Introduction to inotify

inotify is a Linux kernel subsystem that acts to extend filesystems to notice changes to the filesystem, and report those changes to applications. It replaces an earlier facility, dnotify, which had similar goals.

The original developers of inotify were John McCutchan, Robert Love and Amy Griffis. It has been included in the mainline Linux kernel from release 2.6.13 (June 18, 2005), and could be compiled into 2.6.12 and possibly earlier releases by use of a patch.

One major use is in desktop search utilities like Beagle, where its functionality permits reindexing of changed files without scanning the filesystem for changes every few minutes, which would be very inefficient. By being told that a file has changed directly by the kernel, rather than actively looking, Beagle and such utilities can achieve change-to-reindexing times of only about a second.

It can also be used to automatically update directory views, reload configuration files, log changes, backup, synchronize, and upload.

Shell examples
To use the power of inotify you must install the package inotify-tools (available for all the main distribution), after that you can use the function “inotifywait “which makes it easy to integrate it with shell scripts.

inotifywait
This command simply blocks for inotify events, making it appropriate for use in shell scripts. It can watch any set of files and directories, and can recursively watch entire directory trees.

Example 1

#!/bin/sh

# get the current path
CURPATH=`pwd`

inotifywait -mr --timefmt '%d/%m/%y %H:%M' --format '%T %w %f'
-e close_write /tmp/test | while read date time dir file; do

FILECHANGE=${dir}${file}
# convert absolute path to relative
FILECHANGEREL=`echo "$FILECHANGE" | sed 's_'$CURPATH'/__'`

rsync --progress --relative -vrae 'ssh -p 22' $FILECHANGEREL [email protected]:/backup/root/dir &&
echo "At ${time} on ${date}, file $FILECHANGE was backed up via rsync"
done

In this example we watch /tmp/test and if an event of “close_write” is caught we rsync that file to a remote location.

Example 2

Consider the simple example of wanting to monitor files created in /tmp and print on standard output a message any time a file is created.


#!/bin/sh

# CONFIGURATION
DIR="/tmp"
EVENTS="create"
FIFO="/tmp/inotify2.fifo"

# FUNCTIONS
on_exit() {
kill $INOTIFY_PID
rm $FIFO
exit
}

on_event() {
local date=$1
local time=$2
local file=$3

sleep 5

echo "$date $time Fichier créé: $file"
}

# MAIN
if [ ! -e "$FIFO" ]
then
mkfifo "$FIFO"
fi

inotifywait -m -e "$EVENTS" --timefmt '%Y-%m-%d %H:%M:%S' --format '%T %f' "$DIR" > "$FIFO" &
INOTIFY_PID=$!

trap "on_exit" 2 3 15

while read date time file
do
on_event $date $time $file &
done < "$FIFO"

on_exit

Detail:

• Function on_event is launched in the background (a sub-shell process is actually created)
• operations are now performed in parallel processing practical for long or multiple processors eligible
• The use of the named pipe $FIFO can run the command inotifywait background and get its PID
• It is  possible to install a “trap”, ie the function on_exit () “, which will kill the command inotifywait (and also delete the named pipe) when the script is killed by a signal 2 (INT ), 3 (QUIT) or 15 (TERM).
• The events are played through the named pipe, without creating a shell, through the end of “while” done <$ FIFO

Stay “inotified”

There are other inotify-based utilities to consider adding to your bag of tricks. The incron utility is a corollary to cron but reacts to inotify events instead of a schedule. The inoticoming utility is specifically designed to monitor dropboxes. And if you’re a Perl, Ruby, or Python developer, you can find modules and libraries to call inotify from the comfort of your favorite scripting language.

For instance, Perl coders can use Linux::Inotify2 (see Resources for details) to embed inotify features in any Perl application.

This code, taken from the Linux::Inotify2 README file, demonstrates a callback interface to monitor events:


use Linux::Inotify2;

my $inotify = new Linux::Inotify2
or die "Unable to create new inotify object: $!";

# for Event:
Event->io (fd =>$inotify->fileno, poll => 'r', cb => sub { $inotify->poll });

# for Glib:
add_watch Glib::IO $inotify->fileno, in => sub { $inotify->poll };

# manually:
1 while $inotify->poll;

# add watchers
$inotify->watch ("/etc/passwd", IN_ACCESS, sub {
my $e = shift;
my $name = $e->fullname;
print "$name was accessedn" if $e->IN_ACCESS;
print "$name is no longer mountedn" if $e->IN_UNMOUNT;
print "$name is gonen" if $e->IN_IGNORED;
print "events for $name have been lostn" if $e->IN_Q_OVERFLOW;

# cancel this watcher: remove no further events
$e->w->cancel;
});


References:

http://kerlinux.org/2010/08/utilisation-de-inotifywait-dans-des-scripts-shell/

http://www.ibm.com/developerworks/linux/library/l-ubuntu-inotify/index.html?ca=drs-

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