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File cronic.1 of Package cronic

.TH cronic 1 "April 2013" "habilis.net" "User Commands"

.SH NAME
cronic - a cure for Cron's chronic email problem
.SH SYNOPSIS
.B cronic
[COMMAND]
.P
The Disease:
.RS
0 1 * * * backup >/dev/null 2>&1
.RE
.P
The Cure:
.RS
0 1 * * * cronic backup
.RE
.SH DESCRIPTION
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.
.SS
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.
.P
Unfortunately, cron's idea of "output" is simultaneously too broad and too
narrow to actually be useful. Cron considers
.I 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
.I ignores
command result codes, meaning that errors from quiet programs are ignored.
.P
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:
.P
.RS
0 1 * * * backup >/dev/null 2>&1
.RE
.P
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.
.P
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.
.SS
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.
.P
.RS
0 1 * * * cronic backup
.RE
.P
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:
.P
.RS
.nf
#!/bin/bash

set -o errexit -o nounset -o xtace

cp -rp data1 /backup
cp -rp data2 /backup
cp -rp data3 /backup
.fi
.RE
.P
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:
.P
.RS
.nf
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
.fi
.RE
.SH HISTORY
.TP 8
v2
Corrected command evaluation, so shell meta-chars are preserved correctly (Thanks to Frank Wallingford for the fix).
.TP
v1
Initial release. 
.SH AUTHOR
Chuck Houpt
<http://habilis.net/cronic/>
.SH BUGS
Feedback to: chuck@habilis.net
.SH RELATED 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:
.SS Shush
A C-based cron wrapper, with multiple report formats, syslogging, etc.
.br
<http://web.taranis.org/shush/>
.SS Cronwrap
A C-based cron wrapper, with time-out control, logging, etc. 
.br
<http://www.uow.edu.au/~sah/cronwrap.html>
.SH SEE ALSO
.SS Cron
<http://en.wikipedia.org/wiki/Cron>
.br
<http://www.opengroup.org/onlinepubs/009695399/utilities/crontab.html>
.SS "Unix Haters"
<http://www.mindspring.com/~blackhart/>
.SS "Worse is Better"
<http://www.jwz.org/doc/worse-is-better.html>

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