Command substitution

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.