Creates a temporary file and returns an open read-write channel to it.
If varName is given, set that variable to the name of the temporary file (otherwise tries to delete the file at the first opportunity). If template is given, use it to guide the filename generated.
This command was introduced by TIP # 210 in Tcl 8.6.
AMG: How do I make a uniquely-named temporary directory?
AMG: What's a good way to get comparable functionality in Tcl 8.4? Actually, there is: