使用从cron执行的python脚本写入文本文件

Question

Every time I run the script from cron, the file to which i write my stdout is empty, the files gets modified but nothing is written. While running the script from terminal works.

Example of what i'm trying to do:

run.py

#!/bin/python2
import os
os.chdir('/home/user')
import getsmart
import sendemail

getsmart.py

#!/bin/python2

from subprocess import call
import os

os.chdir('/home/user')

f = open("result", "w")
call(["sudo","smartctl", "-a", "/dev/sda"], stdout=f)
call(["sudo","smartctl", "-a", "/dev/sdb"], stdout=f)

sendmail.py

#!/bin/python2

# Import smtplib for the actual sending function
import smtplib
import os

# Import the email modules we'll need
from email.mime.text import MIMEText

os.chdir('/home/user')

# Open a plain text file for reading.  For this example, assume that
# the text file contains only ASCII characters.
fp = open('result', 'rb')
# Create a text/plain message
msg = MIMEText(fp.read())
fp.close()

# me == the sender's email address
# you == the recipient's email address
msg['Subject'] = ''
msg['From'] = ''
msg['To'] = ''

# Send the message via our own SMTP server, but don't include the
# envelope header.
s = smtplib.SMTP('')
s.sendmail('', '', msg.as_string())
s.quit()

cron

* * * * * /home/user/run.py

The script has execute permission.

-rwxr-xr-x. 1 user user 84 Jul  5 12:20 /home/user/run.py

As we can see next in the log the script is being executed:

Jul 5 13:11:01 localhost CROND[10945]: (root) CMD (/home/giumbai/run.py)

---

As of my understanding the problem is something regarding the way the executed commands output is written to stdout or how is read from stdout withing the script. The python documents doesn't helps me, neither searching the www.

Answer 1

It is quite possible that your issue is due to the fact that you have opened the file and then written to it....but left it open. You then attempt to open the file a second time but due to caching the data written to it is not yet available. If you change the first module to do the following instead:

with open("result", "w") as f:
    call(["sudo","smartctl", "-a", "/dev/sda"], stdout=f)
    call(["sudo","smartctl", "-a", "/dev/sdb"], stdout=f)

This will open the file, write to it and then close it. That should help!

Another possibility is the fact that cron runs under a different environment than the normal shell. You may wish to include the full path for the smartctl command. On my machine that is /usr/sbin/smartctl.

In addition, it might be better to install the script under root's crontab instead of the local user and use sudo -- there may be interactions with the environment that don't work under cron there as well.

And one other option to consider: make sure that stderr goes somewhere as well so you can detect errors -- that would probably help you figure out what the trouble is when running with cron. Normally this goes the system log or the local user's mail but depending on your configuration that may not be that helpful!

As a side note, it is good practice not to use the import of a module to do a script's work. A module should have classes and functions defined in it which a script (your run.py) then calls. Or just put the code in run.py directly!