[[document arguments of this [BLT] command]]

Example of [stderr] capture:
        proc DisplayErrors { data } {
            puts "stderr> $data"
        }
        bgexec myVar -onerror DisplayErrors programName &

Also has -onoutput.

onerror sets up a callback so that when output appears on the stderr of programName, DisplayErrors is called with the string, and the proc can then do with it what it likes.  onoutput sets up a similar callback, watching stdout of programName .
----
[gah] writes,
"What I like about 'bgexec' is that it handles all the dumb stuff under
the hood--so you (the Tcl programmer) don't have to worry about it.
It automatically performs non-blocking I/O.  It collects data at
whatever rate, without you doing anything.  It raises the programming
problem from 'readable' events, non-blocking I/O, and buffer lengths,
to Tcl variables and procedure calls.  You don't have to be an expert
in I/O or process semantics.  If you want to kill the pipeline, all
you have to do is set a variable.  It's the same whether you're on
Windows or Unix or MacOSX."

[Vince] writes -- all wonderful reasons why this functionality really should go into Tcl's core.

[US] Ceterum censeo bgexec esse integrandam (Marcus Porcius Cato, slightly modified)
----
[SLB] One point about bgexec that tripped me up recently was in using a command such as this:

    ::blt::bgexec ::bgexecStatus -linebuffer 1 -onerror puts -onoutput puts cat file.txt &

This reported all lines in file.txt except for the blank lines.

If you want the blank lines to can change the command to this:

    ::blt::bgexec ::bgexecStatus -linebuffer 1 -keepnewline 1 -onerror {{puts -nonewline}} -onoutput {{puts -nonewline}} cat file.txt &


keepnewline mode has the
documented effect of adding a trailing newline to each line of output and an undocumented
effect of retaining blank lines.

bgexec.html says this is the keepnewline mode is the default, this seems to be incorrect.

This behaviour is present 2.4z on both Windows and Solaris.

----
For a pure tcl-implementation of bgexec, see [Matthias Hoffmann - Tcl-Code-Snippets]. Sorry, if I had known that a bgexec exists I had used a different proc name...