append - Man Page

append varName ?value value value ...?

Append all of the value arguments to the current value of variable varName.  If varName does not exist, it is given a value equal to the concatenation of all the value arguments. The result of this command is the new value stored in variable varName. This command provides an efficient way to build up long variables incrementally. For example, “append a $b” is much more efficient than “set a $a$b” if $a is long.

Building a string of comma-separated numbers piecemeal using a loop.

set var 0
for {set i 1} {$i<=10} {incr i} {
    append var "," $i
}
puts $var
# Prints 0,1,2,3,4,5,6,7,8,9,10

concat(n), lappend(n)

append, variable

concat(n), dict(n).