/bin/dash -c != /bin/bash -c with pgrep

/bin/dash -c != /bin/bash -c with pgrep

[Richard Retanubun]

Hello,

my dash version is 0.5.5.1-7.4 (from debian/stable I believe)

I ran these two commands on a system where atftpd is not running and I am getting different results

/bin/dash -c "pgrep -f /usr/sbin/atftpd"; echo $?
-vs-
/bin/bash -c "pgrep -f /usr/sbin/atftpd"; echo $?

The dash version returns the PID of the grep itself and thus always succeeds.
The bash version works as expected.

Sorry if this is an old bug that is already fixed...
Has it been reported? (and fixed?) I saw a patch about the "-c" mode in  but it was reverted...

-- 
Richard Retanubun

Re: /bin/dash -c != /bin/bash -c with pgrep

[Jonathan Nieder]

Hi Richard,

Richard Retanubun wrote:

> /bin/dash -c "pgrep -f /usr/sbin/atftpd"; echo $?
> -vs-
> /bin/bash -c "pgrep -f /usr/sbin/atftpd"; echo $?
>
> The dash version returns the PID of the grep itself and thus always succeeds.
> The bash version works as expected.

As the pgrep(1) manual explains, the running pgrep or pkill process
will never report itself as a match.  However, when running pgrep
through dash, it reports the process id of the shell executing pgrep,
while the bash version and recent dash versions optimize "sh -c
'single command'" to

	sh -c 'exec pgrep -f /usr/sbin/atftpd'

so there is no shell process left to report.

If that "exec" is specified explicitly, the result is the same in both
shells.

Hope that helps,
Jonathan

Re: /bin/dash -c != /bin/bash -c with pgrep

[Richard Retanubun]

On 24/01/12 02:43 PM, Jonathan Nieder wrote:
> Hi Richard,
>
> Richard Retanubun wrote:
>
>> /bin/dash -c "pgrep -f /usr/sbin/atftpd"; echo $?
>> -vs-
>> /bin/bash -c "pgrep -f /usr/sbin/atftpd"; echo $?
>>
>> The dash version returns the PID of the grep itself and thus always succeeds.
>> The bash version works as expected.
>
> As the pgrep(1) manual explains, the running pgrep or pkill process
> will never report itself as a match.  However, when running pgrep
> through dash, it reports the process id of the shell executing pgrep,
> while the bash version and recent dash versions optimize "sh -c
> 'single command'" to
>
> 	sh -c 'exec pgrep -f /usr/sbin/atftpd'
>
> so there is no shell process left to report.
>
> If that "exec" is specified explicitly, the result is the same in both
> shells.
>
> Hope that helps,
> Jonathan
It sure does, thanks a lot Jonathan.

As an addendum, I find using "pgrep -x atftpd" in this case also yields the
expected output in both shell.

Thanks for your time.

-- RR --

Re: /bin/dash -c != /bin/bash -c with pgrep

[Richard Retanubun]

On 24/01/12 03:17 PM, Richard Retanubun wrote:
> On 24/01/12 02:43 PM, Jonathan Nieder wrote:
>> Hi Richard,
>>
>> Richard Retanubun wrote:
>>
>>> /bin/dash -c "pgrep -f /usr/sbin/atftpd"; echo $?
>>> -vs-
>>> /bin/bash -c "pgrep -f /usr/sbin/atftpd"; echo $?
>>>
>>> The dash version returns the PID of the grep itself and thus always succeeds.
>>> The bash version works as expected.
>>
>> As the pgrep(1) manual explains, the running pgrep or pkill process
>> will never report itself as a match.  However, when running pgrep
>> through dash, it reports the process id of the shell executing pgrep,
>> while the bash version and recent dash versions optimize "sh -c
>> 'single command'" to
>>
>> 	sh -c 'exec pgrep -f /usr/sbin/atftpd'
>>
>> so there is no shell process left to report.
>>
>> If that "exec" is specified explicitly, the result is the same in both
>> shells.
>>
>> Hope that helps,
>> Jonathan
> It sure does, thanks a lot Jonathan.
>
> As an addendum, I find using "pgrep -x atftpd" in this case also yields the
> expected output in both shell.
>
> Thanks for your time.
>
> -- RR --
Sorry, one more question to clarify, which "recent dash versions" have the optimization?

Debian changelog from wheezy (0.5.7-2) says this:

0005-SHELL-Disable-sh-c-...diff: back out the optimization
that makes dash -c "script" avoid a fork().  It caused
several packages to fail to build from source (thx Stéphane Glondu, closes: #642922).

So it used to, but does not currently?

-- Richard

Re: /bin/dash -c != /bin/bash -c with pgrep

[Jonathan Nieder]

Richard Retanubun wrote:

> Sorry, one more question to clarify, which "recent dash versions"
> have the optimization?

Upstream versions, or you can grab Debian packaging from the
proposed-experimental branch at [1].

Cheers,
Jonathan

[1] git://repo.or.cz/dash/debian/jrn.git