Command Substitution

The second form of substitution is command substitution. A nested command is
delimited by square brackets, [ ]. The Tcl interpreter takes everything between
the brackets and evaluates it as a command. It rewrites the outer command by
replacing the square brackets and everything between them with the result of
the nested command. This is similar to the use of backquotes in other shells,
except that it has the additional advantage of supporting arbitrary nesting of
commands.

Example 1–3 Command substitution.
set len [string length foobar]
=> 6
In Example 1–3, the nested command is:
string length foobar
This command returns the length of the string foobar. The string command
is described in detail starting on page 45. The nested command runs first.
6 Tcl Fundamentals Chap. 1
Then, command substitution causes the outer command to be rewritten as if it
were:
set len 6
If there are several cases of command substitution within a single command,
the interpreter processes them from left to right. As each right bracket is
encountered, the command it delimits is evaluated. This results in a sensible
ordering in which nested commands are evaluated first so that their result can
be used in arguments to the outer command.