/dev/stderr problem

/dev/stderr problem

[Thorsten Kampe]

Hi,

the following bash script results in a different output when 
redirected to a file.

```
printf "FIRST LINE\n" > /dev/stderr
shopt -os xtrace
printf "SECOMD LINE\n" > /dev/stderr
```

```
$ bash script.sh
FIRST LINE
+ printf 'SECOMD LINE\n'
SECOMD LINE
```

```
$ bash script.sh 2> file && cat file
SECOMD LINE
```

When I use `>&2` instead of `> /dev/stderr` the problem does not 
occur.

Since this problem does not occur on Linux but also with Zsh on 
Cygwin, I think this is a Cygwin issue.

Thorsten


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Re: /dev/stderr problem

[Thorsten Kampe]

* Thorsten Kampe (Mon, 17 Oct 2016 08:25:13 +0200)

> the following bash script results in a different output when 
> redirected to a file.
> 
> ```
> printf "FIRST LINE\n" > /dev/stderr
> shopt -os xtrace
> printf "SECOMD LINE\n" > /dev/stderr
> ```

On further inspection: the `xtrace` is not related to the problem:

script.sh
```
printf "FIRST LINE\n" > /dev/stderr
printf "SECOMD LINE\n" > /dev/stderr
```

```
$ bash script.sh
FIRST LINE
SECOMD LINE

$ bash script.sh 2> file && cat file
SECOMD LINE

$ file /dev/stderr
/dev/stderr: symbolic link to /proc/self/fd/2
```

Thorsten


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Re: /dev/stderr problem

[Eric Blake]

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On 10/17/2016 01:32 AM, Thorsten Kampe wrote:
> * Thorsten Kampe (Mon, 17 Oct 2016 08:25:13 +0200)
> 
>> the following bash script results in a different output when 
>> redirected to a file.
>>
>> ```
>> printf "FIRST LINE\n" > /dev/stderr
>> shopt -os xtrace
>> printf "SECOMD LINE\n" > /dev/stderr

Cygwin treats '> /dev/stderr' as a request to truncate /dev/stderr (or,
for that matter, any opening of a file under /proc/self/fd).  Other
platforms treat that as a special file that can never be truncated, but
is instead reopened at the same offset.

Maybe cygwin can be taught that opening a file through /proc/self/fd
should preserve rather than reset offsets, but it will be a tricky
patch, and someone has to write it.

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


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Re: /dev/stderr problem

[Linda Walsh]

Eric Blake wrote:
> On 10/17/2016 01:32 AM, Thorsten Kampe wrote:
>> * Thorsten Kampe (Mon, 17 Oct 2016 08:25:13 +0200)
>>
>>> the following bash script results in a different output when 
>>> redirected to a file.
>>>
>>> ```
>>> printf "FIRST LINE\n" > /dev/stderr
>>> shopt -os xtrace
>>> printf "SECOMD LINE\n" > /dev/stderr
> 
> Cygwin treats '> /dev/stderr' as a request to truncate /dev/stderr (or,
> for that matter, any opening of a file under /proc/self/fd).  Other
> platforms treat that as a special file that can never be truncated, but
> is instead reopened at the same offset.
> 
> Maybe cygwin can be taught that opening a file through /proc/self/fd
> should preserve rather than reset offsets, but it will be a tricky
> patch, and someone has to write it.
---
Is /dev/stderr a POSIX special name that one should expect that rewinding
is disallowed or ignored?

Good analysis, BTW, that sure would have puzzled me.
-l


> 

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