`incron` is an [inotify](http://inotify.aiken.cz/)-based cron system. You can make it do stuff (like trigger a script) when files or directories change, are opened, closed, etc (i.e., "filesystem events").
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

```bash
# 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)