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, updated 2011-03-01)

 T209 - Tcl/Tk - File and Directory Handling
  [779] The fragility of pancakes - and better structures - (2006-06-26)
  [785] Running external processes in Tcl and Tcl/Tk - (2006-06-29)
  [1426] Buffering up in Tcl - the empty coke can comparison - (2007-11-10)
  [1467] stdout v stderr (Tcl, Perl, Shell) - (2007-12-10)
  [2467] Tcl - catching an error before your program crashes - (2009-10-22)
  [3192] Tcl - Some example of HOW TO in handling data files and formats - (2011-03-04)
  [3320] Reading the nth line from a file (Perl and Tcl examples) - (2011-06-09)
  [3429] Searching through all the files in or below a directory - Ruby, Tcl, Perl - (2011-09-09)
  [3617] The fileutil package and a list of file system commands in Tcl - (2012-02-18)
  [4461] Reading from a URL, and reading Json, from your Tcl script - (2015-03-12)
  [4523] Catching failed commands and not crashing the program in Tcl - (2015-10-10)
  [4524] Tcl - a new example for data reformatting - (2015-10-10)

Back to Away or home - which do I prefer?

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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