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AW: Questions to bash "read" builtin functionality


From: Fiedler Roman
Subject: AW: Questions to bash "read" builtin functionality
Date: Mon, 17 Dec 2012 08:34:50 +0000

> -----Urspr√ľngliche Nachricht-----
> Von: Chet Ramey [mailto:address@hidden
> Gesendet: Samstag, 15. Dezember 2012 23:53
> 
> On 12/14/12 6:28 AM, Fiedler Roman wrote:
> > Hello list,
> >
> > One of our bash-scrips failed with very low frequency but randomly. The
> result was that exactly 1 byte was lost, so the string returned by "read -t 1"
> was too short. The culprit seems to be the built-in read function itself, the
> probability of failure was about 1:100000 in our case.
> >
> > After analysis and creation of a reliable reproducer, I'm not sure if this 
> > is a
> programming error in bash or a bug in our script as the result of suboptimal
> documentation.
> >
> > * Description:
> >
> > When "read" builtin is used with timeout setting, both with "-t" option or 
> > by
> setting the "TMOUT" environment variable, and the timeout occurs when
> some bytes of line were already received by bash, then those bytes are
> silently dropped. The next "read" returns the remainder of the line.
> 
> You don't say what version of bash you're using, ...

Sorry, forgot to mention it:

# bash --version
GNU bash, version 4.2.24(1)-release (x86_64-pc-linux-gnu)
Copyright (C) 2011 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

> but the following change  went in with bash-4.0:
>
> k.  Changed the behavior of the read builtin to save any partial input 
> received
>     in the specified variable when the read builtin times out.  This also
>     results in variables specified as arguments to read to be set to the empty
>     string when there is no input available.  When the read builtin times out,
>     it returns an exit status greater than 128.

That is strange: If I understand correctly, following script combined with the 
one from first mail should still fail, but with different error. But it fails 
in a way similar to discarding partial input, not saving it. Could it be, that 
Ubuntu-bash works differently? Output on my  side is:

Script:

#!/bin/bash
line=""
while true; do
  read -t 1 line
  status="$?"
  if [ "${status}" != "0" ]; then
    echo "Read status ${status}, value \"${line}\"" >&2
    continue
  fi
  if [ "${line}" != "Status: OK" ]; then
    echo "FAILED READ: \"${line}\"" >&2
    exit 1
  fi
done

# ./FragmentedSend.py | ./BashReadTest 
Read status 142, value ""
Read status 142, value ""
FAILED READ: "us: OK"
Traceback (most recent call last):
  File "./FragmentedSend.py", line 16, in <module>
    os.write(1, nextSendData[0:sendLength])
OSError: [Errno 32] Broken pipe

>From your description, I would expect 

--- HYPOTHETICAL-OUTPUT---
# ./FragmentedSend.py | ./BashReadTest 
Read status 142, value ""
Read status 142, value "Stat"      << the partial read
FAILED READ: "us: OK"
Traceback (most recent call last):
  File "./FragmentedSend.py", line 16, in <module>
    os.write(1, nextSendData[0:sendLength])
OSError: [Errno 32] Broken pipe
--- HYPOTHETICAL-OUTPUT---

Is this consistent with what you would be expecting!

> That is not clearly stated in the manual's description of `read'.

Perhaps it could be added, at least "see specification [URL] for details on 
read input handling"

Roman



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