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(get-file-buffer filename) returns the buffer visiting filename, or nil if there is none.

https://www.gnu.org/software/emacs/manual/html_node/elisp/Buffer-File-Name.html

Edit: justbur proposes using (find-buffer-visiting file) instead, as it also works when the buffer has a different name than the file.

So one solution would be sth in the lines of

(defun batch (file)
  (let ((keep (find-buffer-visiting file)))
    (find-file file)
    (end-of-buffer)
    (insert "nix")
    (save-buffer)
    (unless keep (kill-buffer)))

but there may be more elegant solutions.

(get-file-buffer filename) returns the buffer visiting filename, or nil if there is none.

https://www.gnu.org/software/emacs/manual/html_node/elisp/Buffer-File-Name.html

Edit: justbur proposes using (find-buffer-visiting file) instead, as it also works when the buffer has a different name than the file.

So one solution would be sth in the lines of

(defun batch (file)
  (let ((keep (find-buffer-visiting file)))
    (find-file file)
    (end-of-buffer)
    (insert "nix")
    (save-buffer)
    (unless keep (kill-buffer)))

but there may be more elegant solutions.

(get-file-buffer filename) returns the buffer visiting filename, or nil if there is none.

https://www.gnu.org/software/emacs/manual/html_node/elisp/Buffer-File-Name.html

So one solution would be sth in the lines of

(defun batch (file)
  (let ((keep (find-buffer-visiting file)))
    (find-file file)
    (end-of-buffer)
    (insert "nix")
    (save-buffer)
    (unless keep (kill-buffer)))

but there may be more elegant solutions.

(get-file-buffer filename) returns the buffer visiting filename, or nil if there is none.

https://www.gnu.org/software/emacs/manual/html_node/elisp/Buffer-File-Name.html

Edit: justbur proposes using (find-buffer-visiting file) instead, as it also works when the buffer has a different name than the file.

So one solution would be sth in the lines of

(defun batch (file)
  (let ((keep (find-buffer-visiting file)))
    (find-file file)
    (end-of-buffer)
    (insert "nix")
    (save-buffer)
    (unless keep (kill-buffer)))

but there may be more elegant solutions.

(get-file-buffer filename) returns the buffer visiting filename, or nil if there is none.

https://www.gnu.org/software/emacs/manual/html_node/elisp/Buffer-File-Name.html

So one solution would be sth in the lines of

(defun batch (file)
  (let ((keep (get-file-buffer file) ))
    (find-file file)
    (end-of-buffer)
    (insert "nix")
    (save-buffer)
    (unless keep (kill-buffer)))

but there may be more elegant solutions.

(get-file-buffer filename) returns the buffer visiting filename, or nil if there is none.

https://www.gnu.org/software/emacs/manual/html_node/elisp/Buffer-File-Name.html

So one solution would be sth in the lines of

(defun batch (file)
  (let ((keep (find-buffer-visiting file)))
    (find-file file)
    (end-of-buffer)
    (insert "nix")
    (save-buffer)
    (unless keep (kill-buffer)))

but there may be more elegant solutions.