you may want boundp: returns t if variable (a symbol) is not void; more precisely, if its current binding is not void. It returns nil otherwise.
(boundp 'abracadabra) ; Starts out void.
=> nil
(let ((abracadabra 5)) ; Locally bind it.
(boundp 'abracadabra))
=> t
(boundp 'abracadabra) ; Still globally void.
=> nil
(setq abracadabra 5) ; Make it globally nonvoid.
=> 5
(boundp 'abracadabra)
=> t
In addition to dfa's answer you may also want to see if it's bound as a function using fboundp:
(defun baz ()
)
=> baz
(boundp 'baz)
=> nil
(fboundp 'baz)
=> t
If you want to check a variable value from within emacs (I don't know if this applies, since you wrote "in Emacs Lisp"?):
M-: starts Eval in the mini buffer. Write in the name of the variable and press return. The mini-buffer shows the value of the variable.
If the variable is not defined, you get a debugger error.
M-: (boundp 'the-variable-name) RET will check without the need for triggering an error.
Remember that variables having the value nil is regarded as being defined.
(progn (setq filename3 nil) (boundp 'filename3)) ;; returns t
(progn (setq filename3 nil) (boundp 'filename5)) ;; returns nil
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symbol-valuefunction to print the actual value. symbol-value-doc. Egs: Using theeval-expressioncommand and then typing this out:(symbol-value 'abracadabra)letblock returnsnilfor me. Does this work with lexical binding?