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Tcl looks for the command to be exec'd in a sequence of directories (PATH) that differs with the host platform. This sequence is documented in the exec man page: http://www.tcl.tk/man/tcl/TclCmd/exec.htm
A potential 'gotcha' occurs when the PATH used by the user's shell differs from that of Tcl. This is a 'gotcha' for all programs that need to execute other programs and isn't restricted to interpreters such as Perl/Python/PHP let alone Tcl.
When this happens calling the command from the shell or commandline (say Dos) works but execing it from Tcl either fails completely or calls a different program with the same name.
E.g. With ImageMagick installed on Windows
# this works dos> convert a.tiff a.jpg #but this results in an error tclsh> exec convert a.tiff a.jpg Invalid Parameter - a.jpg Child process exited abnormally
What's happening here is that Tcl is invoking C:\WINDOWS\system32\convert.exe while dos invokes the convert.exe installed in the ImageMagick directory.
The moral of the story is not to assume that because a command works from the commandline, it will also be exec'ed successfully by Tcl or any other interpreter or program.
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