Run a shell command when a file is added - linux

Question

I have a folder named images on my linux box.
This folder is connected to a website and the admin of the site has the ability to add pictures to this site. However, when a picture is added, I want a command to run resizing all of the pictures a directory.
In short, I want to know how I can make the server run a specific command when a new file is added to a specific location.

Answer 1

I don't know how people are uploading content to this folder, but you might want to use something lower-tech than monitoring the directory with inotify.
If the protocol is FTP and you have access to your FTP server's log, I suggest tailing that log to watch for successful uploads. This sort of event-triggered approach will be faster, more reliable, and less load than a polling approach with traditional cron, and more portable and easier to debug than something using inotify.
The way you handle this will of course depend on your FTP server. I have one running vsftpd whose logs include lines like this:
Fri May 25 07:36:02 2012 [pid 94378] [joe] OK LOGIN: Client "10.8.7.16"
Fri May 25 07:36:12 2012 [pid 94380] [joe] OK UPLOAD: Client "10.8.7.16", "/path/to/file.zip", 8395136 bytes, 845.75Kbyte/sec
Fri May 25 07:36:12 2012 [pid 94380] [joe] OK CHMOD: Client "10.8.7.16", "/path/to/file.zip 644"
The UPLOAD line only gets added when vsftpd has successfully saved the file. You could parse this in a shell script like this:
#!/bin/sh
tail -F /var/log/vsftpd.log | while read line; do
if echo "$line" | grep -q 'OK UPLOAD:'; then
filename=$(echo "$line" | cut -d, -f2)
if [ -s "$filename" ]; then
# do something with $filename
fi
fi
done
If you're using an HTTP upload tool, see if that tool has a text log file it uses to record incoming files. If it doesn't consider adding some sort of logger function to it, so it'll produce logs that you can tail.

Answer 2

Like John commented the inotify API is a starting point, however you might be interested in some tools that use this API to perform file notification tasks:
For example incron which can be used to run cron-like tasks when file or directory changes are detected.
Or inotify-tools which is a set of command-line tools that you could use to build your own file notification shell script.
If you look at the bottom of the Wiki pake for inotify-tools you will see references to even more tools and support for higher-level languages like Python, Perl or Ruby (example code).

Answer 3

You might want to look at inotify
The inotify API provides a mechanism for monitoring file system events. Inotify can be used to monitor individual files, or to monitor directories. When a directory is monitored, inotify will return events for the directory itself, and for files inside the directory.

Answer 4

#!/bin/bash
tail -F -n0 /var/log/vsftpd.log | while read line; do
if echo "$line" | grep -q 'OK UPLOAD:'; then
filename=$(echo $line | cut -d, -f2 |awk '{print $1}')
filename="${filename%\"}"
filename="${filename#\"}"
#sleep 1s
if [ -s $filename ]; then
# do something with $filename
echo $filename
fi
fi
done

Answer 5

Using ghotis work
Here is what I did to get the users free space:
#!/bin/bash
tail -F -n 1 /var/log/vsftpd.log | while read line; do
if echo "$line" | grep -q 'OK LOGIN:'; then
pid=$(sed 's/.*\[\([^]]*\)\].*/\1/g' <<< "$line")
#the operator '<<<' doesnt exist in dash so use bash
if [[ $pid != *"pid"* ]]; then
echo -e "Disk 1: Contains Games:\n" > /home/vftp/"$pid"/FreeSpace.txt; df -h /media/Disk1/ >> /home/vftp/"$pid"/FreeSpace.txt
echo -e "\r\n\r\nIn order to read this properly you need to use a text editor that can read *nix format files" >> /home/vftp/"$pid"/FreeSpace.txt
fi
echo "checked"
# awk '{ sub("\r$", ""); print }' /home/vftp/"$pid"/FreeSpace.txt > /home/vftp/"$pid"/FreeSpace.txt
fi
done

Answer 6

If the file is added through an HTTP upload, and if your server is apache, you might want to check mod_security.
It enables you to run a script for every upload made through HTTP POST.