python and crontab

[nunocv]

Hello,
i'm trying to get the startup time of my raspi, the time that i turn it on.
i create this python script called 'boot_time.py'

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from time import strftime
with open("/home/pi/monitor/boot_time.txt", "w") as log:
  log.write(strftime("%d-%m-%Y,%H:%M:%S"))

the code works ok if execute the python file. it register the date and time that i executed the script

so i created a crontab entry so the script would run on boot
so i executed

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crontab -e

and added the following line

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@reboot python /home/pi/monitor/boot_time.py &

the thing is that on very first boot it doens't work
it only works when i reboot the system
hope that one of you guys can help me figure out what's wrong

[pcmanbob]

Hi.

So using a pi2b running Jessie lite with a wired connection and a fixed IP on the pi, ran your code using the cron line

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@reboot python /home/pi/boot_time.py

and it works perfectly on reboot and a cold start.

So then I tried it with no network connection and it still works but you just get the saved time set by the fake HW clock.

So I don't see why it wont work for you may be some more details which OS and what network connection would help with us trying to reproduce your problem.

Edit.... repeated using same pi but using stretch lite and a fixed IP on the pi same results still works ok.

[rpdom]

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crontab -e

and added the following line

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@reboot python /home/pi/monitor/boot_time.py &

You don't need the "&" on a line in crontab. The code always runs in the background.

To diagnose errors when running you can direct the output to a file like this:

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@reboot python /home/pi/monitor/boot_time.py >/tmp/myprog.log 2>/tmp/myprog.err

That should create two files when the cron job triggers. /tmp/myprog.log, which has any standard output from the script and /tmp/myprog.err which will contain any error mssages.

[nunocv]

thanks for your reply guys.
pcmanbob you said that:

So then I tried it with no network connection and it still works but you just get the saved time set by the fake HW clock

what does this mean?

rpdom i removed the '&' and added the 'log' and 'err', but they both came clean
i think the problem may be related to what pcmanbob said about 'fake HW clock', but i need the learn the real meaning of this

[rpdom]

i think the problem may be related to what pcmanbob said about 'fake HW clock', but i need the learn the real meaning of this

Unlike most computers, the Pi doesn't have an onboard clock with battery power. This means that when it is switched off it forgets the time and date completely.

To get around this the Pi uses two methods.

One is to set the time from an internet time server when it starts up. That only works when it has an internet connection.

The other is to use a program called fake-hwclock which logs the current time in a file every hour (at 17 minutes past the hour, usually), and also during shutdown. When the Pi starts up it reads that file and sets the clock to that time. This means that the time will be wrong, but it will at least be showing as later than the shutdown time so timestamps won't suddenly jump back in time.

The file is /etc/fake-hwclock.data and you can read the contents with the cat command

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pi@raspi3 ~ $ cat /etc/fake-hwclock.data 
2017-10-22 04:17:01

[pcmanbob]

Try this.

add a delay to your cron line so that the pi waits for 1 min before running the python program just to see if it works then.

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@reboot sleep 60 && python /home/pi/monitor/boot_time.py >/tmp/myprog.log 2>/tmp/myprog.err

this might reveal if its some thing that is not ready when you are calling the python program.

[nunocv]

this is owesome, i didn't know that the pi don't have an internal clock, and the fake clock is indeed a great method to try to overcome this problem.
my script was actually working, but it was executed before my raspi (which is connected to the network) updated the time online.
so now i added the delay on crontab

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@reboot sleep 60 && python /home/pi/monitor/boot_time.py >/tmp/boottime.log 2>/tmp/boottime.err

thanks you guys

last question, how do mark this topic as solved?

[DougieLawson]

If you're time critical then create a systemd service sudo nano /etc/systemd/system/python_thing.service

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[Unit]
Description=python thing
DefaultDependencies=no
After=local-fs.target time-sync.target

[Service]
Type=oneshot
ExecStart=/home/pi/monitor/boot_time.py

Edit your boot_time.py program and add

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#!/usr/bin/python

as the first line in the program.

make your python program executable
chmod 755 boot_time.py
no need to use python boot_time.py to run it after that.

Make your service file active
sudo systemctl enable python_thing

[scruss]

If you're time critical then create a systemd service …

Better yet, create a user systemd service. Save this as ~/.config/systemd/user/boot_time_log.service:

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[Unit]
Description=boot time log
DefaultDependencies=no
After=local-fs.target time-sync.target

[Service]
Type=oneshot
ExecStart=/home/pi/monitor/boot_time.py

[Install]
WantedBy=default.target

then enable it with:

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systemctl --user enable boot_time_log.service

Look Ma, no sudo! Also no weird root-owned files in your home folder too!

[DougieLawson]

Thanks Stewart, I've learned a new trick.

Didn't know about --user.

[scruss]

Thought you might like that, Dougie. systemd keeps on giving!

[nunocv]

you're blowing my mind scruss, this looks great
but when i tryed to enabled it, i receve this error

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Failed to execute operation: No such file or directory

to directory you mention does not exist (there was no directory systemd inside ~/.config
so i manually created it
can you give me a step by step?

[scruss]

sure:

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mkdir -p ~/.config/systemd/user

then create the ~/monitor/boot_time.py file and make it executable
now save the service file and enable it like I explained.