What is double substitution

In Tcl the words in a command are always exposed to two layers of interpretation: First, Tcl interprets them and prepares them as arguments to a routine. Second, the routine interprets those arguments according to its own principles. double substitution occurs when Tcl performs substitutions on original command and a routine then performs substitutions on the resulting arguments. Care is required in these cases to avoid errors, including vulnerability to injection attacks.

Explanation:

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