Python - Verifying if variable is empty

Question

I am writing a script that logs into a remote node via SSH and checks for a service being up by saving the output of pgrep -a <service-name> , storing it in a variable and checking if that variable is UP or not.

HOST="172.29.219.110"
COMMAND="pgrep -a haproaxy"
ssh = subprocess.Popen(["ssh", "%s" % HOST, COMMAND], shell=False, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
result = ssh.stdout.readlines()
print ( result )
if result == "":
   print ("The service is NOT running")
else:
   print ("The service is running")

If I run the above as it is, I get the below response:

[b'31318 /usr/sbin/haproxy-systemd-wrapper -f /etc/haproxy/haproxy.cfg -p /run/haproxy.pid\n', b'31319 /usr/sbin/haproxy -f /etc/haproxy/haproxy.cfg -p /run/haproxy.pid -Ds\n', b'31320 /usr/sbin/haproxy -f /etc/haproxy/haproxy.cfg -p /run/haproxy.pid -Ds\n']
The service is running

But when I pgrep a bogus service, the response is

[]
The service is running

Ive checked on the remote server that pgrep -a haaaaaaaaa retuns nothing. But it doesnt seem to register as an empty variable in python. What is it that I am doing wrong ?

Result is a list. You can check for an empty list with if not list: — chatton
– chatton, Commented Apr 3, 2017 at 19:33
That's a pretty crappy way to check whether the service is up, for what it's worth. I can think of multiple failure modes. — wim
– wim, Commented Apr 3, 2017 at 19:36
I'd say swap your conditional and just use if result: #good else: #bad — TemporalWolf
– TemporalWolf, Commented Apr 3, 2017 at 19:36

Rohit Shinde · Accepted Answer · 2017-04-03 19:47:25Z

1

You are equating a list with an empty string. That is why your code fails. The correct answer to this would be:

if not result:
  print("The service is not running")

answered Apr 3, 2017 at 19:47

Rohit Shinde

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