When using Python subprocess, why does Ctrl-C not produce the same behavior as kill -2?

Question

If I kill a Python process via Ctrl-C , all child processes (started by subprocess ) die. However, if I kill the Python process via kill -2 , the child processes keep running. I thought that Ctrl-C was the same as sending a SIGINT (ie, kill -2 ). Why is the behavior different?

Answer 1

The python process, and its children started by multi-process or subprocess, are all attached to the same terminal -- specifically, they are all part of the same process group . When the terminal receives the Ctrl-C, it will send SIGINT to all processes attached to the terminal, which is why you see the main python thread and its children receive the signal.

When you do kill -2 PID you are sending the SIGINT to a specific process of that process group; other processes will not be selected.

If instead you do kill -2 -PGID (notice the minus sign, and the G ), you replicate the action of Ctrl-C. This instructs kill to target the signal at the process group level; all processes within the group will receive the signal.

You can run this command to see the process group ID for each of your python processes:

ps -o pgid,ppid,pid,lwp,sgi_p,fname,cmd -C python

... example output....

 PGID  PPID   PID   LWP P COMMAND  CMD
22706 19662 22706 22706 * python   python parent.py
22706 22706 22707 22707 * python   python /var/tmp/child.py
22706 22706 22708 22708 * python   python /var/tmp/child.py
22706 22706 22709 22709 * python   python /var/tmp/child.py
22706 22706 22710 22710 * python   python /var/tmp/child.py
22706 22706 22711 22711 * python   python /var/tmp/child.py

(PGID: group ID, PPID: parent ID, PID: process ID, LWP: thread ID)

... then to send SIGINT to the entire group:

kill -2 -22706

See here for good explanation of Ctrl-C and process groups: https://unix.stackexchange.com/questions/149741/why-is-sigint-not-propagated-to-child-process-when-sent-to-its-parent-process