stderr is one of the stdio output files opened - applications and functions tend to use it for the output of error messages. no editing is permitted Ok
[Explain common idioms for management of stderr from subprocesses.]
[ exec conventions]
Pure Tcl programs write to stderr in two ways:
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 {}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.
See also stdout, stdin, stdio, and magic names, Tcl syntax help
What category should this page be in? "infrastructure?" "Glossary"?