Bash malfunction, or something else ??

[David Curry]

Nifty Hat Mitch wrote:

>On Mon, Jan 24, 2005 at 12:06:49AM -0500, David Curry wrote:
>  
>
>>Richard Hubbell wrote:
>>    
>>
>>>On Sun, 23 Jan 2005 22:44:09 -0500, David Curry <dsccable at comcast.net> 
>>>      
>>>
>.....
>  
>
>>>Capture stdout and stderr to a file and see if that is what you expect.
>>>
>>>cdrecord -scanbus 2>&1 myfile
>>>
>>>      
>>>
>....
>  
>
>>>>There are six more lines of output sent to the display screen than are 
>>>>sent to a file via use of the > [filename] option in BASH.
>>>>        
>>>>
>....
>  
>
>>Thanks, Richard.  As far as I knew all cdrecord -scanbus output was 
>>displaying on my monitor (stdout) and did not know that the first four 
>>lines omitted from the disk file I created were "errors" (scsidev: "ATA" 
>>-->  Linux sg driver version: 3.5.27).
>>
>>Following Up on the suggestions of Jason Tibbits and Jim Cox, I repeated 
>>the exercise using &> instead of just > and the file produced included 
>>all of the missing lines.
>>
>>Thanks again to all who responded.
>>    
>>
>
>One key point here is that when a process is launched it will
>commonly have three file descriptors by default.
>
>	 standard input  #-0
>	 standard output #-1
>	 standard error  #-2
>
>By convention stdout is used for the 'usefull' output
>and stderr is for the 'exceptional' stuff.  See exec().
>
>"C" programmers, see also 'dup()' and 'dup2()'.
>
>It is common to redirrect the various file descriptors.
>
>This merges stderr on file handle 2 to file handle 1 (stdout).
>
>   2>&1
>
>Or the error and output streams can be seperated/ isolated
>into files.
>
> wc /etc/* > /tmp/junk 2> /tmp/error-junk
>
>Recall that child processes can inherit file descriptors
>from the parrent process.  This makes it possible to do
>something like:
>
>     make something-large  > /tmp/make-out  2> /tmp/make-err &
>
>Note that make may invoke a thousand processes and as long as the make
>files do not do strange things you can....
>in two different windows watch them both.
>
>    tail -f  /tmp/make-out
>    tail -f  /tmp/make-err 
>
>There is also "tee" that can be used to fiddle with IO.
>
>It is also common for modern programs to 'test' their file
>descriptors.  See isatty()... Many common programs do act differently
>if you redirrect IO to a file.
>
>For example "\ls --color=tty / " will not emit the escape characters
>needed to color the terminal.  This type of test can cause confusion
>for QA folks and others.   
>
>Also there is the issue of the LANG environment variable....
>
>
>  
>
Thanks for the info, Tom.