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 huecker.com # Grundlagen der Programmierung | Tcl Tutorial.
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. Evaluation & Substitutions 3: Grouping arguments with [] .

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You obtain the results of a command by placing the command in square brackets ([]). This is the functional equivalent of the back single quote (`) in sh programming, or using the return value of a function in C.

As the Tcl interpreter reads in a line it replaces all the $variables with their values. If a portion of the string is grouped with square brackets, then the string within the square brackets is evaluated as a command by the interpreter, and the result of the command replaces the square bracketed string.

Let's take the following code segment, for example: puts [readsensor [selectsensor]]

  • The parser scans the entire command, and sees that there is a command substitution to perform: readsensor [selectsensor] , which is sent to the interpreter for evaluation.
  • The parser once again finds a command to be evaluated and substituted, selectsensor
  • The fictitious selectsensor command is evaluated, and it presumably returns a sensor to read.
  • At this point, readsensor has a sensor to read, and the readsensor command is evaluated.
  • Finally, the value of readsensor is passed on back to the puts command, which prints the output to the screen.

The exceptions to this rule are as follows:

  • A square bracket that is escaped with a \ is considered as a literal square bracket.
  • A square bracket within braces is not modified during the substitution phase.

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. Example .

   set x "abc"
   puts "A simple substitution: $x\n"

   set y [set x "def"]
   puts "Remember that set returns the new value of the variable: X: $x Y: $y\n"

   set z {[set x "This is a string within quotes within braces"]}
   puts "Note the curly braces: $z\n"

   set a "[set x {This is a string within braces within quotes}]"
   puts "See how the set is executed: $a"
   puts "\$x is: $x\n"

   set b "\[set y {This is a string within braces within quotes}]"
   puts "Note the \\ escapes the bracket:\n \$b is: $b"
   puts "\$y is: $y"
  

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