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Re: Why `echo -n hello | while read v; do echo $v; done' prints nothing?


From: Eric Blake
Subject: Re: Why `echo -n hello | while read v; do echo $v; done' prints nothing?
Date: Thu, 02 Dec 2010 20:11:09 -0700
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On 12/02/2010 07:02 PM, Clark J. Wang wrote:
>> The output from the first command in the pipeline does not end with a
>> newline.  Therefore, 'read' in the second command returns 'failure'
>> (non-zero) when it reads the first line of input, and your loop never
>> iterates.
> 
> But is that reasonable? I think read should return success in this case
> which makes more sense to me. Does the POSIX standards require that?

POSIX requires that the input to read be a text file (and by the
definition of text file in POSIX, it must either be empty or end in a
newline).  By violating POSIX and passing something that does not end in
a newline, you are no longer bound by the rules of POSIX.  Therefore, it
would be a reasonable bash extension that read could return 0 status if
it read data that did not end in a newline, but it would not be a
standard-compliant script that relied on such an extension.  You're
better off supplying the trailing newline, and guaranteeing a compliant
usage.

-- 
Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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