Yesterday in my job I faced a standard assignment in system administration which essentially boiled down to the following:

Create two directories, say, /var/www/dirA and var/www/dirB with the following properties:

1) whenever anything is written to dirA, it has to immediately appear in dirB
2) whenever something is deleted from dirA, it has to stay in dirB.

I came up with this, I wonder if anyone has a better solution:

1) install incron:

aptitude install incron

It is an "inotify cron" system. It works like the regular cron but is driven by filesystem events instead of time events.

2) configure it to watch /var/www/dirA and whenever anything changes, call script /usr/local/bin/incron_script with the first argument being the watched directory and the second - file or directory created there:

incrontab -e
/var/www/dirA IN_CLOSE_WRITE /usr/local/bin/incron_script $@ $#

3) write a simple /usr/local/bin/incron_script:

#!/bin/sh

cp -a $@/$# /var/www/dirB/

Done!
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Incron suffers from one trap, though: the 'mask' ( IN_CLOSE_WRITE in the above example ) and 'command' ( /usr/local/bin/incron_script $@ $#) stanzas in its configuration file have to be separated with exactly ONE SPACE. Two spaces or one tab make it fail silently (the second space becomes the first character of the command to be executed and the command fails). This is documented in bug 456821