Node:Command substitution, Next:Backslash substitution, Previous:Variable substitution, Up:Syntax (Tcl)
Command substitution happens when a word contains an open square bracket, [. The string between the open bracket and matching closing bracket are treated as a Tcl script. The script is evaluated and its result is substituted in place of the original command substitution word.
A simple example of command substitution is:
set a 1 set b [set a]
which does the same as the previous example but using command
substitution. The result of a set a command is to return the
value of a which is then passed as an argument to set b and so
variable b acquires the value of variable a.