set f [open "C:/John/myFile.txt" "a+"]

seek $f 0
set fp [tell $f]

seek $f 0 end
set end [tell $f]

# Restore current file pointer
seek $f $fp

while { $fp < $end } {
    puts -nonewline $f " "
    incr fp
}

close $f

chan truncate $f 0

close [open $thefilename "w"]

close [open $path w]

set blank_file [open "C:/tmp/blank.txt" "w"]
close $blank_file

file rename -force "C:/tmp/blank.txt" "C:/John/myFile.txt"