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Re: handling of test == by BASH's POSIX mode


From: Jon Seymour
Subject: Re: handling of test == by BASH's POSIX mode
Date: Sun, 27 May 2012 20:45:46 +1000

On 27/05/2012, at 17:39, Geir Hauge <address@hidden> wrote:

> 2012/5/27 Jon Seymour <address@hidden>:
>> Is there a reason why bash doesn't treat == as an illegal test
>> operator when running in POSIX mode?
> 
> POSIX does not say == is not allowed.
> 
> POSIX tells you what the shell should at least be able to do. A POSIX
> compliant shell can have whatever other features it likes, as long as
> the POSIX features are covered.
> 

I guess the question is better phrased thus: what use case is usefully served 
by having bash's POSIX mode support a superset of test operators than other 
compliant POSIX shells?  As it stands, I can't use bash's POSIX mode to verify 
the validity or otherwise of a POSIX script because bash won't report these 
kinds of errors - even when running in POSIX mode.

There is an --enable-strict-posix (?) configuration option. Will this do what I 
expect?

> 
>> This is problematic because use of test == in scripts that should be
>> POSIX isn't getting caught when I run them under bash's POSIX mode.
>> The scripts then fail when run under dash which seems to be stricter
>> about this.
> 
> Don't use non-POSIX features in a POSIX script, and you'll be fine.
> http://www.opengroup.org/onlinepubs/9699919799/utilities/contents.html
> 

Which is the exactly the point. Practically speaking when I write scripts I 
expect an interpreter that claims to be running in POSIX mode to give me some 
help to flag usage of non POSIX idioms. Yes, I can second guess the interpreter 
by reading the spec, but is this really the most efficient way to catch these 
kinds of errors?

Jon.


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