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.

Improve this question
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

Reset to default
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.

Improve this answer
Sign up to request clarification or add additional context in comments.

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.

Improve this answer

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.
|

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.