http://purl.org/tcl/home/man/tcl8.4/TclCmd/cd.htm
A frequently made mistake [L1 ] that programmers make is to attempt to exec the cd command. cd stands for change directory. This sets a kernel specific piece of information regarding what files within a filesystem the application will see if they use relative pathnames (that is to say, pathnames either beginning with ./ , ../ , or without any leading directory indicatory).
This kernel value is a unique value per process - changing it in one process won't change it in other existing processes.
Tcl syntax help - Arts and crafts of Tcl-Tk programming - Category Command