A shell toy

grid rowconfigure . 0 -weight 1
grid columnconfigure . 1 -weight 1
grid [text .log] -columnspan 3 -sticky news
grid [label .l -text Command:] -row 1 -column 0 -sticky we
grid [entry .cmd -textvar cmd] -row 1 -column 1 -sticky we
grid [button .exec -text Go! -command go] -row 1 -column 2 -sticky we

bind .cmd <Return> go

switch -- $tcl_platform(platform) {
	unix {
		if {[info exists env(SHELL)]} {
			set shell $env(SHELL)
		} else {
			set shell /bin/sh
		}
	}
	windows {
		if {[info exists env(COMSPEC)]} {
			set shell $env(COMSPEC)
		} else {
			set cmd.exe
		}
	}
	default {
		return -code error "Freaky platform"
	}
}

set fd [open [list | $shell] w+]
fconfigure $fd -blocking no -buffering line
fileevent $fd readable get

proc get {} {
	variable fd

	if {[gets $fd line] < 0} {
		close $fd
		.log insert end "SHELL quit"
	} else {
		.log insert end $line
	}
}

proc go {} {
	variable cmd
	variable fd

	puts $fd $cmd
}

vwait forever