subprocess.call always returns an error when script is run by cron

Question

I have a simple python script which I am using to automate updates to a dhcp config file.

The Idea is that it puts the new config file in the dhcpd directory runs a check and if that returns ok it can restart the service. My code looks like this:


syslog.syslog(syslog.LOG_INFO, 'INFO: file copied to /etc/dhcp/conf.d')
return_code = subprocess.call(['dhcpd -t -cf /etc/dhcp/dhcpd.conf'], shell=True)
if return_code != 0:
    print('dhcp config test failed, exiting script')
    syslog.syslog(syslog.LOG_ERR, 'ERROR: dhcp config test failed, exiting script')
    sys.exit()
else:
    print('dhcp config test passed restarting service')
    syslog.syslog(syslog.LOG_INFO, 'INFO: config check passed, restarting service')
    return_code = subprocess.call(['service', conf['service_name'], 'restart'])
    if return_code != 0:
        print('dhcpd service failed to restart')
        syslog.syslog(syslog.LOG_ERR, 'ERROR: dhcpd service failed to restart')
    else:
        print('dhcpd service restarted')
        syslog.syslog(syslog.LOG_INFO, 'INFO: service restarted')
        email_results()

This script is kicked off by a cron job, when it runs it always fails at this bit:

print('dhcp config test failed, exiting script')

If I run the script manually it always works fine and continues to the end as expected.

If I open the python shell and run the important commands by hand it seems to work fine:

python3
Python 3.5.2 (default, Nov 23 2017, 16:37:01) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import subprocess
>>> return_code = subprocess.call(['dhcpd -t -cf /etc/dhcp/dhcpd.conf'], shell=True)
Internet Systems Consortium DHCP Server 4.3.3
Copyright 2004-2015 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
Config file: /etc/dhcp/dhcpd.conf
Database file: /var/lib/dhcp/dhcpd.leases
PID file: /var/run/dhcpd.pid
>>> print(return_code)
0

I have tried using "shell=True" and also tried without. I have also tried subprocess.check_call with the same results.

Where am I going wrong here?

Maybe it can't find the `dhcp`? What's your PATH in cron? Also check this regarding subprocesses.call problems in cron - https://stackoverflow.com/questions/30766067/using-python-subprocess-call-with-crontab — h4z3, Jun 14 '19 at 11:20
May be better if you use `subprocess.Popen` instead and leave out the shell parameter, let its use its default which is`False`. The commands go as a list rather than a string. [docs](https://docs.python.org/3.6/library/subprocess.html#subprocess.Popen) — cardamom, Jun 14 '19 at 11:31
It it must be started from cron, the most robust way is to set the path inside the script of to use full paths for all files including commands like here `dhcpd`. — Serge Ballesta, Jun 14 '19 at 11:32

score 1 · Accepted Answer · answered Jun 14 '19 at 12:05

1

Use absolute paths instead of just command names like dhcpd in your script.

Try if your script still works when you call it after setting an empty PATH.

answered Jun 14 '19 at 12:05

Roland Weber

1,865
2
17
27

I've made this a community wiki because the answers have actually been provided by other folks in comments. But I don't want this question to remain "unanswered". – Roland Weber Jun 14 '19 at 12:06

score 1 · Answer 2 · answered Jun 14 '19 at 12:09

The arguments to subprocess should be either an array of strings or a single string. Passing in an array of a single string is an error, though it might happen to work on some platforms which are fundamentally broken anyway.

You want

return_code = subprocess.call(['dhcpd', '-t', '-cf', '/etc/dhcp/dhcpd.conf'])   # shell=False implicitly

or

return_code = subprocess.call('dhcpd -t -cf /etc/dhcp/dhcpd.conf', shell=True)

but really, you should avoid shell=True whenever possible; see also Actual meaning of 'shell=True' in subprocess

And of course, if dhcpd is not in the PATH that you get from cron, you want to update the PATH correspondingly, or use an explicit hard-coded path like /usr/sbin/dhcpd (I generally recommend the former).

subprocess.call always returns an error when script is run by cron

2 Answers2