HaO 2011-05-02 Comparisions with == or != do not work with booleans not in canonical form:

% expr {true == 1}
0

This issue may be avoided using the bool() function to bring strings in boolean canonical form:

% expr {bool(true) == bool(1)}
1

This is only recommeded if two variables contain boolean data.

expr {bool($b1) == bool($b2)}

otherwise, one would use:

expr { $b1 }
expr { ! $b1 }

MG The string is command also recognises booleans:

proc trueorfalse {str} {
  if { ![string is boolean -strict $str] } {
       return -1; # not a boolean
     }
  return [string is true $str]
}
% trueorfalse foo
-1
% trueorfalse yes
1
% trueorfalse off
0

HaO Yes, correct, another way to get a value in canonical boolean form is:

string is true -strict $b

hv Here is my little script which lists which token is true, false, or neither:

    #
    # Which token is considered true/false?
    #
    set tokens {-2 -1 0 1 2 yes no Yes No YES NO on off On Off ON OFF
        true false True False TRUE FALSE Y N OK FAIL}

    set trueTokens {}
    set falseTokens {}
    set neitherTokens {}

    foreach token $tokens {
        if {[string is true -strict $token]} {
            lappend trueTokens $token
        } else {
            if {[string is false -strict $token]} {
                lappend falseTokens $token
            } else {
                lappend neitherTokens $token
            }
        }
    }

    puts "True: $trueTokens"
    puts "False: $falseTokens"
    puts "Neither: $neitherTokens"

Output:

    True: 1 yes Yes YES on On ON true True TRUE Y
    False:0 no No NO off Off OFF false False FALSE N
    Neither: -2 -1 2 OK FAIL

AMG: Why does [string is boolean] reject "2" when it valid to use "2" anywhere a Boolean is expected?

Also, in addition to the strings tested above by HV's code, [string is boolean] accepts all capitalizations of: t, tr, tru, f, fa, fal, fals, ye, and of. Notably, o is not accepted, since it is a common prefix shared by on and off.