Don't use cron for this
this post was submitted on 26 Feb 2024
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sudo cat /etc/systemd/system/CUSTOM-networkstuff.service <<EOF
[Unit]
Description=Ping Wikipedia.org
After=network-online.target
Wants=network-online.target
[Service]
Type=oneshot
ExecStart=/usr/bin/ping -c 1 wikipedia.org
EOF
sudo cat /etc/systemd/system/CUSTOM-networkstuff.timer <<EOF
[Unit]
Description=Daily ping to Wikipedia.org
[Timer]
OnCalendar=*-*-* *:*:00
Persistent=true
[Install]
WantedBy=timers.target
EOF
sudo systemctl enable --now CUSTOM-networkstuff
Systemd timers are able to do this. Set it up to run daily and make it require the network-online.target.
network-online.target is not technically about being connected to the internet but just having a network connection.
Yeah, I needed something similar and made a new target that checks whether the system can reach a common website like google
Would it not just be the easiest way to put your scripts under /etc/network/if-up.d/? Then they get run once that connection is brought up.
Cron is basically limited to doing something on a time interval. What you could do is make it so your Cron job checks for internet access and then does the thing if it finds it.
Everyone else is just telling you to do things in a way that is different, and while they are correct (you should use a unit.d/systems script for this depending on your distro), I'm going to actually answer your question since I know sometimes you just need a quick and simple way.
Depending on your version of cron, it may support special statements instead of the * * * * * notation for time.
The one you want is @reboot. Replace all entries of the schedule syntax with that, including the @, and the command will be executed only once when the system boots up.
Use that to start a script that checks for network connectivity on a loop with a sleep statement. Break the loop when you have connectivity, then execute your command, and exit the script.
Don't ignore the correct way though. You're better off executing this as a systemd (or equivalent) script. It's barely more effort, and has the benefit of some nice built in logging and integrations.
Personally I'd write a script to do whatever it is you want to do, checking first whether the internet connection is working. Said script can check the internet is working, if not, sleep for 10 minute or something, and try again, perhaps giving up after a set number of tries. Said script could also check the date and time of the file downloaded and confirm it is out of date.
Then, have the script execute on bootup by adding it to the startup scripts.
Instead of having it sleep 10 minutes just make it a cronjob that runs every 10 minutes and has a lockfile, much more robust that way.
If it should be done say only twice a day, then your way would be completely cumbersome.
If it should be done only twice a day sleeping for 12h would be even more error prone and you would be even less likely to have it actually run at that time.
Also, if it should be done only twice a day you don't really want it to run twice a day only since you can not guarrantee that you have internet exactly at the time when your timer expires, you would want to check some "last_updated" timestamp and check if it was more than 12h ago.