The documentation includes an example. Here is a simple example:

package require cmdline

set parameters {
    {server.arg ""   "Which server to search"}
    {debug           "Turn on debugging, default=off"}
}

set usage "- A simple script to demo cmdline parsing"
array set options [cmdline::getoptions ::argv $parameters $usage]
parray options

Sample runs:

$ tclsh simple.tcl 
options(debug)  = 0
options(server) = 

$ tclsh simple.tcl -server google.com
options(debug)  = 0
options(server) = google.com

$ tclsh simple.tcl -server google.com -debug
options(debug)  = 1
options(server) = google.com

$ tclsh simple.tcl -help
simple - A simple script to demo cmdline parsing
 -server value        Which server to search <>
 -debug               Turn on debugging, default=off
 -help                Print this message
 -?                   Print this message

    while executing
"error [usage $optlist $usage]"
    (procedure "cmdline::getoptions" line 15)
    invoked from within
"cmdline::getoptions ::argv $parameters $usage"
    invoked from within
"array set options [cmdline::getoptions ::argv $parameters $usage]"
    (file "simple.tcl" line 11)

Discussion

• Unlike most Linux utilities, TCL uses single dash instead of double dashes for command-line options
• When a flags ends with .arg, then that flag expects an argument to follow, such as in the case of server.arg
• The debug flag does not end with .arg, therefore it does not expect any argument
• The user defines the command-line parameters by a list of lists. Each sub-list contains 2 or 3 parts:
  • The flag (e.g. debug)
  • The default value (e.g. 0), only if the parameter takes an argument (flag ends with .arg).
  • And the help message
• Invoke usage/help with -help or -?, however, the output is not pretty, see the last sample run.

Update: Help/Usage

I have been thinking about the message output when the user invoke help (see the last sample run above). To get around that, you need to trap the error yourself:

set usage "- A simple script to demo cmdline parsing"
if {[catch {array set options [cmdline::getoptions ::argv $parameters $usage]}]} {
    puts [cmdline::usage $parameters $usage]
} else {
    parray options
}

Sample run 2:

$ tclsh simple.tcl -?
simple - A simple script to demo cmdline parsing
 -server value        Which server to search <>
 -debug               Turn on debugging, default=off
 -help                Print this message
 -?                   Print this message