'''[jrw32982]''' This is the only way I've found that works on a plain vanilla tcl 8.4 to detect if stdin is set to a terminal or a file/pipe/socket

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set interactive [ expr ! [ catch { exec /bin/sh -c { [ -t 0 ] }}]]
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'''[RFox] - 2013-05-29 11:13:27'''

What about checking the value of tcl_interactive?
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'''[jrw32982]''' tcl_interactive doesn't tell you if you're running interactively (if your script can receive input from a terminal).  It tells you if you're running a script.