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.