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Inotify, FAM, Gamin

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Inotify, FAM, Gamin -- comparing filesystem event monitors

This briefly comapres the pros and cons of various filesystem event monitoring tools. These tools monitor the filesystem for changes and alert an application when specified events have occurred. For example, this allows a GUI to automatically update a directory listing if files are added by a background process. Without event based systems like this an application would have to poll the filesystem for changes every few seconds which wastes CPU and is not very responsive -- there can be significant delay between the time a file is delete and the time it disappears from a directory browser window.

  • FAM stands for File Alteration Monitor. It is one of the oldest portable event monitors. It sends events to an application when changes are made to files or directories that an application has registered to me monitored. FAM is complicated and old -- it was last updated 5 years ago. Bad choice.
  • Gamin is newer and simpler than FAM. It tries to be compatible with FAM while not implementing many of the obscure feature. It is moderately maintained and widely available on many distros. It is portable, but the focus of development and testing is on Linux. It has BSD support and can be found in FreeBSD Ports. Decent choice
  • dnotify is ancient and crusty and should be ignored. Bad choice.
  • inotify is a replacement for `dnotify`. It's a Linux kernel module. It's fast and lightweight and should be available for every Linux distro flavor. The downside is that it is Linux-only, so no BSD or OS X portability. If portability is not your top priority then it's the best choice for Linux.

I wish Gamin were a little more polished and up to date. It would be my first choice since it is portable.

Winner: inotify

I do mostly Linux development, so portability is not as much of an issue for me.

There is a handy collection of inotify-tools available that let you easily use inotify from a the shell and in shell scripts. On Ubuntu install the inotify-tools package. I have examples in the next section.

Python developers will want to look at pyinotify. The Ubuntu package that provides this is called python-pyinotify (universe).

The fsniper tool is an utility for watching a directory and processing event scripts based on the filetype of new files that appear in the directory.

The incron tool is a utility for triggering scripts based on filesystem events. It is similar in setup to cron.

Examples with inotify-tools

monitor a directory for changes with inotify

The following example will display filenames created or destroyed in /dev. This examples shows what happens when I inserted a USB flash drive into a USB bus. Note that this only watches the top level of /dev. This example is not recursive, so it does not watch any subdirectories under /dev/. You may add the '--recursive' option to this example. When I tested with this option I was notified of 136 events just from inserting a USB flash drive.

inotifywait --monitor --event CREATE --event DELETE /dev/
Setting up watches.  
Watches established.
/dev/ CREATE usbdev1.12_ep81
/dev/ CREATE usbdev1.12_ep01
/dev/ CREATE usbdev1.12_ep00
/dev/ CREATE sg6
/dev/ CREATE .tmp-block-8:80
/dev/ DELETE .tmp-block-8:80
/dev/ CREATE sdf
/dev/ CREATE .tmp-block-8:81
/dev/ DELETE .tmp-block-8:81
/dev/ CREATE sdf1

Similar with recursive option and formatting.

inotifywait --monitor --recursive --event CREATE --event DELETE --format "%w%f %e" /dev/

Display a window manager notification when a mailbox changes

This example will display a notify popup when a file is updated in ~/Maildir. You must have the `notify-send` command installed (on Ubuntu use `aptitude install libnotify-bin`).

inotifywait -mrq -e create -e modify -e delete -e attrib -e move ~/Maildir | while read file
do
  (notify-send "File updated in Maildir:" "$file"&)
done