问题：Nohup没有将日志写入输出文件
我正在使用以下命令在后台运行python脚本：
nohup ./cmd.py > cmd.log &
但是似乎nohup没有向日志文件写入任何内容。cmd.log已创建，但始终为空。在python脚本中，我使用sys.stdout.write而不是print打印到标准输出。我做错什么了吗？
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?
回答 0
看来您需要定期刷新标准输出（例如sys.stdout.flush()）。在我的测试中，Python不会自动执行此操作，print直到程序退出。
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.
回答 1
您可以使用带有-u标志的Python 来避免输出缓冲：
nohup python -u ./cmd.py > cmd.log &
You can run Python with the -u flag to avoid output buffering:
nohup python -u ./cmd.py > cmd.log &
回答 2
使用-u with nohup对我有用。使用-u将迫使stdout，stderr流是缓冲。它不会影响标准输入。一切都将保存在“ nohup.out ”文件中。像这样-
nohup python -u your_code.py &
您也可以将其保存到目录中。这条路-
nohup python -u your_code.py > your_directory/nohup.out &
另外，您可以使用PYTHONUNBUFFERED。如果将其设置为非空字符串，它将与该-u选项相同。要在运行python代码之前使用以下命令运行此命令。
export PYTHONUNBUFFERED=1
要么
export PYTHONUNBUFFERED=TRUE
PS： 我建议使用cron-job之类的工具在后台运行程序并安排执行时间。
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.
回答 3
回答 4
Python 3.3及更高版本有一个flush参数用于打印，这是唯一对我有用的方法。
print("number to train = " + str(num_train), flush=True)
print("Using {} evaluation batches".format(num_evals), flush=True)
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)
回答 5
我有一个类似的问题，但未与Python进程关联。我正在运行一个脚本，该脚本执行了nohup，并且该脚本通过cron定期运行。
我可以通过以下方法解决问题：
- 重定向标准输入，标准输出和标准错误
- 确保通过nohup调用的脚本没有在后台运行其他任何东西
PS：我的脚本是用RHEL上运行的ksh编写的
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:
- redirecting the stdin , stdout and stderr
- 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
