In Emacs Lisp, how do I check if a variable is defined?

In Emacs Lisp, how do I check if a variable is defined?

From stackoverflow

• 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
  

  Rainer Joswig : sometimes it might also be useful to use INTERN-SOFT to check whether a symbol exists.

• 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.