How do I delete all blank lines in a buffer? [duplicate]

Question

Emacs 26.1

In buffer

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I use command "delete-blank-lines". But it NOT delete ALL BLANK LINES. It's delete ONLY ONE BLANK LINE.

Why?

I need to delete ALL BLANK LINES. The result must be like this:

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Answer 1

From MasteringEmacs.com By Mickey Petersen:

This is a frequent question so I figured I’d mention the solution here:

You want to remove all empty (blank) lines from a buffer. How do you do it? Well, it’s super easy.

Mark what you want to change (or use C-x h to mark the whole buffer) and run this:

M-x flush-lines RET ^$ RET

And you’re done. So what does that mean? Well, M-x flush-lines will flush (remove) lines that match a regular expression, and ^$ contain the meta-characters ^ for beginning of string and $ for end of string. Ergo, if the two meta-characters are next to eachother, it must be a blank line.

We can also generalize it further and remove lines that may have whitespace (only!) characters:

M-x flush-lines RET ^\s-*$ RET

In this case \s- is the syntax class (type C-h s to see your buffer’s syntax table) for whitespace characters. The * meta-character, in case you are not a regexp person, means zero or more of the preceding character.