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I would like to set up a cron job that recusively does a chmod/chown on a directory (the dropbox directory).

This works when I execute the commands manually like:

sudo chown -R $USER ~/Dropbox
sudo chmod -R u+rw ~/Dropbox

I tried to convert it into a cron job like this:

10      *       *       *       *       sudo chown -R $USER ~/Dropbox

But it doesn't seem to do the job.

How to do this?

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incron + inotify would be cleaner solutions :) –  franzlorenzon Jan 23 at 11:50

5 Answers 5

up vote 4 down vote accepted

You want your root cron script (edit by running sudo crontab -e) to be:

 55 * * * * /bin/chown -R somename /home/somename/Dropbox && /bin/chmod u+rw /home/somename/Dropbox

Assuming the user is named somename and that /home/somename/Dropbox is the full path of your Dropbox directory. As root user, ~ goes to /root.

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Thanks for the clear answer! I think this shows me some of the missing links I was looking for. I'll test it first thing tomorrow :) –  tvgemert Feb 16 '12 at 19:55
Tested it, and it's working! –  tvgemert Feb 17 '12 at 11:14
Great; glad its working. The lessons are (a) don't expect environmental variables like $PATH to be defined equivalent to those of a signed in user in a cronjob (e.g., on my linuxbox $HOME is defined, but $PATH is set to /usr/bin and /bin from crontab and $USER is not defined; so in this case you didn't need to specify /bin before chmod/chown), (b) avoid sudo in scripts (run cron task as root user), (c) as root user $HOME will refer to /root not your normal user, (d) ~ should be avoided in cron scripts (by default run as #!/bin/sh. –  dr jimbob Feb 17 '12 at 12:57
I'd also recommend putting in a script per David's advice if you ever do anything more complicated than this; a script seems overkill for a combination of a chmod / chown. However, if you begin applying to other users's Dropboxes or other directories or decide to only run on recently changed files (e.g., result of find), etc than it makes sense to put in a script. –  dr jimbob Feb 17 '12 at 13:02

Two issues:

1) Paths aren't normally set up in cron the same way they are when you log in. Try /usr/bin/sudo /bin/chown ... (or whatever the right paths are to those programs on your system).

2) sudo normally asks for your password, or may otherwise not be happy running noninteractively. I suggest you put the commands in root's crontab without the sudo instead, then the commands run as root.

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sudo should almost never be used in scheduled tasks. It expects to be able to talk to a terminal, and requires specific flags to avoid trying to do so.

Create your cron job as root (in /etc/crontab - Note that the format of this file is slightly different: minute hour mday month wdayusercommand) instead.
This also has the benefit of working on systems where sudo isn't installed.

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Thanks! I created this, but I'm still unsure about it. I didn't get the expected result: 55 * * * * root chown -R somename ~/Dropbox –  tvgemert Feb 16 '12 at 16:57
somename could be anywhere; it is a relative path. Secondly, ~ is only understood by the shell; use $HOME. Use a command like this: chown -R /some/path/somename $HOME/Dropbox (this will use root's home directory...) –  Mei Feb 16 '12 at 18:18
@David: somename is not a path, but the user name (and should also contain the group name: somename:somegroup) ;) –  knittl Feb 16 '12 at 19:10
@tvgemert What David said above (and in his answer) regarding paths/relative paths/symbolic paths (~) is CRITICALLY IMPORTANT -- Please spend some time with the cron and crontab man pages and be sure you understand what's going on (if in doubt, create a cron job that does echo "what you want to do" > /tmp/what-i-would-run and look at the results to be sure they're sane -- MUCH less chance of destroying the universe.) –  voretaq7 Feb 16 '12 at 19:11
Thank for the useful tips, gave me more insight already! –  tvgemert Feb 16 '12 at 19:57

There are multiple issues with your crontab:

10 * * * sudo chown -R $USER ~/Dropbox

The issues:

  • sudo should not be used here; it requires terminal input
  • chown should be fully specified (i.e., /bin/chown)
  • USER as an actual variable may not exist; some systems use LOGNAME
  • ~ (tilde) will only be recognized by a shell - a bare chown will not understand it
  • specifying HOME in root's crontab goes to root's home

I think I'd actually script it:


# FILE: /usr/local/bin/myscript

eval chown -R $1 ~$1/Dropbox
eval chmod -R u+rw ~$1/Dropbox

(The eval is needed to convert ~$1 to ~user then to /home/user.)

Run this script from root's crontab:

# root's crontab
10 * * * /usr/local/bin/myscript someuser
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As an alternative, you might be able to enable ACL support and use setfacl to achieve the same/similar result.

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