format - Format a string in the style of sprintf 

 format formatString ?arg arg ...?  

This command generates a formatted string in the same way as the ANSI C sprintf procedure (it uses sprintf in its implementation). FormatString indicates how to format the result, using % conversion specifiers as in sprintf, and the additional arguments, if any, provide values to be substituted into the result. The return value from format is the formatted string.  

More at: http://purl.org/tcl/home/man/tcl8.4/TclCmd/format.htm
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Tips and Tricks with format

You can use [[format]] to produce unsigned integers for display (but don't reckon with them - for expr they're still signed!):
 % format %u -1
 4294967295

See [floating-point formatting] for discussion on how to write format strings to handle floats...
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To make numbers look nice:

 set fah [format "%0.2f" [expr $temperature_cel * 9 / 5 + 32]]
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Color formatting:
 set color [format #%02x%02x%02x $r $g $b]
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A limited formatting of decimals to characters is available in other languages, e.g. CHR() in Basic. If you use that more often, here's a cute shortcut:
 interp alias {} chr {} format %c
 % set a [chr 49][chr 48]
 10
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See [Narrow formatting] for short rendering of big integers, with powers of 1024:
 % fixform 12345678
 11.7M

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