Nohup没有将日志写入输出文件

I am using the following command to run a python script in the background:

nohup ./cmd.py > cmd.log &

But it appears that nohup is not writing anything to the log file. cmd.log is created but is always empty. In the python script, I am using sys.stdout.write instead of print to print to standard output. Am I doing anything wrong?

It looks like you need to flush stdout periodically (e.g. sys.stdout.flush()). In my testing Python doesn’t automatically do this even with print until the program exits.

You can run Python with the -u flag to avoid output buffering:

nohup python -u ./cmd.py > cmd.log &

• Using -u with nohup worked for me. Using -u will force the stdout, stderr streams to be unbuffered. It will not affect stdin. Everything will be saved in “nohup.out ” file. Like this-

  nohup python -u your_code.py &
  

  You can also save it into your directory. This way-

  nohup python -u your_code.py > your_directory/nohup.out &
  

• Also, you can use PYTHONUNBUFFERED. If you set it to a non-empty string it will work same as the -u option. For using this run below commands before running python code.

  export PYTHONUNBUFFERED=1
  

  or

  export PYTHONUNBUFFERED=TRUE
  

P.S.- I will suggest using tools like cron-job to run things in the background and scheduled execution.

export PYTHONUNBUFFERED=1
nohup ./cmd.py > cmd.log &

or

nohup python -u ./cmd.py > cmd.log &

https://docs.python.org/2/using/cmdline.html#cmdoption-u

Python 3.3 and above has a flush argument to print and this is the only method that worked for me.

print("number to train = " + str(num_train), flush=True)
print("Using {} evaluation batches".format(num_evals), flush=True)

I had a similar issue, but not connected with a Python process. I was running a script which did a nohup and the script ran periodically via cron.

I was able to resolve the problem by:

1. redirecting the stdin , stdout and stderr
2. ensuring the the script being invoked via nohup didn’t run anything else in the background

PS: my scripts were written in ksh running on RHEL