If you want to read from another process on the same system that you're running your code on in Tcl can be done using a pipe - you can use open with the command name instead of the file name, preceeded by a pipe character (|) - thus:

set said [open |df r]
 
while {[gets $said this] >= 0} {
  puts $this
}

You can also talk to a process on another system if it's running as a server using sockets. In Tcl all you need to do in the simplest case is to use a socket command instead of an open, and you can use puts and gets on the resource that's returned. Here's an example of a Tcl script that connects to a web server and downloads a page of html:

set talkto [socket 192.168.200.67 80]
 
puts $talkto "GET /index.html HTTP/1.0\n"
flush $talkto
set lines 0
 
while {[gets $talkto stuffing] > -1} {
  incr lines
  puts $stuffing
}
puts $lines
(written 2007-10-26 05:48:04)

 T209 - Tcl/Tk - File and Directory Handling

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Buffering of inputs to expect, and match order
Tcl / regsub - changing a string and using interesting bits
What is Expect?
Wireless hotel tips - FTP and Skype connections failing
Reading from another process in Tcl (pipes and sockets)
Away or home - which do I prefer?
Sorting in Tcl - lists and arrays
Tcl - global, upvar and uplevel.
Square Bracket protection in Tcl
Tcl - append v lappend v concat