[mdlug] [HOWTO request] filter stderrr without altering stdout

[Aaron Kulkis]

Robert Meier wrote:
> Carl
> 
> Thank you for your response.
> 
>>> How can I write a wrapper that filters stderr (fd 2) without altering
>>> stdout (fd 1) using ksh?
> 
>> I'm not sure I understand what you're trying to do.  It sounds
>> like you want a script that will call a function.  And I'm guessing
>> that you don't want to see any errors from the function, but not
>> filter errors from the script.

I recall once seeing a method for doing exactly that sort of thing

It involved something like this:

(foo | foo_postprocess ) &2>1 | foo_error_postprocess

It's been about 5 years, so I'll have to play with the syntax
to get it right... when I finally get off duty in another
4 to 8 hours (never know around here....)

> 
> Actually I do want to see errors from the function, except for
> a few known errors that have been handled by the script.
> 
> Detail:
>    # I have an interactive function, part of a large package.
>    # It reports to both stdout and stderr
>    # Unfortunately, it also reports a warning to stderr even though
>    #   the warning has been successfully dealt with.
>    ksh> echo help | func 1>stdout 2>stderr
>    ksh> cat stdout
>    This is a nice function that does something interactively.
>    ksh> cat stderr
>    Warning: Unable to find my config file in /etc/func
>    Found the config file in /opt/etc/func
>    usage: func ...
> 
>    # I don't wish to alter stdout at all, so the user can continue
>    #   normal use.
>    # I do want to remove the warning successfully dealt with lest the
>    #   the user ignore all errors and warnings.
>    # I do want the remaining warnings and errors to be seen, so the
>    #   user can take corrective action.
>    ksh> echo help | func-wrapper 1>wrapped-stdout 2>wrapped-stderr
>    ksh> cat wrapped-stdout
>    This is a nice function that does something interactively.
>    ksh> cat wrapped-stderr
>    usage: func ...
> 
>    # The necessary feature is that only a few well-defined warnings
>    # be removed from stderr, while all of stdout and the remainder of
>    # stderr are passed unaltered.
>    ksh> diff stdout wrapped-stdout
>    #    no difference
>    ksh> diff stderr wrapped-stderr
>    < Warning: Unable to find my config file in /etc/func
>    < Found the config file in /opt/etc/func
> 
>    
> 
>    ksh> cat func-wrapper
>    #!/bin/sh
>    ????
>    func $*
>    ????
> 
> For instance, is there a redirection usage that can carry interactive stdout
> past grep, and apply grep to stderr only?
> THE FOLLOWING DOES NOT WORK AS WRITTEN:
>    ksh> cat func-wrapper
>    #!/bin/sh
>    func $* 1>3 2>1 |
>      grep -v 'Warning: Unable to find my config file in /etc/func' |
>      grep -v 'Found the config file in /opt/etc/func' |
>      cat 1>2 3>1
> THE ABOVE DOES NOT WORK AS WRITTEN
> 
> TIA,