Version 11 of stderr

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]

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).

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).

Glenn Jackman: 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:

 set somecmd {sh -c {echo "to stdout" ; echo >&2 "to stderr"}}
 set f [open "| $somecmd" r]
 set std_out [read $f]
 catch {close $f} std_err
 puts "no redirection: std_out is '$std_out', std_err is '$std_err'"
 set f [open "| $somecmd 2>@ stdout" r]
 set std_out [read $f]
 catch {close $f} std_err
 puts "redirected: std_out is '$std_out', std_err is '$std_err'"