I now use dwim-shell-command, which reduces the logic to:
(defun dwim-shell-commands-image-to-jpg ()
"Convert all marked images to jpg(s)."
(interactive)
(dwim-shell-command-on-marked-files
"Convert to jpg"
"convert -verbose '<<f>>' '<<fne>>.jpg'"
:utils "convert"))
Shell one-liners are super handy for batch-processing files. Say you'd like to convert a bunch of images from HEIC to jpg, you could use something like:
for f in *.HEIC ; do convert "$f" "${f%.*}.jpg"; done
Save the one-liner (or memorize it) and pull it from your toolbox next time you need it. This is handy as it is, but Emacs dired is just a file-management powerhouse. Its dired-map-over-marks function is just a few elisp lines away from enabling all sorts of batch processing within your dired buffers.
Dired already enables selecting and deselecting files using all sorts of built-in mechanisms (dired-mark-files-regexp, find-name-dired, etc) or wonderful third-party packages like Matus Goljer's dired-filters.
Regardless of how you selected your files, here's a snippet to run ImageMagick's convert on a bunch of selected files:
;;; -*- lexical-binding: t; -*-
(defun ar/dired-convert-image (&optional arg)
"Convert image files to other formats."
(interactive "P")
(assert (or (executable-find "convert") (executable-find "magick.exe")) nil "Install imagemagick")
(let* ((dst-fpath)
(src-fpath)
(src-ext)
(last-ext)
(dst-ext))
(mapc
(lambda (fpath)
(setq src-fpath fpath)
(setq src-ext (downcase (file-name-extension src-fpath)))
(when (or (null dst-ext)
(not (string-equal dst-ext last-ext)))
(setq dst-ext (completing-read "to format: "
(seq-remove (lambda (format)
(string-equal format src-ext))
'("jpg" "png")))))
(setq last-ext dst-ext)
(setq dst-fpath (format "%s.%s" (file-name-sans-extension src-fpath) dst-ext))
(message "convert %s to %s ..." (file-name-nondirectory dst-fpath) dst-ext)
(set-process-sentinel
(if (string-equal system-type "windows-nt")
(start-process "convert"
(generate-new-buffer (format "*convert %s*" (file-name-nondirectory src-fpath)))
"magick.exe" "convert" src-fpath dst-fpath)
(start-process "convert"
(generate-new-buffer (format "*convert %s*" (file-name-nondirectory src-fpath)))
"convert" src-fpath dst-fpath))
(lambda (process state)
(if (= (process-exit-status process) 0)
(message "convert %s ✔" (file-name-nondirectory dst-fpath))
(message "convert %s ❌" (file-name-nondirectory dst-fpath))
(message (with-current-buffer (process-buffer process)
(buffer-string))))
(kill-buffer (process-buffer process)))))
(dired-map-over-marks (dired-get-filename) arg))))
The snippet can be shorter, but wouldn't be as friendly. We ask users to provide desired image format, spawn separate processes (avoids blocking Emacs), and generate a basic report. Also adds support for Windows.
The snippet isn't currently capping the number of processes, but hey we can revise in the future…
Thanks to Philippe Beliveau for pointing out a bug in snippet (now updated) and changes to make it Windows compatible.