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Re: "read" builtin: timeout vs. EOF

From: Eric Blake
Subject: Re: "read" builtin: timeout vs. EOF
Date: Mon, 19 Dec 2011 07:07:35 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:8.0) Gecko/20111115 Thunderbird/8.0

On 12/18/2011 09:45 AM, Stephen Gildea wrote:
> The "read" built-in documentation says,
>     Exit Status:
>     The return code is zero, unless end-of-file is encountered, read times 
> out,
>     or an invalid file descriptor is supplied as the argument to -u.
> I would like to be able to distinguish the no-data conditions.  In
> particular I want to know whether I got a timeout or an end-of-file,
> but as you can see, Bash is not documented to behave differently in
> these two cases.
> As it happens, in Bash 4.2, "read" exits with status 1 on EOF and
> SIGALRM on timeout.  This is just what I want.  Can I count on this
> behavior?  If so, please commit to it with documentation.


POSIX made a change to require the following on read:

An error in setting any variable (such as if a var has previously been
readonly) shall be considered an error of read processing, and shall result
in a return value greater than one.

At line 103977 [XCU read EXIT STATUS], change

>0 End-of-file was detected or an error occurred.


1 End-of-file was detected.
>1 An error occurred.

Getting a timeout or invalid file descriptor should definitely be
considered error cases that return > 1 (SIGALRM for timeout works well).

Eric Blake   eblake@redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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