incron FAQ (Frequently Asked Questions)

Q: What is incron?
incron is an inotify cron system. See About for more details.
Q: What can I use incron for?
incron can be used for many situations where something depends on file system events. See Why to use for more details.
Q: Are there some distribution (binary) packages?
There are some packages provided by third parties. Please see Links for such packages. You can also check your distribution whether there is a package named 'incron'.
Q: How does it work?
incron has two parts: a daemon and a table manipulator. The daemon reads rules from system and user tables, creates appropriate watches on files and waits for events. If an event occurs the daemon run a command (an executable file with parameters). Any changes in the tables are reflected immediately.
Q: How to add/modify/remove a system table?
System tables are located in /etc/incron.d. These files are intended to be modified directly. Simply add, edit or remove such file.
Q: How to add/modify/remove an user table?
User tables are located in /var/spool/incron but they are not intended to be modified directly (although root can do so). Please use incrontab for any manipulation.
Q: Why such complex solution? I want to have my table in my home directory!
incron use the same model as cron. There are more reasons why its better to have user tables in one special directory. Some users may share one home directory and some of them have / as their home (especially some system accounts). If an user want to maintain his table in his home directory, he can. But it will always be only a "work copy" with must be (using incrontab) copied into the regular location.
Q: How to write incron rules?
incron tables have one rule per row. Each rule has three parts. The first one is an absolute path to the file which have to be watched. The second one is an event mask containing all events which will be passed in (one or more comma separated symbols or eventually a numeric value). And the last one is a command to run. The parts are separated by one or more spaces. If you want to use spaces inside the file path or the command you must prepend each such characted by a backslash. Each real backslash must be doubled of course.
Q: What is IN_NO_LOOP?
In some cases, looping may occur (e.g. if you run 'chmod' as a reaction on IN_ATTRIB). To avoid this behavior you can use a special symbol, IN_NO_LOOP. It temporarily disables watching (all events for the given rule) and after processing the event it is enabled again. This disabling means that such events are not reported by the kernel to incron (ie. they are lost).
Q: How does incron handles IN_ONESHOT?
Immediately after the first event is occurred the watch is disabled. It means this event can occur only once until incrond is restarted or the table is reloaded.
Q: Is it better to use IN_MODIFY or IN_CLOSE_WRITE?
Solution of this dilemma (and some related things) is described in the inotify FAQ
Q: How to use event properties in commands?
There are a few special symbols starting with a dollar sign ($). These symbols may be used everywhere in command definitions. See Documentation for more details.
Q: Is it possible to specify an user to run a command under?
Not yet. It will be implemented in future versions. But regular users will never have this right of course.
Q: Can I catch outputs of executed commands?
It can be currently done only indirectly. You create a script which runs the appropriate program and redirects the outputs (stdout and stderr). Direct implementation is planned for future versions, probably including the possibility to send this data via e-mail.

Q: What privileges does incron use for commands?
Commands generated by system tables have root privileges. Commands from user tables have privileges of the appropriate user.
Q: Can I run incrond on foreground?
Yes (since 0.4.0). Specify -n or --foreground in the command line.
Q: Can I simply terminate a running instance of incrond?
Yes (since 0.4.0). Specify -k or --kill in the command line.
Q: Can I use my own editor?
Yes. Export the EDITOR variable (e.g. export EDITOR=nano). Since 0.4.0 also VISUAL is accepted (with lower priority than EDITOR) and if an "alternative" editor is defined (/etc/alternatives/editor) it is used in preference to the hard-wired one.
Q: How does incrond log its actions?
incrond actions are logged through syslog as the cron facility. The typical log file is /var/log/cron but it may differ across distributions.
Q: How the environment variables are treated?
There is currently no feature how to specify enviroment variables for executed commands directly (if you want to do this, please write a script and set those variables here) although it's planned for the future. For system tables, incrond's enviroment variables are used without any changes. The same applies for root's user table. For non-root user, the whole environment is cleared and then some of the variables are set (LOGNAME, USER, USERNAME, HOME, SHELL and PATH) using the values from the user database. PATH is set to /usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin.
Q: What about licensing?
incron is licensed under the GNU General Public License, version 2. Some parts (e.g. inotify-cxx) may be licensed under additional licenses too.
Q: How to report bugs?
If you find a bug or have a suggestion, you can report it through the Bug Tracking System.
Q: incron is nice! I want to support its development.
You can make a donation through PayPal. If you do this then 5 % of the total amount will be tranferred to Electronic Frontier Foundation (a non-profit group established to protect freedom in the digital world). Thanks for any help!