Thursday, November 5, 2009

Running cron on Ubuntu

Automating regular admin tasks with cron is a great way to handle all sorts of chores. Every day systems around the globe e-mail me cron reports showing their backups were successful and a report of how many bad guys tried to break in (by the obvious front door of ssh at least). I run an Ubuntu desktop at home, and I'd like to automate tasks on it with cron as well. Here's a quick guide to the Ubuntu-specific bits required to get cron going, presuming you're already familiar with it on other systems.

The Ubuntu wiki has a Cron How-To. When I started my night, the description in the "Enable User Level Cron" section was wrong. It suggested that the default setup didn't allow regular users to add jobs with "crontab -e" and have them execute, and that you first had to setup cron.allow for them. This may be true on some other system, but it's not the Debian or Ubuntu default. If you run into this restriction, you'll know it--running crontab will throw the error right in your face. So if that doesn't happen, but your jobs aren't running, the allow/deny section isn't your issue. I updated that section to match the description of the behavior here you'll see with "man crontab".

I also updated the how-to with the troubleshooting bits I either needed this time or have run into in the past. I'm saving the parts I added here in case somebody decides they're not worthy and deletes them:

Troubleshooting cron problems
  • Edits to a user's crontab and jobs that are run on their behalf are all logged by default to /var/log/syslog and that's the first place to check if things are not running as you expect.
  • When adding a new entry to a blank crontab, forgetting to add a newline at the end is a common source for the job not running. If the last line in the crontab does not end with a newline, no errors will be reported at edit or runtime, but that line will never run. See man crontab for more information. This has already been suggested as a bug.
  • If a user was not allowed to execute jobs when their crontab was last edited, just adding them to the allow list won't do anything. The user needs to re-edit their crontab after being added to cron.allow before their jobs will run.
  • When creating a crontab for the root user, the user name must be specified as a parameter after the date/time parameters. Accidentally including the user name that way in a user-specific crontab will result in trying to run the user's name as a command, rather than what was expected.
  • Entries in cron may not run with the same environment, in particular the PATH, as you expect them to. Try using full paths to files and programs if they're not being located as you expect.
  • The "%" character is used as newline delimiter in cron commands. If you need to pass that character into a script, you need to escape it as "\%".
Mail output

The behavior I expect from a UNIX system is that if I run a job in a crontab, and that job writes something to standard output, that output will be e-mailed to me. You're supposed to get that on Ubuntu, via the Exim4 Mail Transport Agent; see Setting Up Your System To Use E-Mail for details. If that's what you've got, you may get mail that cron sends just fine, to the root-ish user setup by the default installer.

But, if like me you installed something that pulled in the postfix package as a prerequisite (but didn't bother configuring it at the time), you'll discover cron output doesn't get mailed to you. Instead you'll see something like this in /var/log/syslog:
Nov  5 20:33:01 gsmith-desktop /USR/SBIN/CRON[26173]: (root) CMD (/home/gsmith/test)
Nov 5 20:33:01 gsmith-desktop postfix/sendmail[26176]: fatal: open /etc/postfix/ No such file or directory
Nov 5 20:33:01 gsmith-desktop /USR/SBIN/CRON[26166]: (root) MAIL (mailed 5 bytes of output but got status 0x004b )
This tells us that even though postfix, the mail server, is installed, because it hasn't been configured it isn't doing anything. That's kind of poor behavior just because I was too busy to fool with it at the time that prompt flashed by; what I'd expect is that the default would provide local delivery only until I wanted something better. Here's a simple guide to a minimally install and configure postfix:
sudo apt-get install postfix
sudo dpkg-reconfigure postfix
You can answer its questions like this to setup what cron needs to deliver to your system:
  • Choose "local only"
  • Enter your user name for "Root and postmaster mail recipient"
  • Use the defaults for the rest of the questions
Now you should see that mail is being delivered in the log files when jobs execute. In order to read the mail, you might want the standard UNIX 'mail' utility, which also isn't installed by default; fix that like this:
sudo apt-get install mailutils
And that might also help random UNIX programs you install that expect 'mail' is available to send you things. Alternative text-based mail programs include alpine and mutt. GUI mail programs like thunderbird and evolution could also be used, although those are pretty heavyweight just to read the kind of local mail cron generates.

If you want your server to mail to instead deliver mail to the outside world, that's a whole nother level of complexity altogether. I suggest getting local delivery going first, to confirm that cron is working normally, before launching into that adventure. Then you can deal with the fun that is the Postfix setup documentation. Good luck with that.

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