Setting up and using Cron

Cron jobs are a mainstay of Unix and Linux. Cron basically lets you run a shell script or other program according to some schedule. The system has been around forever and it just plain works.. even if it is just a little on the cryptic side. The system is ubiquitous and so widely used. A common use of this is to automate backups, for instance. When administering a server I would say it is of the things to think about when setting up a system — what kinds of tasks will you need to run when? (This will, of course, evolve over time. But still….)

Pretty much every implementation of Linux/Unix does the same thing. There is a cron daemon that will go through all the cron files on the system and check in every minute to see what it needs to run. You can’t really do cron jobs any more precisely than at the minute level — there is way to say ‘run a script at 5:33:45 on Monday’ for instance (and why would you ever want to do that anyway?)

Basic Usage

Cron operates on crontab files. You don’t access these files directly to view or edit them. The system maintains these in some location. Whenever you want to do anything with a crontab file, you use the crontab command.

At the command line you can view your crontab with crontab -l. This simply prints out your crontab to stdout. If you haven’t set one up yet then you’ll most likely see a message saying something like ‘crontab: no crontab for you’ (where ‘you’ is your username on the system, of course.)

In general every user with an account has access to setting up cron jobs. To do this, run ‘crontab’ with the -e flag to edit your crontab file. That will kick you into your editor (whichever it is set to, or vi by default.) Initially this will be empty but here we’ll show you how to set that up.

[me@myserver ~]$ crontab -e

However, if you don’t want to edit the crontab file interactively, you can also edit the file in whatever editor you choose and set it up with the crontab command by giving the filename. Usually when doing that I check that I loaded in what I thought by running crontab -l right after.

[me@myserver ~]$ crontab my_crontab.txt

In a crontab file lines that start with a hash mark (‘#’) are comments. The cron program will ignore those lines.

One entry in a crontab file is set up per line. This is a space-delimited list that has the information about what program or script to run and how often. The format is:

mm hh dd tt ww command

Where each field is defined as such:

mm Number of minutes past the hour to run the cron job.

(having the minutes field first is usually the first thing that throws you off. We’re so used to using the hour figure first whenever we talk about time.)
hh Hour of day to run the cron job.
dd day of month (00 to 31)
tt month (1 to 12)
ww day of week (0 to 6, with 0=Sunday, 1=Monday, etc.)
command the absolute path to the script/program to run, along with any redirection to send output wherever.

The basic usage is pretty straight-forward. For example, we might want to run a script at 10pm every night…

0 22 * * * /backup/ > /dev/null 2>&1

Where cron can get a bit cryptic is when you want to run things multiple times a day or every few days. To run a command at two different times, say 10am and 10pm, you could put each hour value separated by a comma:

# Run at 10:15am and 10:15pm every night and send the output to
* /dev/null (ie. throw it away)
15 10,22 * * * /home/me/ > /dev/null 2>&1

Or to run a command every four hours you would use an asterisk, but then specify the multiple of ‘every four hours’ with ‘/4’.

# Run every 4 hours
0 */4 * * * /home/me/ > /dev/null 2>&1

You can also specify a range of values with a hyphen. In practice this probably only every makes sense in the column indicating which days of the week to run a script.

# Run at 5:30pm every workday (mon-fri), and save the output to a logs directory…
30 17 * * 1-5 /home/me/ >> /home/me/logs/bar.log 2>&1

Version control of your crontab files

Since you can both load a crontab from a text file as well as list it out easily, it is not too much work to set up crontab files under a version control system (ie. git or subversion). Then as you add more things to your crontab you’ll have a log of these changes over time.

Considerations for scripts run via cron

There’s nothing to say you can’t run a compiled program with cron — we do this often enough. But probably the main use of cron is to run a script of some kind — BASH, Python, Perl, PHP scripts can all be run via cron.

1. At the risk of stating something kinda obvious, your script/program should start up, do it’s thing, and exit. It shouldn’t hang around running forever. If you want a daemon, set up a daemon (that’s a future blog post.)

2. Your script should use absolute paths — you can’t assume the script is run from your home directory, for instance.

3. Some jobs might take a long time to run. You should take this into account so that one invocation of your script does not overlap the next invocation by cron. Putting in a check of some kind — a sentinel or lock file — to see if your script is still running can be effective. Or, if they do overlap, that each instantiation of your script can coexist happily with the others.

*** Update: (2013-03-15) Just came across a link to Chronos, which is a replacement for Cron developed by the folks at AirBnB. I haven’t looked at it much yet but wanted to throw in a link to that here.