the same process has different PIDs in different namespaces

PID in host / PID in container
23512 / 1 (PID 1 is special)
23513 / 4
23518 / 12

PID namespaces are in a tree

Diagram showing “host PID namespace (the root)” with three arrows coming down from it, each pointing to a label that says “child”.

Often the tree is just 1 level deep (every child is a container)

you can see processes in child PID namespaces

Illustration of a host, represented by a box with heart eyes and a big smile.

host: aw! look at all those containers running!

if PID 1 exits, everyone gets killed

Illustration of PID 1, represented by a box with a smiley face, and Linux, represented by its penguin mascot.

PID 1: ok I’m done!
Linux: I’m kill -q’ing everyone else in this PID namespace IMMEDIATELY

Killing PID 1 accidentally would be bad

Illustration of a container process, represented by a box with a smiley face, and Linux, represented by its penguin mascot.

container process: kill 1
Linux: do you WANT everyone to die? I’m not gonna let you do that

rules for signaling PID 1

• from same container: only works if the process has set a signal handler
• from the host: only SIGKILL and SIGSTOP are ok, or if there’s a signal handler