[jdp] - An 8 line server. Serves text files out of, or below, the current directory. As no headers are sent, it depends on the browser what he does with it - IE6 seems to expect HTML 0.9 (i.e. linebreaks are ignored, etc.)

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proc s {c a p} {
        set o [lindex [split [gets $c] " "] 1]
        puts $c [read [set f [open ".$o"]]]
        close $f
        close $c
}
set s [socket -server s 3210]
vwait forever
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[jdp] - After spending time in the chatroom, with the help of [stu] and [suchenwi], it became a 1-line, 140-char server. (Interactive mode only, though.)

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vw [soc -server {appl {{c a p} {puts $c [read [set f [open .[regsub {\.\.+} [lindex [split [gets $c] \ ] 1] x]]]][close $f];close $c}}} 80]
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[RS] 2009-05-29: The [fcopy] version is still a few bytes shorter:
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vw [soc -server {appl {{c a p} {fcopy [set f [open .[regsub {\.\.+} [lindex [split [gets $c] \ ] 1] x]]] $c;close $f;close $c}}} 80]
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In fact, I'm pretty sure the [split] isn't needed, as {} etc can't occur in URLs. Down to 121 characters:
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vw [soc -server {appl {{c a p} {fcopy [set f [open .[regsub {\.\.+} [lindex [gets $c] 1] x]]] $c;close $f;close $c}}} 80]
======
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!!!!!!
%| Category Example |%
!!!!!!