[Bug 1429285] Re: feature request: apt-get update --if-necessary

[Julian Andres Klode]

I think automatic updating on old caches is not really the best idea,
because it will always happen when you expect it the least. Or maybe you
do in fact want the old one.

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Title:
  feature request: apt-get update --if-necessary

Status in apt package in Ubuntu:
  Confirmed

Bug description:
  In many cases (juju, lxc containers .. ) we find ourselves in the
  position of not knowing if the apt-cache has been udpated recently.
  So, you either risk not doing it, or do it and it takes some time and
  generates load.

  so long story short, you always run 'apt-get update' which is quite often
  unnecessary.

  Would it be possible to add (or is there now) something like
  '--if-necessary' or '--if-necessary=5m'.  I could imagine that that would
  look at /var/lib/apt/lists and check timestamps on files for each url that
  /etc/apt/sources.list[.d/*] would hit.  If nothing was needed and
  reasonably recent, then it would not do the update.

  There exist other solutions to this like:
    https://coderwall.com/p/0xtstw/doing-an-apt-get-update-in-ansible-only-if-a-condition-is-met
    https://blog.kumina.nl/2010/11/puppet-tipstricks-running-apt-get-update-only-when-needed/

  
  It'd be nice if we had a sane way to say:
    update if you need to, otherwise don't waste time and resources

  ProblemType: Bug
  DistroRelease: Ubuntu 15.04
  Package: apt 1.0.9.3ubuntu1
  ProcVersionSignature: Ubuntu 3.19.0-7.7-generic 3.19.0
  Uname: Linux 3.19.0-7-generic x86_64
  ApportVersion: 2.16.2-0ubuntu1
  Architecture: amd64
  CurrentDesktop: Unity
  Date: Fri Mar  6 17:06:22 2015
  EcryptfsInUse: Yes
  InstallationDate: Installed on 2015-01-02 (63 days ago)
  InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Alpha amd64 (20150101)
  SourcePackage: apt
  UpgradeStatus: No upgrade log present (probably fresh install)

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