stderr is one of the [stdio]  output files opened - applications and functions tend to use it for the output of error messages. 





[[Explain common idioms for management of stderr from subprocesses.]]




[bgexec]


[[ [exec] conventions]]




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Pure Tcl programs ''write'' to stderr in two ways:
   * "puts stderr $msg" gives fine-grained control;
   * "[error] $message" generally has the effect of reporting a diagnostic [traceback] to stderr (except if prevented by an outer [catch], or in [wish] which pops up an error info window instead).



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Often people ask how to [open] a [pipeline] to a command and read the command's [stdout] and stderr.

One recent example of how to do this was:

 set fd1 [open "|somecmd |& cat" "r"]

(if your system has a command named cat in the default path).

[glennj]: Or, without having to open a cat process (see http://www.tcl.tk/man/tcl8.4/TclCmd/exec.htm):

 set fd1 [open "|somecmd 2>@ stdout" r]

Here's a quick test sequence that takes advantage of [close] returning the standard error for ''blocking'' pipes:

    set somecmd {sh -c {echo "to stdout" ; echo >&2 "to stderr"}}

    set errorCode ""
    puts "no redirection:"
    set f [open "| $somecmd" r]
    set std_out [read -nonewline $f]
    catch {close $f} std_err
    puts "  std_out is {$std_out}"
    puts "  std_err is {$std_err}"
    puts "  errorCode is {$errorCode}"

    set errorCode ""
    puts "redirected:"
    set f [open "| $somecmd 2>@ stdout" r]
    set std_out [read -nonewline $f]
    catch {close $f} std_err
    puts "  std_out is {$std_out}"
    puts "  std_err is {$std_err}"
    puts "  errorCode is {$errorCode}"

The output should be:

 no redirection:
   std_out is {to stdout}
   std_err is {to stderr}
   errorCode is {NONE}
 redirected:
   std_out is {to stdout
 to stderr}
   std_err is {}
   errorCode is {}

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'''Enrico''' 03-03-25: I have tried this on Windows 2000 with 8.4.1, but it doesn't work properly. The error message I've got, is
  channel "console1" wasn't opened for writing
The same result it takes, enforcing to show the console with the tcl-command ''[console] show''. Redirection of stderror to a file works:
  open "| $somecmd 2> errfile.txt" r

Who have an idea - or should I use the [bgexec] command?

Is there someone with a critcl implementation of bgexec (like the one for busy)?
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Another option of dealing with stderr, (or other file descriptors for that matter) that I have seen mentioned and used may be considered ''a cheat'':

  exec /bin/ksh -c "command 2>&1 > /dev/null"

This provides you with the stderr on the ''stdout'' file descriptor.
This specific example throws away the original stdout data.  If that data
is also going to be needed, then replace "> /dev/null" with "> /some/file".

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Unfortunately, the 'cat' solution means you lose the exit status of the process
whereas the non-cat solution doesn't let your script read the text, it only redirects it
to the stdout channel of the tclsh process. It seems that [bgexec] provides the only
complete solution.

[LV] March 26, 2003 - I don't understand this comment.  Here's a transcript of
a tclsh session:

 $ cat test.ksh
 #! /bin/ksh

 echo "This is stdout"
 echo "This is stderr" >&2
 exit 0
 $ tclsh
 % set fd [open "|/home/lwv26/test.ksh 2>@ stdout" "r"]
 file3
 % gets $fd
 This is stdout
 % gets $fd
 This is stderr


In other words, the 2>@ stdout allows tcl to read the stderr on the same
[channel] as stdout.  What it _doesn't_ allow is for you to be able to tell
what is stderr and what is stdout.  That's a nuisance.

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See also [stdout], [stdin], [stdio], and [magic names],
[Tcl syntax help]
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[Category Glossary]