Just a note to self for future reference: One of our source documents came in the form of full-page-spread images, and rather than manually crop them (there are 370), I wrote a bit of JS to generate a bash script which uses ImageMagick to do it. First I renamed all the input files so that they were -0001, -0003, -0005 etc. to allow room for the new ones, then used this:

function writeCropLines(num){
  let padNum = num.toString().padStart(4, '0');
  let output = 'convert mgchau-' + padNum + '.jpg -crop ';
  output += '1377x1936+0+0 out/mgchau-' + padNum + '.jpg ';
  output += '\u000a';
  padNum2 = (num + 1).toString().padStart(4, '0');
  output += 'convert mgchau-' + padNum + '.jpg -crop ';
  output += '1497x1936+1096+0 out/mgchau-' + padNum2 + '.jpg ';
  output += '\u000a';
  return output;
}

function doAll(){
  let output = '';
  for (var i=1; i<740; i+=2){
    output += writeCropLines(i);
  }
  return output;
}

console.log(doAll());

in the browser dev tools to generate the script.