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.]
[ exec conventions]
There are a few ways to write to stderr:
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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