Process uploaded images with cron

Let's say that you're uploading images to a server and for whatever crazy reason you're not immediately processing them with ruby, or io.js/node.js or whatever you're using to upload them.

Here's how you might routinetly process the images (or whatever type of file) in as a batch job with cron:

Create the Processing Script

Let's say that you just want to resize every file to 300px, you're script might look like this:

#!/bin/bash
set -e
set -u
IN_FILE=$1
OUT_FILE=/srv/processed_images/$(basename "${IN_FILE}")
convert ${IN_FILE} -resize 300 -quality 85 ${OUT_FILE}

Then maybe through that script into /usr/local/bin/process-one-upload.sh

Find only the unprocessed files

Keep a timestamp file and only process files that are newer than the timestamp using find. Use find's -exec option to run the proccessing script on each file.

#!/bin/bash
TS_FILE="./LAST_UPDATE"
PIC_DIR="./Pictures/"
if ! [ -e "${TS_FILE}" ]
then
  touch -t 197001010000.00 "${TS_FILE}"
fi
find "${PIC_DIR}" \
  -newer "${TS_FILE}" \
  -iname '*.jpg' \
  -exec /usr/local/bin/process-one-upload.sh {} \;
touch "${TS_FILE}"

Then maybe through that script into /usr/local/bin/process-new-uploads.sh

NOTE: {} is a kinda like a macro which will be replaced with the current file and the \; is not a typo, it's still part of -exec.

Create the cron job

If you wanted this processor to run every 5 minutes you could use a line like this:

# MIN HOUR DOM MON DOW CMD
*/5  *  *  *  *  /usr/local/bin/process-new-uploads.sh /srv/www/example.com/uploads/

And put it into /etc/crontab

FIN

There you have it: A cron job to iterate over a directory of uploads and run some operation on the ones that have been uploaded since the last time the script ran.

By AJ ONeal


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