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.

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}]"
# 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"

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