Conditionals in Tcl

Tcl's if command evaluates a boolean expression and executes a block of code enclosed in braces when the condition is true. Unlike many languages, Tcl treats if as an ordinary command: the condition and each body are just arguments, which is why the opening brace { of the body must appear on the same line as if -- otherwise Tcl's command parser sees an incomplete command and waits for more input. The condition itself is evaluated by the expr command internally, using C-like operators such as ==, !=, <, and >, as well as string comparison operators like eq and ne.

Multiple related conditions chain together with elseif: Tcl checks each condition in the order it's written and executes the body of the first one that's true, then skips every remaining elseif and else clause entirely. A trailing else acts as a catch-all body that runs only if none of the preceding conditions matched. Because evaluation stops at the first true branch, the order in which you write your conditions matters -- a more specific condition placed after a broader one that would also match it will never be reached.

Tcl's expr engine keeps numeric and string comparisons as separate operator families: ==, !=, <, >, <=, and >= coerce their operands to numbers (and error if that's impossible), while eq and ne always compare the raw string representation regardless of whether it looks numeric. The logical operators && and || short-circuit, meaning the second operand is never evaluated once the overall result is already determined by the first, which matters if that second operand has side effects like a command call. Tcl also provides the in operator for compact list-membership tests, e.g. $status in {"open" "pending"}, avoiding a chain of || comparisons.

The switch command matches a value against a series of patterns and runs the body of the first arm that matches, similar to an if/elseif chain but often more readable for many discrete cases. Its matching mode is controlled by a flag: -exact (the default) requires character-for-character equality, -glob allows wildcard patterns with * and ?, and -regexp allows full regular expressions. A default arm catches anything unmatched, and writing a bare - as an arm's body tells Tcl to fall through and execute the next matching arm's body instead, letting several patterns share one action.