TCL/Expect - exec - how to execute program with parameters - arguments

Question

I am experimenting with TCL command exec in tclsh and here are my results:
% set show_me_dir "ls"
ls
% exec $show_me_dir
VboxSharedFolder
% set show_me_dir "ls -la"
ls -la
% exec $show_me_dir
couldn't execute "ls -la": no such file or directory
% set show_me_dir {ls -la}
ls -la
% exec $show_me_dir
couldn't execute "ls -la": no such file or directory
% ls -la
total 141
d---------+ 1 wakatana Domain Users 0 Jan 22 19:12 .
d---------+ 1 wakatana Domain Users 0 Apr 16 2014 ..
----------+ 1 wakatana Domain Users 20214 Jan 23 18:43 .bash_history
----------+ 1 wakatana Domain Users 1494 Apr 15 2014 .bash_profile
----------+ 1 wakatana Domain Users 7593 Jan 22 19:03 .bashrc
d---------+ 1 wakatana Domain Users 0 Jan 15 14:56 VboxSharedFolder
%
Can somebody please explain how can I execute command with arguments?
Edit:
The following example from Expanding a list of parameters in Tcl and eval article was big eye opener of what is going on here:
The variable $action is only expanded into the string "piemiddle apple" AFTER the command line has been split into its individual parameters:
% set action {piemiddle apple}
% set $action
can't read "piemiddle apple": no such variable
Result: set command "sees" one argument, equivalent to:
% set {piemiddle apple}
The expand operator allows you to specify that a variable is to be expanded BEFORE the command line is split into individual parameters:
% set action {piemiddle apple}
% set {*}$action
apple
Result: set command "sees" two arguments, equivalent to:
% set piemiddle apple
In earlier versions of Tcl, the eval command was the recommended alternative and it remains available today.
% set action {piemiddle apple}
% eval set $action
apple
Another examples which proves functionality of expansion operator:
% set {*}"name Linus"
Linus
% puts $name
Linus
%
%
% set distro Unbuntu
Unbuntu
% set {*}"linux $distro"
Unbuntu
% puts $linux
Unbuntu
%
%
Finally the discovery that exec needs command as it's first argument and first command option as it's second argument etc.
% exec "ls" "-la"
total 137
d---------+ 1 wakatana Domain Users 0 Jan 22 19:12 .
d---------+ 1 wakatana Domain Users 0 Apr 16 2014 ..
----------+ 1 wakatana Domain Users 20214 Jan 23 18:43 .bash_history
----------+ 1 wakatana Domain Users 1494 Apr 15 2014 .bash_profile
----------+ 1 wakatana Domain Users 7593 Jan 22 19:03 .bashrc
d---------+ 1 wakatana Domain Users 0 Jan 15 14:56 VboxSharedFolder
%
%
% exec "ls -la"
couldn't execute "ls -la": no such file or directory

Answer 1

The safest way to build a command for exec is to use Tcl's list. For example:
% set tcl_version
8.5
% set cmd [list ls -l tmp]
ls -l tmp
% eval exec $cmd
total 32
-rw-r--r-- 1 pynexj staff 1176 Jan 23 23:24 file.txt
-rw-r--r-- 1 pynexj staff 1176 Jan 23 23:24 foo-1.dat
-rw-r--r-- 1 pynexj staff 1176 Jan 23 23:24 foo-2.dat
-rw-r--r-- 1 pynexj staff 1176 Jan 23 23:24 foo-3.dat
% exec {*}$cmd
total 32
-rw-r--r-- 1 pynexj staff 1176 Jan 23 23:24 file.txt
-rw-r--r-- 1 pynexj staff 1176 Jan 23 23:24 foo-1.dat
-rw-r--r-- 1 pynexj staff 1176 Jan 23 23:24 foo-2.dat
-rw-r--r-- 1 pynexj staff 1176 Jan 23 23:24 foo-3.dat
%
Note that {*} is a new syntax of Tcl 8.5 which can help reduce the uses of eval.

Answer 2

As example for ls command you can do:
exec {*}ls -lsa {*}[glob *.cpp]
Please have a look at What does {*} do in TCL?