How to create thumbnails in batch with ImageMagick

A directory of product photos, gallery images, or release screenshots often needs matching thumbnails before it can be uploaded or handed to another workflow. ImageMagick can process the whole batch from a shell and write the smaller files to a separate directory so the originals remain available.

The safest batch pattern is to create the output directory first and run magick mogrify with -path. Plain mogrify normally rewrites each matched file, while -path sends the generated thumbnails to another directory and preserves the source filenames.

A *.jpg glob and a 320x320 thumbnail box keep the first run narrow enough to review before applying the same pattern to a larger image set. Landscape files fit by width, portrait files fit by height, and ImageMagick preserves proportions instead of stretching images into a square.

1. Open a terminal in the directory that contains the source images.
2. Create a separate output directory for the thumbnail batch.

   $ mkdir -p thumbnails

   magick mogrify writes into the directory named by -path. Create it first so a missing directory does not stop the batch.

3. Check the source images that match the batch glob.

   $ magick identify -format '%f %m %wx%h\n' *.jpg
   photo-01.jpg JPEG 1200x800
   photo-02.jpg JPEG 900x600
   photo-03.jpg JPEG 800x1200

   Use one glob that matches the files you intend to process. Repeat the command with another pattern, such as *.png, only when those files need the same thumbnail geometry.

4. Create thumbnails in the output directory.

   $ magick mogrify -verbose -path thumbnails -thumbnail 320x320 *.jpg
   photo-01.jpg JPEG 1200x800 1200x800+0+0 8-bit sRGB 44578B 0.000u 0:00.004
   photo-01.jpg=>thumbnails/photo-01.jpg JPEG 1200x800=>320x213 8-bit sRGB 6481B 0.010u 0:00.005
   photo-02.jpg JPEG 900x600 900x600+0+0 8-bit sRGB 30631B 0.000u 0:00.001
   photo-02.jpg=>thumbnails/photo-02.jpg JPEG 900x600=>320x213 8-bit sRGB 6723B 0.010u 0:00.010
   photo-03.jpg JPEG 800x1200 800x1200+0+0 8-bit sRGB 39604B 0.000u 0:00.002
   photo-03.jpg=>thumbnails/photo-03.jpg JPEG 800x1200=>213x320 8-bit sRGB 6297B 0.000u 0:00.005

   The -verbose output shows each input file and the matching file written under thumbnails/. Remove -verbose after the command is proven and the batch becomes routine.

5. Verify the thumbnail dimensions from the output directory.

   $ magick identify -format '%f %m %wx%h\n' thumbnails/*.jpg
   photo-01.jpg JPEG 320x213
   photo-02.jpg JPEG 320x213
   photo-03.jpg JPEG 213x320

   Each thumbnail should fit within the 320x320 box. A portrait image reports 213x320 because the height reaches the box limit first.

6. Open one thumbnail before replacing gallery, catalog, or site references.

   Keep the originals outside the thumbnail directory. Running magick mogrify without -path can overwrite the files matched by the glob.