How to schedule a job that depends on network availability?

Question

I have cloned a git repository of some project. I want to automate git pull and compile the project once in a week. I am using a laptop which wont be on 24x7. Now, I cannot use cron as I should keep the system running at that exact moment. I cannot use anacron too as it might start before I connect to network. Is there some option in anacron that will run this particular job only when I am connected to internet? Or should I be using some other different tool for this?

Answer 1

Run the job when you connect to the network. Most distributions have a scripting infrastructure that you can plug into, though you will need root permissions. If you connect with NetworkManager or Wicd, they have their own hook infrastructure as well. Add a cron job that only runs if the network is available (and, optionally, only if the job hasn't been run performed in a long time), in case the network remains connected for a long time.

You don't specify your distribution, so I'll give an example for a Debian-based distribution. The scripts in /etc/network/if-up.d are executed after an interface is brought up; see the interfaces(5) man page for more information. Write a script /etc/network/if-up.d/zzzz_balki_git_pull such as this:

#!/bin/sh
su balki -c /home/balki/bin/pull_repository_if_old

with a pull_repository_if_old that does something like this (run git pull if the network is available unless there is a timestamp that is less than 7 days old):

#!/bin/sh
set -e
if [ -z "$(find /path/to/.repository.pull.timestamp -mtime -7)" ] &&
   ping -c 1 -w 1 repository.example.com >/dev/null 2>/dev/null
then
  cd /path/to/repository
  git pull
  touch /path/to/.repository.pull.timestamp
fi
EOF

And a crontab entry on your account:

28 4 * * * /home/balki/bin/pull_repository_if_old

Answer 2

Use anacron, because you won't be up 24/7, then add some magic to ensure the network is up (by pinging the default gateway), and if not delay the execution.

Here's an example script that will get you close:

#!/bin/bash
cd repo
JOB="git pull"
ping -c 2 $(netstat -rn | awk '/^0.0.0.0/ {print $2}') 2>&1 > /dev/null && $JOB || echo $JOB | at $(date --date="+1 hour" +%H:%M)