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@xxxxxxxxxxx>
.....
> >
> >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....
--
T o m M i t c h e l l
