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

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See also [stdout], [stdin], [stdio], and [magic names],
[Tcl syntax help]
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What category should this page be in?  "infrastructure?"  "Glossary"?