[Tutor] command in menu and button

[Double Six]

Hi all, 

I'm puzzled by the 'command' option in menu and Button of
Tkinter. With the following lines,

menu.add_command(label="Open Viewer", command=os.system("Open my
viewer &"))

Button(toolbar, text='Open Viewer', command=os.system("Open my
viewer &")).pack(side=LEFT)	

I wanted to open a graphical viewer from the prompt (that is why
I used os.system("Open viewer &")) once the user selects the
option from the menu, or clicks the button, but this gets
implements whenever the GUI is launched instead. Why is this?
And how can I make it work?

Many thanks,
Joe


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