Re: [Help-bash] How to merge stdout and stderr yet distinguish what is from stdout and what is from stderr?

[Tadeus Prastowo]

On Mon, Feb 5, 2018 at 4:22 PM, Greg Wooledge <address@hidden> wrote:
> On Mon, Feb 05, 2018 at 03:31:12PM +0100, Tadeus Prastowo wrote:
>> On Mon, Feb 5, 2018 at 2:54 PM, Greg Wooledge <address@hidden> wrote:
>> > On Sun, Feb 04, 2018 at 06:03:37PM -0600, Peng Yu wrote:
>> >> I tried it multiple times. But the order is not guaranteed to be
>> >> maintained,
>> >
>> > Correct!  THAT is the real issue here.
>>
>> Won't `stdbuf' help?
>
> No, because it's not a buffering issue.  It's a race condition.
>
> When you take stream A and run it through filter program A, filter
> program A introduces some nondeterministic delay between the time the
> input is received and the time the filtered output is written.
>
> Then you take stream B and run it through filter program B, which
> introduces *its* own delay.
>
> If the delay of line 1 (stream A) is just a few milliseconds longer
> than expected for any reason, then it may occur *after* line 2
> (stream B) is written.
>
> If you want to work around that, you could timestamp each line in
> addition to filtering it, and then run both timestamped-and-filtered
> streams through a third program that merges the lines back into the
> correct order by using the timestamps, and then removes the timestamps.

Then, timestamping won't help either, will it?  Suppose cmd output
line L at time t_1 at stdout and were still be able to output line L'
at time t_1+D at stderr.  The scheduler then happened to decide that
the process waiting at stderr were to go first.  So, the process
stamped L' with time t_1+D+D'.  Afterwards, the scheduler gave the
processor to the process waiting at stdout, which would stamp L with
time t_1+D+D'+D''.  So, the correct relative order of L and L' as
output by cmd is screwed because by their timestamps, L' would be
ordered before L.

--
Best regards,
Tadeus