Cronic
A cure for Cron's chronic email problem
By Chuck Houpt
- The Disease:
0 1 * * * backup >/dev/null 2>&1- The Cure:
0 1 * * * cronic backup
Cronic is a shell script to help control the most annoying feature of cron: unwanted emailed output, or "cram" (cron spam). If the Unix Haters list was still active, I would submit the rant below to gain membership. (feedback to: chuck@habilis.net)
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:
0 1 * * * backup >/dev/null 2>&1
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
Download: cronic v3
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.
0 1 * * * cronic backup
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 xtrace 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
Version History
- v3 - Use mktemp-d to avoid race-conditions and security problems.
- v2 - Corrected command evaluation, so shell meta-chars are preserved correctly (Thanks to Frank Wallingford for the fix).
- v1 - Initial release.
Releated Projects
Cronic isn't the only cure for cron. Cronic's main advantage it is small, simple and a shell script. There are several C-based cron wrapper programs with many additional capabilities and options: