How do I join all lines in a file into one, or use a custom separator when joining?

Explanation

Using \n in the pattern of :substitute matches the newline character at the end of each line, letting you join lines with any separator you choose — something the J command and :join cannot do.

How it works

• % — apply to every line in the file
• s/\n/ /g — replace each line-ending newline with a space
• Change the replacement to suit your needs: use , to join with commas, or leave it empty (s/\n//g) to concatenate with no gap

Because Vim's :substitute can match across the newline at the end of each line, every line is collapsed into the first in a single command pass.

Example

Given a buffer with three lines:

foo
bar
baz

Running :%s/\n/ /g produces:

foo bar baz

To join with a comma separator instead:

:%s/\n/,/g

Result:

foo,bar,baz

Tips

• Use a line range to join only part of the file: :1,5s/\n/ /g
• J and :[range]join always join with a single space; this substitution is the only built-in way to specify a custom separator
• To keep blank lines as paragraph separators while joining within paragraphs, use :g/./,/^$/join instead