The Disease¶

One of the best features of cron is its automatic email - it is also its worst feature. Cron automatically emails the output of a cron job to the user. On the face of it, this sounds like a great idea. Cron jobs can run automatically in the background for months at a time - so getting an email when a problem occurs sounds useful.

Unfortunately, cron's idea of "output" is simultaneously too broad and too narrow to actually be useful. Cron considers any output to be significant - including standard output. This interacts badly with many unix commands, which often send status info to standard out. Some commands have a quiet options, but that can turn off all error output too. To make matters worse, cron ignores command result codes, meaning that errors from quiet programs are ignored.

It is almost impossible to create a non-trivial cron job that is quiet enough to run without output, but still reports all errors. Following the principle of "Worse is Better", the typical solution is to sweep it all under the carpet by redirecting all output to /dev/null, and hoping for the best:

Now when your cron job fails, you will never know about it. Using cron to backup your files? Sorry, the cron job has been failing due to permission errors for months - all your files are gone.

Could cron be fixed? Although almost all current implementation of cron are open source, cron's pathological behavior has been petrified into the Unix standards. So if it isn't broken, it isn't cron. The only solution left is a work-around.

The Cure: Cronic¶

Cronic is a small shim shell script for wrapping cron jobs so that cron only sends email when an error has occurred. Cronic defines an error as any non-trace error output or a non-zero result code. Cronic filters Bash execution traces (or anything matching PS4) from the error output, so jobs can be run with execution tracing to aid forensic debugging. Cronic has no options, it simply executes its arguments.

With cronic, you can turn on Bash's strict error handling and debug options (exit on error, unset variable detection and execution tracing to make sure problems are caught early. For example:

#!/bin/bash
set -o errexit -o nounset -o xtace
cp -rp data1 /backup
cp -rp data2 /backup
cp -rp data3 /backup

When an error is detected, Cronic outputs a report listing the result code, error output, and combined trace and error output. The combined output can help put error messages in context. An example:

From: user@example.net (Cron Daemon)
To: user@example.net
Subject: Cron <user@server> cronic backup
Cronic detected failure or error output for the command:
backup
RESULT CODE: 1
ERROR OUTPUT:
cp: data2: Permission denied
STANDARD OUTPUT:
TRACE-ERROR OUTPUT:
+ cp -rp data1 /backup
+ cp -rp data2 /backup
cp: data2: Permission denied

Shush¶

A C-based cron wrapper, with multiple report formats, syslogging, etc.
<http://web.taranis.org/shush/>

Cronwrap¶

A C-based cron wrapper, with time-out control, logging, etc.
<http://www.uow.edu.au/~sah/cronwrap.html>

Cron¶

<http://en.wikipedia.org/wiki/Cron>
<http://www.opengroup.org/onlinepubs/009695399/utilities/crontab.html>

Unix Haters¶

<http://www.mindspring.com/~blackhart/>

Worse is Better¶

<http://www.jwz.org/doc/worse-is-better.html>