtx}
section indicates that you may have at most one function.  Usually it is
advisable to consult the manpage if the printed usage message isn't
helpful enough.

khm