143 lines
4.9 KiB
Plaintext
143 lines
4.9 KiB
Plaintext
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What is Anacron ?
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-----------------
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Anacron is a periodic command scheduler. It executes commands at
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intervals specified in days. Unlike cron, it does not assume that the
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system is running continuously. It can therefore be used to control
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the execution of daily, weekly and monthly jobs (or anything with a
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period of n days), on systems that don't run 24 hours a day. When
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installed and configured properly, Anacron will make sure that the
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commands are run at the specified intervals as closely as
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machine-uptime permits.
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Every time Anacron is run, it reads a configuration file that
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specifies the jobs Anacron controls, and their periods in days. If a
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job wasn't executed in the last n days, where n is the period of that
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job, Anacron executes it. Anacron then records the date in a special
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timestamp file that it keeps for each job, so it can know when to run
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it again. When all the executed commands terminate, Anacron exits.
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It is recommended to run Anacron from the system boot-scripts.
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This way the jobs "whose time has come" will be run shortly after the
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machine boots. A delay can be specified for each job so that the
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machine isn't overloaded at boot time.
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In addition to running Anacron from the boot-scripts, it is also
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recommended to schedule it as a daily cron-job (usually at an early
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morning hour), so that if the machine is kept running for a night,
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jobs for the next day will still be executed.
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Why this may be useful ?
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------------------------
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Most Unix-like systems have daily, weekly and monthly scripts that
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take care of various "housekeeping chores" such as log-rotation,
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updating the "locate" and "man" databases, etc. Daily scripts are
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usually scheduled as cron-jobs to execute around 1-7 AM. Weekly
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scripts are scheduled to run on Sundays. On machines that are turned
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off for the night or for the weekend, these scripts rarely get run.
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Anacron solves this problem. These jobs can simply be scheduled as
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Anacron-jobs with periods of 1, 7 and a special target called @monthly.
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What Anacron is not ?
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---------------------
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Anacron is not an attempt to make cron redundant. It cannot
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currently be used to schedule commands at intervals smaller than days.
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It also does not guarantee that the commands will be executed at any
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specific day or hour.
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It isn't a full-time daemon. It has to be executed from boot
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scripts, from cron-jobs, or explicitly.
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For more details, see the anacron(8) manpage.
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Requirements
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------------
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- A Linux system. (maybe other *NIX systems)
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- A functioning syslog daemon.
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- A functioning /usr/lib/sendmail command. (all MTAs should have
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that).
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Compilation and Installation
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----------------------------
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- Untar the source package.
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- Check the Makefile. Edit as required.
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- Check the top of "global.h". You may want to change the syslog
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facility and priorities, and the path to your MTA's sendmail
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compatible command (/usr/lib/sendmail).
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- cd to the directory.
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- Type "make".
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You can safely ignore warnings of the form: "*.d: No such file or
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directory"
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- Become root. Type "make install".
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Setup
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-----
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1. Locate your system's daily, weekly and monthly cron-jobs.
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See your cron documentation for more details.
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2. Decide which of these jobs should be controlled by Anacron.
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Remember that Anacron does not guarantee execution at any specific
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day of the month, day of the week, or time of day. Jobs for which
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the timing is critical should probably not be controlled by
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Anacron.
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3. Comment these jobs out of their crontab files. (You may have to
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use the "crontab" command for this. See the cron documentation.)
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4. Put them in /etc/anacrontab. Note that the format is not the same
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as the crontab entries. See the anacrontab(5) manpage. Here's an
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example from a typical Debian system:
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-----Cut
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# /etc/anacrontab example
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SHELL=/bin/sh
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PATH=/sbin:/bin:/usr/sbin:/usr/bin
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# format: period delay job-identifier command
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1 5 cron.daily run-parts /etc/cron.daily
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7 10 cron.weekly run-parts /etc/cron.weekly
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@monthly 15 cron.monthly run-parts /etc/cron.monthly
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-----Cut
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5. Put the command "anacron -s" somewhere in your boot-scripts.
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Make sure that syslogd is started before this command.
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6. Schedule the command "anacron -s" as a daily cron-job (preferably
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at some early morning hour). This will make sure that jobs are run
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when the systems is left running for a night.
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That's it.
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It is a good idea to check what your daily, weekly and monthly scripts
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actually do, and disable any parts that may be irrelevant for your
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system.
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Credits
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-------
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Anacron was originally conceived and implemented by Christian Schwarz
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<schwarz@monet.m.isar.de>.
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The current implementation is a complete rewrite by Itai Tzur
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<itzur@actcom.co.il>.
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Current code base maintained by Sean 'Shaleh' Perry <shaleh@(debian.org|valinux.com)>.
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