When issuing an "exec dir" command or an "exec echo Hello World" command, the following error occurs:
Exec is a Linux command that is not fully supported in Windows in the Vivado 2013.4 Tcl compiler.
Modifications to the Vivado Tcl compiler in Vivado2014.1 will resolve this issue.
To work around this in 2013.4 and older, use the dir and echo commands by themselves without the exec command as a precursor.
When the Vivado Tcl shell does not recognize these commands, it will automatically pass them to the OS shell.