1

Hardware setup (computer, etc)

  • Ubuntu server 18.04.1
  • Serial To Usb Converter with 8 ports

Python version

  • 2.7.15r1

Python program description

When the program starts to create some threads:

  1. Create one thread for the Modbus server.
  2. Run 1 thread for each serial port connected (/dev/ttyUSBn) and start read the data.

Problem explanation

When I run the script using the normal command (python2.7 myProgram.py) it work, so the modbus server start and I can read the value, and I can also see the usb-serial convert blink on the TX-RX leds. If I check the readed data they are correct, so the program is working properly.

The problem come out when I set up a crontab job that run my python script!

The modbus server start properly, but I can't see the usb-serial converter leds blink and the python program don't print the readed data. That means the program is not working on the "serial" side.

To create the job I has used the following commands:

  1. crontab -e
  2. selected nano (default option)
  3. added at the end of the file the cron command: @reboot /usr/bin/python2.7 /myProgram.py

I can't figure out where the problem is, the program is not catching the exception and the process is still running until I stop it manually. If I stop it and run it manually after that it start and work properly.

To help you:

I have also tried to run it using **systemctl**, the problem is the same. At boot the service start and if I check it I can read: Active(running), but the software is not reading from the serial port.

The questions are:

  • How can I solve it?
  • There is something wrong with the crontab job?
  • Maybe crontab job can't access the /dev/ directory? How can I solve this?

I'm very confused about that, I hope the question is properly created and formatted.

EDIT 30/11/18:

I have removed the crontab command, and create a service to run the program using this procedure.

If I run the command: service supervision start I can see that the process is running correctly and on htop I have only 4 processes.

htop output

service status output

In this case, the program is not reading from the serial port, but the modbus server is working. You can see that I have just 4 processes and the cpu load is too high.

If I run it manually with the command: python2.7 LibSupervisione.py

The output of the htop command is: outupt of working program

Here you can see that I have more processes, 1 for each thread that I create and the load on the cpu is properly distributed.

asked Nov 29, 2018 at 17:25
6
  • Change to ... myProgram.py >>/tmp/myProgram.log 2>&1, to get the error message and read crontab-not-running-my-python-script Commented Nov 29, 2018 at 20:09
  • Add this #!/usr/bin/env python shebang on top of your code, and set chmod 755 to your code, but the best way is that you create a service from your code, I using from this procedure for that. Commented Nov 29, 2018 at 20:32
  • @stovfl I have tryed to add the >>/tmp/myProgram.log 2>&1 in my crontab command, but nothing where printed in the file. As I told the problem is non that the cron job isn't running, but the problem is that it run, start just a piece of program. Thanks anyway for the help! Commented Nov 30, 2018 at 7:22
  • @BenyaminJafari I have tryed to create the service using systemctl, I'll try with your guide! At the top of the file I alredy have #!/usr/bin/python2.7 and I have updated the permission of the folder to 777 using this command: chmod 777 /programFolder/ -R Commented Nov 30, 2018 at 7:26
  • @CarloZanocco Could you run your script using a service? Commented Nov 30, 2018 at 9:00

2 Answers 2

1

Your script probably requires a Console or some Environment variables, but in a systemd started process you dont have these automatically.

The easiest way would be to prepend /usr/bin/bash -c "your command" in your System unit in the field ExecStart to enable a Shell like Environment likle this:

ExecStart=/bin/bash -c "/usr/bin/python2.7 /myProgram.py" 
WorkingDirectory=yourWorkingDir

Why do you Need to use cron? Use a systemd timer instead.

Carlo Zanocco
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answered Nov 30, 2018 at 10:03
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5 Comments

I don't need to use cron, I have tested it, because the problem persist with systems, systemctl, service and cron command. I have created a service and the problem persist. The program START but is not reading from the serial port, but the modbus server is working.
Did you start your program with a prepended Shell? Like bash? I know this problem from own experience.
What it means? Sorry but I don't know what you mean with prepended Shell.. I log in to the remote computer using ssh root@ip, after that I use the shell of the Ubuntu Server computer
you are right. I understand what you mean! But in the script I have to read one config file. I have the path that is not absolute! In the program I have : ` path = "Utility/config.txt" , but this is wrong, because the folder is in another folder, so the path is not absolute. The correct way is to use: path = "/Monitoraggio/Utility/config.txt". So I have to start from the /` directory!
Try to Change the systemd unit to: `` ExecStart=/bin/bash -c "/usr/bin/python2.7 /myProgram.py" WorkingDirectory=yourWorkingDir ``
1

If you could run your code with the service like this: sudo service <service-name> start and get a good status using sudo service <serivice-name> status, you can test it in crontab -e like this (run every 5 minutes for test):

*/5 * * * * service <service-name> start
*/10 * * * * service <service-name> stop

Then using @rebote after with the above test.

OR:

Finally, if you want to run your code/service at the system startup, do it instead of cron jon:

Edit the rc.local file with an editor with the sudo permission, then:

#!/bin/sh -e
# rc.local
#
# This script is executed at the end of each multiuser runlevel.
# Make sure that the script will "exit 0" on success or any other
# value on error.
#
# In order to enable or disable this script just change the execution
# bits.
#
# By default this script does nothing.
service <service-name> start
exit 0

[NOTE]:

This is the procedure of creating a service from your code.

answered Nov 30, 2018 at 9:48

7 Comments

The problem is still here. The process is running correctly in any case, I always have a good status. But the problem is that the program not work the same as when I run it manually! Using service test start it run, I have a good status, but the serial port is not reading in any case. And the server Modbus is working correctly. If I run the code manually with python2.7 myProgram.py it work and read from the serial port.
I have added more info to the question, check the edit please
I did it, please put on your code for better sense.
I can't put on the code, I can create a sample code to share with you. But why you need it? The code is working correctly, as I told if I run it not as service it work.
Don't use this, why start initd from system when you can call it directly.
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