True is a [boolean] value. It is recognised by [expr]. 

   1. True values are '''yes''', '''on''', '''true''', their ''unique'' abbrevations, and non-zero integers, 
   2. False values are '''no''', '''off''', '''false''', their ''unique'' abbrevations, and the integer '''0'''.

The function '''[string is] true''' returns true if its argument obeys the condition 1. above (and '''[string is] false''' does the opposite).

Unless the '''-strict''' flag is passed both '''[string is] true ""''' and '''[string is] false ""''' (i.e., testing the empty string) return 1.  This is the case to facilitate entry validation in Tk.

The canonical form for true, like in C, is integer 1. To wit:
 % expr 42==42
 1

[1S] See also: [Boolean], [magic names]


----
[Category Glossary] [Category Internals]