Python script not running correctly when launched with crontab

Question

I'm running a telegram bot on my Raspberry Pi 3. When I run it in my console or using Thonny, everything runs smoothly.

python3 /home/pi/Documents/telegrambot.py

The problem occurs when I set the program to start on boot.

sudo crontab -e
[last line] @reboot sudo python3 /home/pi/Documents/telegrambot.py &

I know that the code is running because I set the bot to send me "I'm online" and it answer me if the reply isn't related to sending images... but when I send to it /tempgraph, it should answer with the the picture of its readings. But I only get the sensor's name and no image at all.

...
    chatHist = readCommandHistory(db,chat_id,1)
    lastCommand = chatHist[0]['msg']
    choice = command.split(".")
    if command in lastCommand and len(choice) <= 2:
        Fonte = re.search('(\s|\n)'+str(command.split('.')[0])+'\)\s(.*)', lastCommand).group(2)
        bot.sendMessage(chat_id, Fonte) #this works
        if len(choice) == 1:
            table = sdc.readTableServer(db, senstable, 'Fonte', Fonte)
            for sensor in table.Sensor.unique():
                bot.sendMessage(chat_id, sensor) 
                tbl = table[table.Sensor == sensor]
                #until here everything is ok
                for ts in tiposensores:
                    try:
                        gop.plotSetup(tbl[['Data',ts]]) 
                        #save the image in the folter ... Images/Imagem.png, 
                        #but only works when not on stratup...
                        bot.sendPhoto(chat_id,open('/home/pi/Documents/Images/Imagem.png','rb'))
                        pif.cleanImages()
                    except OSError:
                        pass

...

@edit:

I found out that the problem happens after i call

gop.plotSetup(tbl[['Data',ts]]) 

inside the plotSetup function, in this step:

f= plt.figure(figsize=(12, 9))

This is the first time that i use the variable f inside the function.

@edit2

As I said before, the error happens in the plt.figure.

THis is the gpo.plotSetup(), with some changes to catch the error as instructed:

def plotSetup(Data):
...[some code here]...
    with open("/home/pi/Desktop/log.csv","a+") as o:
        fop.writeCSVFile(o, ['before first plt.figure'])

        try:
            f= plt.figure(figsize=(12, 9))
        except IOError as e:
            fop.writeCSVFile(o, e)
        except:
            for e in sys.exc_info():
                fop.writeCSVFile(o, e)

...[some code here]...
    f.savefig("/home/pi/Documents/Images/Imagem.png", bbox_inches="tight") 

and i got this errors:

<class '_tkinter.TclError'>
no display name and no $DISPLAY environment variable
<traceback object at 0x6e3c8d50>

@edit3

That error led me to this post:

_tkinter.TclError: no display name and no $DISPLAY environment variable

this post suggested use this in the very beginning of the code:

import matplotlib
matplotlib.use('Agg')

and then VOILA!! It's alive.

Really appreciate all the help that you guys gave me, thanks! ;p

Answer 1

Try this:

$ sudo chmod 777 /home/pi/Documents/Images 

After you've done that, re-boot to check if that fixes it.
If not, try changing your crontab entry to this:

@reboot ( /bin/sleep 30; /usr/bin/sudo /usr/bin/python3 /home/pi/Documents/telegrambot.py >> /home/pi/Desktop/log.txt 2>&1)

You should also consider adding a debug statement to your Python code to collect and save any errors that are thrown during execution.

Answer 2

You have two problems: the problem of the file not being found, and the problem preventing you from debugging the first problem.

First let's make a general improvement. You have

try:
    [...]
except OSError e:
    pass

This means if the try part fails with OSError, it will just ignore it, tell you nothing about it, and carry on. At very least, log the error:

try:
    [...]
except OSError e:
    print(e)

Now when it goes wrong you should be able to find out why from the system logs (/var/log/syslog I think).

Now the real problem. I don't know what this gop thing is, and your extract doesn't show it, but you say it generates a file, which you indicate is in Images/Imagem.png -- although I suspect it's really in Documents/Images/Imagem.png. This is a relative path - relative to your current directory. When you load the file in the next line, you use an absolute path - relative to the filesystem root.

When you run the file as user pi you are probably doing so from /home/pi (or ~pi, which is equivalent), your file goes into /home/pi/Documents/Images/Imagem.png, the absolute and relative paths happen to match up, and the next line can find the file. However when you run it from crontab, it will have a different current directory. I don't know offhand what current directory crontab will give it, but there's no way it'll be /home/pi.

Either change gop (whatever that is) to put the file in an absolute location (I don't have the code to help with that), or change your code to read it from the same relative location:

bot.sendPhoto(chat_id,open('Documents/Images/Imagem.png','rb'))

Lastly, unless there is a good reason to run the script as root, don't. It should have the least privilege it requires to work. Just good practice.

Answer 3

This might be a permission problem. When running under the user's (your) credentials it works and when under the crontab's credentials is does not.

Try a test: WRT to your images, temporarily open permissions to all images files and the associated directory and see if the application works when run by the crontab.

Don't leave the permissions opened to all as it is a security risk.

Answer 4

like i said in the last edition that i made in the question...

After adding the print sentence to some file after the exception, i got this:

<class '_tkinter.TclError'>
no display name and no $DISPLAY environment variable
<traceback object at 0x6e3c8d50>

Thit error led me to this post:

_tkinter.TclError: no display name and no $DISPLAY environment variable

that suggested to use this in the very beginning of the code:

import matplotlib
matplotlib.use('Agg')

and then VOILA!! It's alive.

Really appreciate all the help that you guys gave me, thanks @Seamus, @Mark Smith and @st2000!! ;p