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Listing 1: The syntax of Tcl commands is quite simple.

A. Command names begin lines, and arguments possibly grouped by quotes or braces follow:

1 set name {Brent Welch}
2 puts stdout "My name is $name"

B. Command substitution occurs inside of square brackets:

1 set d [exec date]
2 puts stderr "Error occurred at $d"

Listing 2: Using Tcl/Tk, it is easy to build a graphical user interface to the ping command.

A. The first version runs ping once and displays the result:

 1 #!/usr/local/bin/wish -f
 2 # Ping example #1
 3 frame .buttons -borderwidth 10
 4 pack .buttons -side top -fill x
 5
 6 button .buttons.quit -text Quit -command exit
 7 button .buttons.ping -text Ping -command Ping
 8 pack .buttons.quit .buttons.ping -side right
 9
10 frame .f ; pack .f -side top
11 label .f.l -text Host:
12 entry .f.host -width 20 -relief sunken
13 pack .f.l .f.host -side left
14
15 text .log -width 60 -height 10 -bd 2 -relief raised
16 pack .log -side top
17
18 proc Ping {} {
20     set hostname [.f.host get]
21     catch {exec ping $hostname} result
22     .log insert end $result
23     .log insert end \n
24 }

B. The helper script continuously sends ping's output to the front end for display:

 1 #!/usr/local/bin/wish -f
 2 # Install this as "pinghelper"
 3 wm withdraw .
 4 set hostname [lindex $argv 0]
 5 set otherInterp [lindex $argv 1]
 6 set in [open "|ping -s $hostname" r]
 7 while {[gets $in line] >= 0} {
 8     send $otherInterp [list Insert $line]
 9 }
10 exit

C. The lines in Listing 2A to change to integrate the pinghelper:

18 proc Ping {} {
19     global pid
20     set hostname [.f.host get]
21     set pid [exec pinghelper $hostname [winfo name .] &]
22     .buttons.ping configure -text Stop -command Stop
23 }
24 proc Insert { text } {
25     .log insert end $text
26     .log insert end \n
27     .log yview -pickplace end
28 }
29 proc Stop {} {
30     global pid
31     catch {exec kill $pid} result
32     .log insert end $result\n
33     .buttons.ping configure -text Ping -command Ping
34 }