The PATH environment variable is used by the OS when exec calls
are made.  Tcl invokes exec during both the [exec] command, as well
as the [open] ''pipe'' style call.

If you specify a full pathname for the command in either of these
cases, exec directly invokes the command.  Otherwise, the system
call works as documented for your system, which is typically
to walk through the path, a directory at a time, reading through
the directory until a '''suitable''' (defined by the OS)
file is located for execution.