Apply scanner effect to a PDF with imagemagick (using nix-shell)

dirtypdf.zsh

#!/usr/bin/env zsh

# Apply scanner effect to a PDF
# Credits:
# https://gist.github.com/andyrbell/25c8632e15d17c83a54602f6acde2724
# https://github.com/NixOS/nixpkgs/issues/138638#issuecomment-1068569761

dirtypdf () {
  if [ $# -ne 2 ]; then
    echo "Usage: dirtypdf <input_pdf> <output_pdf>"
    return 1
  fi

  input_file="$1"
  output_file="$2"

  if [ ! -f "$input_file" ]; then
    echo "Error: Input file '$input_file' does not exist"
    return 1
  fi

  nix-shell --packages 'imagemagickBig' --run "magick -density 90 \"$input_file\" -rotate 0.5 -attenuate 0.2 +noise Multiplicative -colorspace Gray \"$output_file\""
  
  # open the output file in macOS Finder
  open -R "$output_file"
}