How to run inotifywait continuously and run it as a cron or deamon?

Question

I've built a shell script that uses inotifywait to automatically detect file changes on a specific directory. When a new PDF file is dropped in the directory this script should go off and it should then trigger ocropus-parser to execute some commands on it. The code:

#!/bin/sh
inotifywait -m ~/Desktop/PdfFolder -e create -e moved_to |
while read path action file; do
    #echo "The file '$file' appeared in directory '$path' via '$action'"
    # Check if the file is a PDF or another file type.
    if [ $(head -c 4 "$file") = "%PDF" ]; then
        echo "PDF found - filename: " + $file
        python ocropus-parser.py $file
    else
        echo "NOT A PDF!"
fi
done

This works pretty well when I run this script through the terminal with ./filenotifier.sh but when I reboot my Linux (Ubuntu 14.04) my shell will no longer run and it will not restart after a reboot. I've decided to create an init script that starts at boot time (I think). I did this by copying the file filenotifier.sh to init.d:

sudo cp ~/Desktop/PdfFolder/filenotifier.sh /etc/init.d/

I've then gave the file the correct rights:

sudo chmod 775 /etc/init.d/filenotifier.sh

and finally I've added the file to update-rc.d:

sudo update-rc.d filenotifier.sh defaults

However when I reboot and drop a PDF in the folder ~/Desktop/PdfFolder nothing will happen and it seems that the script does not go off. I'm really not experienced with init.d, update-rc.d and deamon so I'm not sure what is wrong and if this is even a good approach or not.

Thanks, Yenthe

Answer 1

• Being an init-script, you should add the LSB header to your script, like this:

  #!/bin/sh
  ### BEGIN INIT INFO
  # Provides:          filenotifier
  # Required-Start:    $remote_fs $syslog
  # Required-Stop:     $remote_fs $syslog
  # Default-Start:     2 3 4 5
  # Default-Stop:      0 1 6
  # Short-Description: Something
  # Description:       Something else
  ### END INIT INFO
  
  inotifywait -m ...
  

  This way, you can ensure that your script runs when all mount points are available (thanks to Required-Start: $remote_fs). This is essential if your home directory is not on the root partition.

• Another problem is that in your init-script you're using ~:

  inotifywait -m ~/Desktop/PdfFolder ...
  

  The ~ expands to the current user home directory. Init-scripts are run as root, so it'll expand to /root/Desktop/PdfFolder. Use ~<username> instead:

  inotifywait -m ~yenthe/Desktop/PdfFolder ...
  

  (Assuming that your username is yenthe.)

  Or perhaps switch user before starting (using sudo).

• $file is the basename without the path to the directory. Use "$path/$file" in your commands:

  "$(head -c 4 "$path/$file")"
  python ocropus-parser.py "$path/$file"
  

  Maybe consider using name instead of file, to avoid confusion.

• If things are not working, or if in general you want to investigate something, remember to use ps, like this:

  ps -ef | grep inotifywait
  

  ps will tell you, for example, whether your script is running and if inotifywait was launched with the correct arguments.

• Last but not least: use "$file", not $file; use "$(head -c 4 "$file")", not $(head -c 4 "$file"); use read -r, not read. These tips can save you a lot of headaches in the future!

Answer 2

For that purpose the developers of inotify created incron. It is a cron like daemon which executes scripts based on changes in a watched file/directory rather than on time events.