CODEWITHSUNDEEP
pythonlinuxnohup
user2399453
user2399453

Reputation: 3081

Nohup for Python script not working when running in the background with &

Boiling down to the smallest problem here is a simple python script that I want to run using nohup on linux. I run it using the following (on linux):

nohup python test.py &

The command seems to just not do anything, nothing is appended to nohup.out. If I run it without the & the output shows correctly on the terminal window. What am I missing?

import time

def test():
    while(True):
      print "Woke up!"
      time.sleep(5)

if __name__ == "__main__":
    test()

Upvotes: 15

Views: 13998

Answers (4)

AugustoM
AugustoM

Reputation: 11

I had the same problem, however, my algorithm was a little more complex than yours demonstrated. As I had created a python virtual environment by installing all the necessary packages for its execution, it only runs if the virtual environment is activated source env/bin/activate. After activating the environment, I was able to run normally, not resulting in any errors: nohup python3 -u scriptShow.py > logScriptNohup.log &.

If you want to make sure it's running, do in the terminal: ps aux | grep scriptShow. If the running process appears, congratulations, you did it

Upvotes: 1

SandRock
SandRock

Reputation: 5573

Just had a similar issue. My script worked well without nohup. With nohup, the script would crash with a SyntaxError.

The problem was the execution context of nohup which would use an alias of python mapped to python2 instead of python3.

Fixed it by specifying python3 instead of python.

Upvotes: 3

Metropolis
Metropolis

Reputation: 2128

Pass python the -u flag for unbuffering stdout

nohup python -u test.py &

Python will buffer stdout otherwise. This doesn't require a code change.

From the man page:

       -u     Force  stdin,  stdout and stderr to be totally unbuffered.  On systems where it matters, also put stdin, stdout
          and stderr in binary mode.  Note that there is internal buffering in xreadlines(), readlines() and  file-object
          iterators ("for line in sys.stdin") which is not influenced by this option.  To work around this, you will want
          to use "sys.stdin.readline()" inside a "while 1:" loop.

Upvotes: 31

Dan D.
Dan D.

Reputation: 74675

You need to flush stdout after printing: sys.stdout.flush(); otherwise it'll take awhile to fill the stdout buffer.

Upvotes: 4

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