forked from pool/cronic
168b5a6fa3
Fixed license string, as requested. OBS-URL: https://build.opensuse.org/request/show/247453 OBS-URL: https://build.opensuse.org/package/show/utilities/cronic?expand=0&rev=1
146 lines
4.3 KiB
Groff
146 lines
4.3 KiB
Groff
.TH cronic 1 "April 2013" "habilis.net" "User Commands"
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.SH NAME
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cronic - a cure for Cron's chronic email problem
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.SH SYNOPSIS
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.B cronic
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[COMMAND]
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.P
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The Disease:
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.RS
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0 1 * * * backup >/dev/null 2>&1
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.RE
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.P
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The Cure:
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.RS
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0 1 * * * cronic backup
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.RE
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.SH DESCRIPTION
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Cronic is a shell script to help control the most annoying feature of cron:
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unwanted emailed output, or "cram" (cron spam). If the Unix Haters list was
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still active, I would submit the rant below to gain membership.
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.SS
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The Disease
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One of the best features of cron is its automatic email - it is also its worst
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feature. Cron automatically emails the output of a cron job to the user. On the
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face of it, this sounds like a great idea. Cron jobs can run automatically in
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the background for months at a time - so getting an email when a problem occurs
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sounds useful.
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.P
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Unfortunately, cron's idea of "output" is simultaneously too broad and too
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narrow to actually be useful. Cron considers
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.I any
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output to be significant - including standard output. This interacts badly with
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many unix commands, which often send status info to standard out. Some commands
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have a quiet options, but that can turn off all error output too. To make
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matters worse, cron
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.I ignores
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command result codes, meaning that errors from quiet programs are ignored.
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.P
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It is almost impossible to create a non-trivial cron job that is quiet
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enough to run without output, but still reports all errors. Following
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the principle of "Worse is Better", the typical solution is
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to sweep it all under the carpet by redirecting all output to /dev/null,
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and hoping for the best:
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.P
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.RS
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0 1 * * * backup >/dev/null 2>&1
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.RE
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.P
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Now when your cron job fails, you will never know about it. Using cron
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to backup your files? Sorry, the cron job has been failing due to
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permission errors for months - all your files are gone.
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.P
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Could cron be fixed? Although almost all current implementation of cron are
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open source, cron's pathological behavior has been petrified into the Unix
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standards. So if it isn't broken, it isn't cron. The only solution left is a
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work-around.
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.SS
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The Cure: Cronic
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Cronic is a small shim shell script for wrapping cron jobs so that cron only
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sends email when an error has occurred. Cronic defines an error as any
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non-trace error output or a non-zero result code. Cronic filters Bash execution
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traces (or anything matching PS4) from the error output, so jobs can be run with
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execution tracing to aid forensic debugging. Cronic has no options, it simply
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executes its arguments.
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.P
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.RS
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0 1 * * * cronic backup
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.RE
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.P
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With cronic, you can turn on Bash's strict error handling and debug
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options (exit on error, unset variable detection and execution tracing
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to make sure problems are caught early. For example:
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.P
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.RS
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.nf
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#!/bin/bash
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set -o errexit -o nounset -o xtace
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cp -rp data1 /backup
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cp -rp data2 /backup
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cp -rp data3 /backup
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.fi
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.RE
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.P
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When an error is detected, Cronic outputs a report listing the result
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code, error output, and combined trace and error output. The combined
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output can help put error messages in context. An example:
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.P
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.RS
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.nf
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From: user@example.net (Cron Daemon)
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To: user@example.net
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Subject: Cron <user@server> cronic backup
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Cronic detected failure or error output for the command:
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backup
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RESULT CODE: 1
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ERROR OUTPUT:
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cp: data2: Permission denied
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STANDARD OUTPUT:
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TRACE-ERROR OUTPUT:
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+ cp -rp data1 /backup
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+ cp -rp data2 /backup
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cp: data2: Permission denied
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.fi
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.RE
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.SH HISTORY
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.TP 8
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v2
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Corrected command evaluation, so shell meta-chars are preserved correctly (Thanks to Frank Wallingford for the fix).
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.TP
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v1
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Initial release.
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.SH AUTHOR
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Chuck Houpt
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<http://habilis.net/cronic/>
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.SH BUGS
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Feedback to: chuck@habilis.net
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.SH RELATED PROJECTS
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Cronic isn't the only cure for cron. Cronic's main advantage it is
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small, simple and a shell script. There are several C-based cron wrapper
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programs with many additional capabilities and options:
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.SS Shush
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A C-based cron wrapper, with multiple report formats, syslogging, etc.
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.br
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<http://web.taranis.org/shush/>
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.SS Cronwrap
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A C-based cron wrapper, with time-out control, logging, etc.
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.br
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<http://www.uow.edu.au/~sah/cronwrap.html>
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.SH SEE ALSO
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.SS Cron
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<http://en.wikipedia.org/wiki/Cron>
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.br
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<http://www.opengroup.org/onlinepubs/009695399/utilities/crontab.html>
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.SS "Unix Haters"
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<http://www.mindspring.com/~blackhart/>
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.SS "Worse is Better"
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<http://www.jwz.org/doc/worse-is-better.html>
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