info exists varName
Returns 1 if the variable named varName exists in the current context (either as a global or local variable) and has been defined by being given a value, returns 0 otherwise.
See also:
Called with a variable or array element name (no $ in front!), returns 1 if the variable or array element exists, else 0.
info exists a 0 set a 1 1 info exists a 1 info exists b 0 set b(2) 2 2 info exists b 1 info exists b(1) 0 info exists b(2) 1 info exists $a 0
Notice the last one - if the user thought they were testing to see if the a variable exists, they used the wrong syntax - saying $a means read the contents of the a variable, and see if there is a varible by that name.
-- It doesn't have to be wrong syntax, actually: it's often useful to have one variable reference others, like this:
foreach var {a b c} {
if {[info exists $var]} {
puts "$var does indeed exist"
} else {
puts "$var sadly does not exist"
}
}Then again -
set a b b info exists $a 1
Unintentionally setting a to a value that equals the name of an existing variable yields a false positive when erroneously (in this case) checking the value of a.
KPV Another thing to keep in mind is that linking variable with upvar can do funny things with the existence of variables.
% set a 1 1 % upvar #0 a b % info exists b 1 % unset a % info exists b 0 % set a 1 1 % info exists b 1