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I am trying to set the system clock at a specific time each day (1:49 AM local time) so that I can send a command to close a relay at exactly 2:00 AM local time to reset the clock on a external device that can't access NTP servers. When I try to edit the crontab file, it will not let me save changes. I believe the code that I want to add is:

49 1 * * * ntpd -s 3.us.pool.ntp.org 2.us.pool.ntp.org

Can someone verify this for me? How do I edit the crontab file and save changes?

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    crontab -e will open vi. To start editing type i. To save in vi press escape, type :wq, and press enter.
    – Gerben
    Dec 18, 2013 at 19:33
  • If you only want to sync the clock occasionally ntpdate might be more what you are looking for.
    – hildred
    Dec 19, 2013 at 3:25

2 Answers 2

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Just login as root and run crontab -e. A editor will appear, letting you add the cronjob.

The syntax for the cronjob is correct, running at 1:49 AM.

Crontab syntax overview (source):

# * * * * *  command to execute
# ┬ ┬ ┬ ┬ ┬
# │ │ │ │ │
# │ │ │ │ │
# │ │ │ │ └───── day of week (0 - 7) (0 to 6 are Sunday to Saturday, or use names; 7 is Sunday, the same as 0)
# │ │ │ └────────── month (1 - 12)
# │ │ └─────────────── day of month (1 - 31)
# │ └──────────────────── hour (0 - 23)
# └───────────────────────── min (0 - 59)
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  • Why run as root? Does ntpd require root access?
    – Gerben
    Dec 18, 2013 at 19:35
  • Actually I have no idea if ntpd requires root. I would just assume so, since I would not let non-root users adjust my clock. Alteast in a server environment nonroot users should absolutely not be able to tamper with the system clock. But please look it up, I am not sure. :-)
    – Ragnar123
    Dec 18, 2013 at 22:26
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    @Gerben ntpd only requires root access to set the date, and to get a low port number.
    – hildred
    Dec 19, 2013 at 3:24
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why do you need to set the system clock exactly at 2AM ? install "ntpdate" package and forget about clock syncing, it will be done totally automatically.

the problem you need to solve is to send a command to your devices to set their clocks, and that command would be, as you wrote, something along the lines of:

49 1 * * * /your/command/to/control_the_remote_device

you may enter this command in "crontab -e", which will install this as a cron job for your user (no need for root). if you don't like "vi", use "export EDITOR=nano" to set your favourite editor to be used when editing the cron file.

ps. just don't touch ntp, it works perfectly as it is =)

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  • Thanks everyone for the info. I need to send a contact closure to school consoles that have an internal clock that is drifting a few seconds a day. In the days before cell phones, etc. no one cared if the systems were off by even a few minutes. Now, everyone at the schools has a fit that the school clocks are not EXACTLY on. The school consoles are older technology that can only have the clock reset by a contact closure at exactly 2AM local time. So I first need to make sure the RaspberryPi is exactly on time to the second each day and then send a relay command to close at exactly 2AM. Thanks!
    – John
    Feb 10, 2014 at 14:56

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