Can't run python script from cron

Question

Running latest version of Rasbian and want to execute /home/pi/temp.py from cron every 2 min. Have chmod 755 on the file

When I run the command from the prompt it executes fine and the script does what it supposed to: pi@raspberrypi ~ $ python temp.py

I have added it in cron like: */2 * * * * /usr/bin/python /home/pi/temp.py

I never get any output from the script. I have other scripts the run fine from cron. What do I miss?

This is what temp.py looks like

!/usr/bin/python
import time
 # Create a unique timestamp for inclusion in the log...
timestamp = time.strftime("Time: %H:%M, Date: %Y%m%d, ")
 # append the filename with the date (this will start a new log daily)...
logname = time.strftime("")
filename = "".join(["/home/pi/bredhult_temp.txt"])
 # If already created, this will open the current log file, if not it will create a new on
datafile = open(filename, "a")
 # open the sensor log file as seen in the tutuorial (replacing 28-000003f1cede with the n
tfile = open("/sys/bus/w1/devices/28-000006879f89/w1_slave")
 # read the data and close the file...
text = tfile.read()
tfile.close()
 # Split the data and extract the temperature value...
temperature_data = text.split()[-1]
temperature = float(temperature_data[2:])
temperature = temperature / 1000
 # write our newly formatted date, time and sensor value to our log file...
datafile.write(timestamp + "Temp: " + str(temperature) + "\n")
datafile.close()

Answer 1

SUGGESTION:

First create a simple Python script - that just outputs something to a file.

with open("/tmp/hello.txt", "w") as outfile:
        outfile.write("hello")

You don't need the hashbang (#!/usr/bin/python) here (although there is no harm in adding it if you want) : since you will be using the Python executable to call a script in your cron.

To simplify the cron (and you need to decide whether to use 'root' or 'pi' (or another) crontab here) : I assume you will be using the 'root' cron here - use the @reboot instead of a specific time.

$ sudo crontab -e

Make the change, and save the crontab:

@reboot     /usr/bin/python /home/pi/simple.py

$ sudo reboot

Now check that /tmp contains the 'hello.txt' file after the reboot. If this works: swap out the (crontab -e again) this simple script for your existing one , reboot ) - does this work ?

Note: you can also check your system logs for evidence that the cron is running your script or not:

See this other post for information.

Answer 2

Looking at your pasted example, the shebang is missing #; your first line should be:

#!/usr/bin/python

Once that is fixed I would try to make your python script executable with the following line:

chmod +x /home/pi/temp.py

And then simply call it with the following in your crontab:

*/2 * * * * /home/pi/temp.py

Answer 3

This is how I run a python program as root every five minutes.

johnma@raspberrypi ~ $ sudo crontab -l
#
# For more information see the manual pages of crontab(5) and cron(8)
#
# m h  dom mon dow   command
*/5 * * * * /home/johnma/do-DHT-once

Here is the source of do-DHT-once which automatically outputs sysout and syserr to temp files in /tmp and checks to see if a previous copy of the script is still running and tries to kill it if it is. The python program outputs its PID to file /tmp/DHT.pid. The script can also be disabled by creating a file called DHT.disabled in $DIR:

#!/bin/bash

# Set home directory where script exists
DIR=/home/johnma

if [ -f $DIR/DHT.disabled ]
then
  exit 1
fi

if [ -f /tmp/DHT.pid ]
then
  echo Attempting to kill process `cat /tmp/DHT.pid` >>/tmp/DHT.log 2>>/tmp/DHT.err.log
  kill -HUP `cat /tmp/DHT.pid` >>/tmp/DHT.log 2>>/tmp/DHT.err.log
fi

# change to home directory
cd $DIR

# run the program using the debug version of python
python2.7-dbg $DIR/DHT_googledocs.twitter.once.py >>/tmp/DHT.log 2>>/tmp/DHT.err.log

if [ -f /tmp/DHT.pid ]
then
  echo Attempting to delete /tmp/DHT.pid >>/tmp/DHT.log 2>>/tmp/DHT.err.log
  rm /tmp/DHT.pid >>/tmp/DHT.log 2>>/tmp/DHT.err.log
fi

Finally, the file do-DHT-once has permissions 755 to allow it to be executed:

johnma@raspberrypi ~ $ ls -l do-DHT-once
-rwxr--r-- 1 johnma johnma 703 May 12  2014 do-DHT-once

Answer 4

I had problems with Python script as cronjob as well. I recommend to write a simple wrapper bash script which sets PYTHONPATH (or whatever) correctly, put it in $HOME/bin and run a script like the following example:

#!/bin/bash

# Optionally set PYTHONPATH
export PYTHONPATH=/path/to/python/libraries:$PYTHONPATH

# Run your script with your preferred Python installation
# in case you have more than one
/path/to/your/python /path/to/yourscript.py