lappend varName [ value ... ]
This command treats the variable given by varName as a list and appends each of the value arguments to that list as a separate element, with spaces between elements. If varName doesn't exist, it is created as a list with elements given by the value arguments. lappend is similar to append except that the values are appended as list elements rather than raw text. This command provides a relatively efficient way to build up large lists. For example,
lappend a $bis much more efficient than
set a [concat $a [list $b]]when $a is long.