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For a quick batch resize of JPEGs in the current folder, ImageMagick’s magick mogrify can process them with one command: magick mogrify -resize 256x256 *.jpg. That command changes the matching originals in place. To preserve them, write converted or compressed copies to a separate location, then check both the output files’ sizes and their appearance.
Choose what “compress” should do
ImageMagick can reduce image file sizes in several ways, and they do not have the same effect:
- Resize: changes pixel dimensions, which can reduce file size but also removes resolution. The example command resizes images to fit within 256 by 256 pixels while preserving their aspect ratio.
- Change encoding settings: can trade visual detail, file size, and processing time. JPEG quality settings are lossy; the result depends on the image and encoder settings.
- Optimize losslessly: seeks a smaller file without changing the decoded pixels. No particular size reduction is guaranteed.
- Convert format: can create a smaller file in some cases, but may change image characteristics. JPEG, for example, is lossy, so inspect converted images before using them.
There is no universal quality value or compression ratio that works for every image. Choose settings for the target format and intended use.
Batch-process JPEGs in one folder
ImageMagick’s documented example resizes matching JPEGs in the current directory:
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magick mogrify -resize 256x256 *.jpgThe *.jpg wildcard selects matching files in the current directory; it does not search nested folders. The command overwrites those originals. ImageMagick notes that “Mogrify overwrites the original image file, whereas, magick writes to a different image file.” See the ImageMagick command-line tools documentation and Mogrify documentation.
Run the command on a backed-up sample or copies first. If the originals matter, do not use an in-place command on your only copy.
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Keep originals by creating copies
Convert matching PNGs to JPEG copies
To leave the PNG inputs in place and create JPEG output files in the current directory, ImageMagick documents:
magick mogrify -format jpg *.pngThe input PNGs remain, while JPEG copies are written. Because JPEG uses lossy encoding, check that the converted images are suitable for their intended purpose. For a different output location, use an output-path approach supported by ImageMagick and make sure the destination directory exists.
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Use a separate output directory for a batch
For more control over output names and location, use magick to read each source and write a distinct destination file. In a POSIX-compatible shell such as Bash or Zsh, this loop writes JPEGs to an existing compressed directory:
mkdir -p compressed
for file in ./*.jpg; do
[ -f "$file" ] || continue
magick "$file" -quality 80 "compressed/$(basename "$file")"
doneThis example uses JPEG quality 80; it does not promise a fixed file-size reduction. The loop matches lowercase .jpg files in the current directory only, and writes them to compressed. It does not recurse into subdirectories or handle other extensions. Keep the source files until you have verified the output.
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Set quality for the output format
ImageMagick’s -quality option has format-specific meaning, so a number should not be treated as a universal compression scale. See the ImageMagick command-line options documentation.
| Format | What -quality controls |
Practical implication |
|---|---|---|
| JPEG | A scale from 1 (lowest image quality and highest compression) to 100 (best quality and least effective compression). Default behavior can depend on whether the input quality can be estimated. | Lower settings can reduce file size by discarding more visual information. Compare representative output images rather than assuming a setting will suit every source. |
| PNG | Zlib compression level and filter selection. | The value is not comparable to JPEG quality: it controls PNG compression behavior rather than a JPEG-style visual-quality scale. |
| WebP | WebP has additional encoding options that trade encoding speed, compressed file size, and quality. | Choose settings according to whether speed, size, or visual quality matters most. Consult the ImageMagick WebP documentation. |
Before using a less common format, verify that your installed ImageMagick build supports the required input and output codec.
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Handle nested folders and filenames with spaces
A shell wildcard such as *.jpg is a convenient selector for one directory, not a portable recursive file-discovery method. For nested folders, mixed extensions, or a large collection with varied filenames, use a file-list or loop approach appropriate to your operating system and shell. Quote each filename when passing it to ImageMagick so spaces are not treated as separators. The exact recursive command depends on the shell and platform; do not assume the single-folder wildcard covers subdirectories.
Check the results before using them
- Start with a small sample. Copy a few representative images to a test folder and run the intended command there.
- Inspect appearance and dimensions. Check for unwanted artifacts, lost detail, unexpected format changes, or dimensions that are too small.
- Compare file sizes. Confirm that the output is smaller for your sample and that the trade-off is acceptable; compression results vary by source, format, and settings.
- Process the full set only after validation. Keep a backup or write to a separate output location, and retain originals until the output is checked.
ImageMagick’s documentation establishes how these commands and format settings work, but it does not establish a typical percentage of storage savings. Measure your own images rather than relying on a promised reduction.
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