Incrond

incron is an inotify-based cron system.
You can make it do stuff (like trigger a script) when files or
directories change, are opened, closed, etc (i.e., ‚Äúfilesystem events‚ÄĚ).
You need a 2.6.13 kernel or higher to use it.

Installation

# CentOS/RHEL  
sudo yum install incron --yes  

# Ubuntu  
sudo apt-get install incron --yes

The global config file is /etc/incron.conf. Now make sure you start
the service (might be a good idea to chkconfig this too):

# CentOS/RHEL  
service incrond start  

# Ubuntu  
/etc/init.d/incron start

Events

You can get a listing of events by issuing:

[root@example ~]# incrontab -t  
IN_ACCESS,IN_MODIFY,IN_ATTRIB,IN_CLOSE_WRITE,IN_CLOSE_NOWRITE,IN_OPEN,  
IN_MOVED_FROM,IN_MOVED_TO,IN_CREATE,IN_DELETE,IN_DELETE_SELF,IN_CLOSE,  
IN_MOVE,IN_ONESHOT,IN_ALL_EVENTS,IN_DONT_FOLLOW,IN_ONLYDIR,IN_MOVE_SELF

You can find an explanation of each in /usr/include/linux/inotify.h.
For example:

26 /* the following are legal, implemented events that user-space can watch for */  
27 #define IN_ACCESS               0x00000001      /* File was accessed */  
28 #define IN_MODIFY               0x00000002      /* File was modified */  
29 #define IN_ATTRIB               0x00000004      /* Metadata changed */  
30 #define IN_CLOSE_WRITE          0x00000008      /* Writtable file was closed */  
31 #define IN_CLOSE_NOWRITE        0x00000010      /* Unwrittable file closed */  
32 #define IN_OPEN                 0x00000020      /* File was opened */  
33 #define IN_MOVED_FROM           0x00000040      /* File was moved from X */  
34 #define IN_MOVED_TO             0x00000080      /* File was moved to Y */  
35 #define IN_CREATE               0x00000100      /* Subfile was created */  
36 #define IN_DELETE               0x00000200      /* Subfile was deleted */  
37 #define IN_DELETE_SELF          0x00000400      /* Self was deleted */  
38 #define IN_MOVE_SELF            0x00000800      /* Self was moved */

Creating traps

Start editing your incrontab with incrontab *-e*. Here are some
examples of traps

# Restart the NTP daemon when its config file changes  
/etc/ntp/ntp.conf IN_MODIFY /sbin/service ntpd restart  

# Run a script with the absolute path to filename as parameter when it changes (and is closed)  
/home/nanand/thesis.txt IN_WRITE_CLOSE /home/nanand/bin/log_changes.sh $@/$#

Here is a full list of wildcards:

$$ - a dollar sign  
$@ - the watched filesystem path   
$# - the event-related file name  
$% - the event flags (textually)  
$& - the event flags (numerically)

Sources

  • /usr/share/doc/incron/README (Ubuntu)
  • /usr/share/doc/incron-0.5.5/README (CentOS/RHEL)