bash strerr pipe problem - workaround ?

[dave selby]

2008/6/14 James Gray <james at gray.net.au>:
>
> On 14/06/2008, at 6:16 PM, dave selby wrote:
>> I have a app that outputs to stderr, what I need is a filtered version
>> to be saved in a file
>>
>> motion -c %s/core/motion_conf/motion.conf 2> saved
>>
>> works as expected but is unfiltered. So to filter it I am trying ...
>>
>> motion -c %s/core/motion_conf/motion.conf | grep -E 'error|not
>> found' > saved
>
> Hi Dave :)
>
> Yep - because by the time you're redirecting STDERR to "saved", the
> output has already been sent to /dev/stderr (usually the screen).  The
> key is to remember that with shell redirection, ORDER IS IMPORTANT ;)
>
> Try something like this:
>
> motion -c %s/core/motion_conf/motion.conf 2>&1 | egrep 'error|not
> found' > saved

That what I need - works a treat. I could not think how to redirect
stderr to stdout, many thanks :)

> The magic is the "2>&1" which tells the shell, send STDERR to the same
> place as STDOUT.  As pipes (in the example above) read from STDIN, by
> redirecting STDERR->STDOUT we can then pipe the error stream as
> normal :)
>
> Consider this:
>
> some_command > output.file 2>&1
>
> STDOUT is going to a file called "output.file", we then redirect
> STDERR to the same place as STDOUT which means we merge the streams
> into a single output file.  However, the following WILL NOT WORK AS IT
> FIRST APPEARS:

That makes sense too

> some_command 2>&1 > output.file
>
> Think about it ;)  .... and read the "REDIRECTION" section in "man
> bash".
>
> HTH,
>
> James

Thanks James for explaining this, that sorts my problem

cheers

Dave



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