what are the usage of equalprg and formatprg

Question

If I want to format a few lines, should I choose equalprg (using = operator) or formatprg (using gq operator)?

Both of them are for some kind of formatting. But what are the differences.

Answer 1

From :help equalprg:

'equalprg' 'ep'               string  (default "")
                      global or local to buffer |global-local|
      External program to use for "=" command. ...

From :help =:

={motion}               Filter {motion} lines through the external program
                        given with the 'equalprg' option.  When the 'equalprg'
                        option is empty (this is the default), use the
                        internal formatting function |C-indenting| and
                        |'lisp'|.  But when 'indentexpr' is not empty, it will
                        be used instead |indent-expression|.  When Vim was
                        compiled without internal formatting then the "indent"
                        program is used as a last resort.

I agree that it would be nice if the "intent" of the motion was written more clearly, but it does seem relatively clear that the intent of = and thus equalprg is indentation.

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Similarly we can find from :help formatprg and :help gq that this is a more general purpose formatting feature. It allows to use external programs or internal methods based on the formatexpr and formatprg settings. With internal formatting it typically just reflows text within a given textwidth, i.e. the textwidth setting.