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27.2.1 Examining and Setting Variables

C-h v
M-x describe-variable

Print the value and documentation of a variable.

M-x set-variable

Change the value of a variable.

To examine the value of a single variable, use C-h v (describe-variable), which reads a variable name using the minibuffer, with completion. It prints both the value and the documentation of the variable.

C-h v fill-column RET

prints something like:

fill-column's value is 75

*Column beyond which automatic line-wrapping should happen.
Automatically becomes local when set in any fashion.

The star at the beginning of the documentation indicates that this variable is an option. C-h v is not restricted to options; it allows any variable name.

If you know which option you want to set, you can use M-x set-variable to set it. This prompts for the variable name in the minibuffer (with completion), and then prompts for a Lisp expression for the new value using the minibuffer a second time. For example,

M-x set-variable RET fill-column RET 75 RET

sets fill-column to 75, as if you had executed the Lisp expression (setq fill-column 75).

Setting variables in this way, like all means of customizing Emacs except where explicitly stated, affects only the current Emacs session.