GNU Emacs Manual: Examining

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AD.2.1 Examining and Setting Variables

C-h v var RET
Display the value and documentation of variable var (describe-variable).
M-x set-variable RET var RET value RET
Change the value of variable var to value.

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 displays both the value and the documentation of the variable. For example,

C-h v fill-column RET
displays something like this:

fill-column's value is 70 Documentation: *Column beyond which automatic line-wrapping should happen. Automatically becomes buffer-local when set in any fashion.
The star at the beginning of the documentation indicates that this variable is a user option. C-h v is not restricted to user options; it allows any variable name.
The most convenient way to set a specific user option is with M-x set-variable. This reads the variable name with the minibuffer (with completion), and then reads 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.
M-x set-variable is limited to user option variables, but you can set any variable with a Lisp expression, using the function setq. Here is a setq expression to set fill-column:

(setq fill-column 75)
To execute an expression like this one, go to the `*scratch*' buffer, type in the expression, and then type C-j. See section V.9 Lisp Interaction Buffers.
Setting variables, like all means of customizing Emacs except where otherwise stated, affects only the current Emacs session.

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