Automating image resizing and format conversion with ImageMagick

Kevin van Zonneveld

Co-founder · Amsterdam, The Netherlands · Show bio

Use ImageMagick to fit JPEGs inside an 800 × 600 box and save PNG copies while keeping the originals. Start with a flat folder, then use the Bash script below to process nested folders without losing files that share a name. Each example creates a new output directory and refuses to reuse it.

PNG is useful when the next step in your workflow needs lossless image storage. Converting a JPEG to PNG does not recover detail already lost to JPEG compression or guarantee a smaller file. If your goal is smaller photographs for a website, see the JPEG recipe.

Set up ImageMagick 7

These examples require Bash, find, and ImageMagick 7 with JPEG and PNG support. They were tested on macOS with ImageMagick 7.1.2-31 and Bash 3.2.57 and 5.3.20. Run the command blocks in Bash; macOS Terminal may otherwise start a different shell.

With Homebrew installed, use ImageMagick’s macOS installation instructions⁠:

brew install imagemagick

On Linux, check your distribution’s version before installing. For example, Ubuntu 24.04’s imagemagick package⁠ provides version 6. These commands require version 7; consult the official downloads⁠ if your package does not provide it.

ImageMagick also has a Windows installer⁠, but this walkthrough does not cover PowerShell or Command Prompt. A Windows installation alone does not provide the Bash environment used here.

Check the installed version and available formats:

magick -version && magick -list format

Look for ImageMagick 7 in the version line and rw in the mode column for both JPEG and PNG. If magick is missing, fix the installation or PATH before continuing.

Resize and convert a flat folder

In your working directory, create an input folder and put a few JPEGs in it. This setup command also accepts an existing input directory and does not change its contents:

mkdir -p input_images

The short commands process non-hidden, lowercase .jpg files directly inside input_images. For nested folders, other JPEG extensions, or names containing brackets or wildcard characters, use the recursive script instead.

mogrify normally overwrites its input files⁠. Here, -path directs the PNG copies into a new directory. The && prevents conversion if that directory already exists or cannot be created:

mkdir resized_png &&
magick mogrify -regard-warnings -define registry:filename:literal=true \
  -auto-orient -resize '800x600>' -format png -path resized_png ./input_images/*.jpg

-auto-orient applies the JPEG’s EXIF orientation before resizing. -regard-warnings⁠ makes warnings such as a truncated JPEG fail the command. A failure can leave partial output; inspect it before using the batch. Fix the input and choose a new output directory for another run.

The literal filename setting⁠ stops output names such as photo%03d.png from becoming photo000.png. Quoting shell arguments alone does not disable ImageMagick’s filename formatting.

The resize geometry⁠ 800x600> preserves aspect ratio and only shrinks images that exceed the box. Quote it so Bash does not interpret > as output redirection. To limit width alone, use '800x>' instead.

Check what was written:

magick identify -format '%f: %m %wx%h\n' ./resized_png/*.png

For example, a 1600 × 900 wide.jpg produces wide.png: PNG 800x450. The bounding box is a limit, so the output need not be exactly 800 × 600.

Automate nested folders

A recursive find followed by mogrify -path would flatten the directory tree: trip/photo.jpg and work/photo.jpg could overwrite each other. This script preserves subdirectories and appends .png to the complete source filename, keeping photo.jpg and photo.jpeg distinct too.

Save this as a new file named process_images.sh beside input_images. Keep the output directory outside the input tree. It must not already exist.

#!/usr/bin/env bash
set -euo pipefail

INPUT_DIR="input_images"
OUTPUT_DIR="output_images"

if [[ ! -d "$INPUT_DIR" ]]; then
  printf 'Input directory not found: %s\n' "$INPUT_DIR" >&2
  exit 1
fi

mkdir "$OUTPUT_DIR"

find "$INPUT_DIR" -type f \( -iname '*.jpg' -o -iname '*.jpeg' \) -print0 |
(
  found=false
  while IFS= read -r -d '' file; do
    found=true
    relative=${file#"$INPUT_DIR"/}
    output="$OUTPUT_DIR/$relative.png"
    mkdir -p "${output%/*}"

    # Capture diagnostics separately from PNG bytes; warnings also reject the image.
    if ! diagnostics=$(magick -regard-warnings jpeg:- -auto-orient -resize '800x600>' png:- \
      < "$file" 2>&1 > "$output") || [[ -n "$diagnostics" ]]; then
      rm -f "$output"
      printf 'Could not convert: %s\n%s\n' "$file" "$diagnostics" >&2
      exit 1
    fi
  done

  if [[ "$found" == false ]]; then
    printf 'No JPEG files found in %s\n' "$INPUT_DIR" >&2
    exit 1
  fi
)

printf 'Images have been resized and converted into %s\n' "$OUTPUT_DIR"

Run it from the directory containing the script and input_images:

bash process_images.sh

For example, input_images/trip/photo.jpg becomes output_images/trip/photo.jpg.png. The script accepts .jpg and .jpeg in any letter case. find -print0 and read -d '' preserve spaces and line breaks, while shell redirection keeps names such as -cover.jpg and photo%03d.jpg literal on both input and output. ImageMagick’s standard streams⁠ carry the JPEG and PNG bytes.

The script checks conversion diagnostics as well as exit status, so it rejects a truncated JPEG even if ImageMagick only prints a warning. It reports completion only after the whole pipeline succeeds. pipefail also catches an unreadable directory reported by find. A missing input directory, no matching JPEGs, or a failed conversion returns a nonzero status. The failed image’s output is removed; earlier successful outputs remain. Review a failed batch and choose a new OUTPUT_DIR before retrying. Use this workflow for local batches of ordinary still JPEGs.

Adapt the output for your website

These two optional commands use the same flat input_images folder as the first example. Each requires its own new destination and leaves the originals alone. A failed command can leave a partial batch, just as with resized_png.

Creating thumbnails

For square thumbnails, fill the square with '150x150^', then crop from the center using -gravity center -extent 150x150. This can enlarge small images and crop away subjects near an edge:

mkdir thumbnails &&
magick mogrify -regard-warnings -define registry:filename:literal=true \
  -auto-orient -resize '150x150^' -gravity center -extent 150x150 \
  -format jpg -path thumbnails ./input_images/*.jpg

Optimizing images for the web

To keep photographs as JPEGs, combine the bounding box with a JPEG quality setting:

mkdir optimized &&
magick mogrify -regard-warnings -define registry:filename:literal=true \
  -auto-orient -resize '800x600>' -quality 85 -path optimized ./input_images/*.jpg

-quality 85⁠ is an encoder setting on the JPEG quality scale, not a promise of 85% visual quality or a particular file-size reduction. Re-encoding JPEGs is lossy. Compare the resulting bytes and appearance with your originals before choosing a setting; a source that is already heavily compressed may become larger.

If the output looks wrong, check orientation metadata first, then confirm whether you wanted a bounding box or a square crop. If a command refuses to run again, check its destination: these examples deliberately require a new output directory to protect the previous result.