proc bexec {command input} {
    # Execute shell "command", send "input" to stdin, return stdout.
    # Ignores stderr (but "2>@1" can be part of "command").
    # Supports binary intput and output. e.g.:
    #     set flac_data [bexec {flac -} $wav_data]

    # Run "command" in background...
    set f [open |$command {RDWR BINARY}]
    fconfigure $f -blocking 0

    # Connect read function to collect "command" output...
    set ::bexec_done.$f 0
    set ::bexec_output.$f {}
    fileevent $f readable [list bexec_read $f]

    # Send "input" to command...
    puts -nonewline $f $input
    close $f write

    # Wait for read function to signal "done"...
    vwait ::bexec_done.$f

    # Retrieve output...
    set result [set ::bexec_output.$f]
    unset ::bexec_output.$f
    unset ::bexec_done.$f

    fconfigure $f -blocking 1
    try {
        close $f
    } trap {CHILDSTATUS} {options info} {
        dict set info -errorinfo $result
        return -options $info $result
    }

    return $result
}


proc bexec_read {f} {
    # Accumulate output in ::bexec_output.$f.

    append ::bexec_output.$f [read $f]
    if {[eof $f]} {
        fileevent $f readable {}
        set ::bexec_done.$f 1
    }
}