Run python programs in the background in Linux Systems

Firstly, you need to add the shebang line in the Python script which looks like the following:

#!/usr/bin/env python3

This path is necessary if you have multiple versions of Python installed and /usr/bin/env will ensure that the first Python interpreter in your $$PATH environment variable is taken. You can also hardcode the path of your Python interpreter (e.g. #!/usr/bin/python3), but this is not flexible and not portable on other machines. Next, you’ll need to set the permissions of the file to allow execution:

chmod +x test.py

Now you can create a new session with setsid and run the script with nohup which ignores the hangup signal. This means that you can close the terminal without stopping the execution. Also, don’t forget to add & so the script runs in the background. setsid is used to run a program in a new session.

setsid nohup /path/to/test.py &

If you did not add a shebang to the file you can instead run the script with this command:

nohup python /path/to/test.py &

The output will be saved in the nohup.out file, unless you specify the output file like here:

setsid nohup /path/to/test.py > output.log &
setsid nohup python /path/to/test.py > output.log &

You can find the process and its process Id with this command:

ps ax | grep test.py

If you want to stop the execution, you can kill it with the kill command:

kill -9 PID

Output Buffering

If you check the output file nohup.out during execution you might notice that the outputs are not written into this file until the execution is finished. This happens because of output buffering. If you add the -u flag you can avoid output buffering like this:

setsid nohup python -u ./test.py &

Or by specifying a log file:

setsid nohup python -u ./test.py > output.log &