I personally have not seen a 'standard' that denotes that a switch is optional (like how there's a standard that defines how certain languages are written for example), as it really is personal choice, but according to IBM's docs and the Wiki, along with numerous shell scripts I've personally seen (and command line options from various programs) the 'defacto' is to treat square bracketed (
) parameters as optional parameters. Example from Linux:
ping (output trimmed...)
usage: ping [-c count] [-t ttl] host
[-c count] and
[-t ttl] are optional parameters but
host is not.
I personally follow the defacto as well by using
 to mean optional parameter.
Yes, the square brackets indicate optional arguments in Unix man pages.
From "man man":
[-abc] any or all arguments within [ ] are optional.
I suppose this is as much a standard as anything.
The Open Group Base Specifications Issue 7
IEEE Std 1003.1, 2013 Edition
Copyright © 2001-2013 The IEEE and The Open Group
Ch. 12 - Utility Conventions
Although it doesn't seem to mention many things I have commonly seen over the years used to denote various meanings:
- square brackets
- angle brackets
- curly braces
Edit: I should add, that these are just conventions. The important thing is to pick a convention which is sensible, clearly state your convention, and stick to it consistently. Be flexible and create conventions which seem to be most frequently encountered on your target platform(s). They will be the easiest for users to adapt to.