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 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).

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See also [stdout], [stdin], [stdio]
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What category should this page be in?  "infrastructure?"  "Glossary"?