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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.