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.
puts [readsensor [selectsensor]]
The exceptions to this rule are as follows:
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:" puts " X: $x Y: $y\n" set z {[set x "String within quotes within braces"]} puts "Note curly braces: $z\n" set a "[set x {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}]" # Note the \ escapes the bracket, and must be doubled to be a # literal character in double quotes puts "Note the \\ escapes the bracket:\n \$b is: $b" puts "\$y is: $y"
A simple substitution: abc Remember that set returns the new value of the variable: X: def Y: def Note curly braces: [set x "String within quotes within braces"] See how the set is executed: String within braces within quotes $x is: String within braces within quotes Note the \ escapes the bracket: $b is: [set y {This is a string within braces within quotes}] $y is: def