GNU bug report logs - #22205
tee bug?

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Nacho Esteban <juanignacio.esteban <at> alliance-healthcare.net>
To: bug-coreutils <at> gnu.org
Subject: tee bug?
Date: Fri, 18 Dec 2015 14:18:07 +0100

[Message part 1 (text/plain, inline)]

Hi team,

I detected an strange behaviour of tee command that may bee a bug.

Please consider this script snippet

   VAR=""

   A() {
        VAR="This is my value"
        echo "VAR in $FUNCNAME: $VAR"
   }

   BA() {
        A | tee /tmp/kk
        echo "VAR in $FUNCNAME: $VAR"
   }

This is the output when when executed:

   $ BA
   VAR in A: This is my value
   VAR in BA:


I miss the value of VAR inside BA function.



In fact if "| tee /tmp/kk" is removed value of VAR is properly visible 
from BA function:

   VAR=""

   A() {
        VAR="This is my value"
        echo "VAR in $FUNCNAME: $VAR"
   }

   BA() {
        A
        echo "VAR in $FUNCNAME: $VAR"
   }


   $ BA
   VAR in A: This is my value
   VAR in BA: This is my value


Many thanks for your priceless effort,
Nacho


*Juan Ignacio Esteban*

Integration Architect & Security Officer | Integration Centre of Expertise


*Walgreens Boots Alliance - Enterprise IT Shared Services - Retail 
Pharmacy International & Pharmaceutical Wholesale*

Pol. Ind. San Miguel Sector 4 | Villanueva de Gállego | Zaragoza (Spain) 
| 50830

*Member of Walgreens Boots Alliance *




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Message #12 received at 22205-done <at> debbugs.gnu.org (full text, mbox):

From: Eric Blake <eblake <at> redhat.com>
To: Nacho Esteban <juanignacio.esteban <at> alliance-healthcare.net>,
 22205-done <at> debbugs.gnu.org
Subject: Re: bug#22205: tee bug?
Date: Fri, 18 Dec 2015 09:43:51 -0700

[Message part 1 (text/plain, inline)]

tag 22205 notabug
thanks

On 12/18/2015 06:18 AM, Nacho Esteban wrote:
> Hi team,
> 
> I detected an strange behaviour of tee command that may bee a bug.

Not a bug in tee, but intentional behavior of your shell.

> 
> Please consider this script snippet
> 
>    VAR=""
> 
>    A() {
>         VAR="This is my value"
>         echo "VAR in $FUNCNAME: $VAR"
>    }
> 
>    BA() {
>         A | tee /tmp/kk

You are using a pipeline.  Shells implement pipelines by running the
left command (here, the function A) in a subshell. (POSIX also says that
the right command can also be a subshell, but there it is not required;
ksh runs the right command in the current shell, and bash used to run it
in a subshell but new enough bash now has a knob you can turn on to
emulate ksh behavior).

Therefore, all changes to the environment made during the execution of A
are local to that subshell, and are lost as soon as the subshell exits.

>         echo "VAR in $FUNCNAME: $VAR"

Thus, here you print the value of $VAR as it was before A ran, since
running A does not affect the current shell.


> In fact if "| tee /tmp/kk" is removed value of VAR is properly visible
> from BA function:

Because then you no longer have a subshell, so now the execution of
function A affects the current shell.  Again, not tee's fault.  You
could replace it with "| :" and see the same effect.

> This e-mail (including any attachments) is confidential and may be
> privileged or

Such a disclaimer is unenforceable on publically-archived lists, and you
may want to consider not using your employer's spammy email gateway when
sending mail to public lists.

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

[signature.asc (application/pgp-signature, attachment)]

Message #15 received at 22205 <at> debbugs.gnu.org (full text, mbox):

From: Pádraig Brady <P <at> draigBrady.com>
To: Nacho Esteban <juanignacio.esteban <at> alliance-healthcare.net>,
 22205 <at> debbugs.gnu.org
Subject: Re: bug#22205: tee bug?
Date: Fri, 18 Dec 2015 16:45:25 +0000

tag 22205 notabug
close 22205
stop

On 18/12/15 13:18, Nacho Esteban wrote:
> Hi team,
> 
> I detected an strange behaviour of tee command that may bee a bug.
> 
> Please consider this script snippet
> 
>     VAR=""
> 
>     A() {
>         VAR="This is my value"   
>         echo "VAR in $FUNCNAME: $VAR"
>     }
> 
>     BA() {
>         A | tee /tmp/kk
>         echo "VAR in $FUNCNAME: $VAR"
>     }
> 
> This is the output when when executed:
> 
>     $ BA
>     VAR in A: This is my value
>     VAR in BA:
> 
> 
> I miss the value of VAR inside BA function.
> 
> 
> 
> In fact if "| tee /tmp/kk" is removed value of VAR is properly visible from BA function:

The reason is due to the "|", not tee.
You can see that by replacing the tee with cat.
The pipe is implemented in a subshell and thus
can't propagate the VAR back to the current shell.

cheers,
Pádraig

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