The [exec] command treats ampersands (&) in its arguments specially, e.g. for making execution asynchronous. This can be a problem if an argument, like an URL, contains ampersamds.

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petterik@gol.com (Petteri K) writes in [the comp.lang.tcl newsgroup]:
Would anybody know a workaround in case $url contains
character '&' e.g. http://127.0.0.1/cgi-bin/script?var1=abc&var2=xyz

[Benjamin Riefenstahl] replied:
[exec] plays tricks with quoting which make it impossible for me to
get the right command executed in Tcl 8.3.4.  What works on W2K here
though is this:

   set url "http://www.tcl.tk?hello&hi"
   set shell [open "|[file join $env(COMSPEC)]" w]
   fconfigure $shell -buffering line
   puts $shell "start \"\" \"$url\""
   puts $shell exit
   close $shell

I hope this works on Windows 9x/Me, too.

[CL] suggests: In the context of avoiding exec's
unescapable special characters, I prefer to use the
far-too-little-understood << argument, in the manner of
[[slightly edited]]:

   set url http://ats.nist.gov/cgi-bin/cgi.tcl/echo.cgi?hello=1&hi=2
   exec [file join $env(COMSPEC)] << "start \"\" \"$url\" \n exit \n"

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Here is the simplest way that I've found, and I use it in my mindweb [http://www.rohanpall.com/mindweb/].  It relies on the [rundll] utility available with some versions of Windows.

  proc goUrl x {
    global tcl_platform
    if {$tcl_platform(platform) eq "windows"} {
      set x [regsub -all -nocase {htm} $x {ht%6D}]
      exec rundll32 url.dll,FileProtocolHandler $x &
    }
  } ;# This definitely works on Win95 and Win98. 
  
See [invoking browsers] ''--[Ro]''

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For Unix/Linux, it is highly probable that the line
   set shell [open "|/bin/sh" w]
works as well (just in place of the "start" command, you'd have to substitute a more specific one...)
Actually:
        
        set shell [ open "|/bin/sh -s 2>@stdout" w+ ]
        fconfigure $fid -blocking off
        fconfigure $fid -buffering line

Comes closer to doing what is expected... ''-sluggo''

And when time comes to read it off, something like:

  while { [ gets $shell line ] >= 0 } {
     append retval "$line\n"
     after 1
  }

Will get you past things that buffer, like ps. The `after 1' seems to be quite magical in the case of buffering, whereas `after 0' does nothing useful.

But one idea for more cross-platformity: Unix has (guaranteed?) env(SHELL) to give the default shell; Windows has env(COMSPEC), Mac I don't know. Would it be worthwile to unify these into a, say, ''::[tcl_platform](shell)'' element? ([RS])

No, [auto_execok] should cover that.