Batch Resize Images on Linux with ImageMagick

Resizing a single image is straightforward, but processing dozens or hundreds requires automation. ImageMagick’s convert command (or the modern magick command) paired with a bash loop is the standard approach for batch image resizing on Linux.

Using ImageMagick

ImageMagick remains the most reliable tool for this task. Here’s a practical script to resize all images in a directory:

mkdir -p resize
for i in *.{jpg,jpeg,png,gif}; do
    [ -f "$i" ] || continue
    echo "Processing $i"
    magick "$i" -resize 600 "resize/$i"
done

Key points:

• mkdir -p resize creates the output directory (the -p flag prevents errors if it already exists)
• The glob pattern *.{jpg,jpeg,png,gif} targets common image formats
• [ -f "$i" ] || continue skips non-matching patterns safely
• magick is the modern ImageMagick command (replaces deprecated convert)
• -resize 600 scales the image to 600px width, maintaining aspect ratio

To resize to a specific width and height without maintaining aspect ratio, use:

magick "$i" -resize 600x400\! "resize/$i"

The backslash-escaped ! forces exact dimensions.

Handling Filenames with Spaces

The script above already handles spaces correctly because we’re using proper quoting. The old IFS trick is unnecessary with modern bash:

mkdir -p resize
for i in *; do
    [ -f "$i" ] || continue
    magick "$i" -resize 600 "resize/$i"
done

The quotes around "$i" protect filenames with spaces, newlines, and special characters.

Using Parallel Processing for Speed

For large batches, process multiple images simultaneously:

mkdir -p resize
find . -maxdepth 1 -type f \( -iname "*.jpg" -o -iname "*.png" \) | \
    parallel magick {} -resize 600 "resize/{/}"

Install parallel if needed (sudo apt install parallel on Debian/Ubuntu). This dramatically speeds up processing on multi-core systems.

Using FFmpeg for Additional Control

FFmpeg can handle batch image resizing with more advanced options:

mkdir -p resize
for i in *.{jpg,png}; do
    [ -f "$i" ] || continue
    ffmpeg -i "$i" -vf "scale=600:-1" "resize/$i" -y
done

The -vf "scale=600:-1" filter scales to 600px width with proportional height. Use scale=600:400 for fixed dimensions.

Converting Between Formats

Resize and convert formats in one step:

mkdir -p resize
for i in *.png; do
    [ -f "$i" ] || continue
    magick "$i" -resize 600 -quality 85 "resize/${i%.png}.jpg"
done

The ${i%.png} removes the .png extension before adding .jpg.

Checking Results

Before processing hundreds of images, test on a single file:

magick original.jpg -resize 600 test.jpg
identify test.jpg  # Check dimensions and file size

Use identify to verify the output dimensions and confirm quality settings produce acceptable file sizes.

Common Gotchas

• Progressive JPEG: Use magick -interlace Plane for web-optimized JPEGs
• Transparency: PNG resizing preserves alpha channels by default; JPEGs don’t support them
• Disk space: Keep the original directory and verify the resize folder has enough space before running large batches
• ImageMagick policies: Some systems restrict ImageMagick’s functionality via /etc/ImageMagick-6/policy.xml; adjust if needed

ImageMagick remains the standard because it handles edge cases well, produces consistent output, and integrates seamlessly with shell scripts.