process kill signals in Docker

sigkills.md

Proccess and kill signals

These are best practiced in a virtual environment. Docker will do.

$ docker run --rm -it ubuntu /bin/bash

Here we create a background process, capture its process id ( PID ), and then kill it.

# sleep 100 &

# pid=$( jobs -p )

# echo pid=${pid}
pid=10

# ps -fux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.4  0.1   4624  3756 pts/0    Ss   23:23   0:00 /bin/bash
root        10  0.0  0.0   2788  1024 pts/0    S    23:23   0:00 sleep 100
root        12  0.0  0.0   7060  1576 pts/0    R+   23:24   0:00 ps -fu

# kill ${pid}

# ps -fux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.3  0.1   4624  3760 pts/0    Ss   23:23   0:00 /bin/bash
root        13  0.0  0.0   7060  1576 pts/0    R+   23:24   0:00 ps -fu

Here we do the same with one change: we "trap" the TERM signal and ignore it.

# trap '' TERM

# sleep 100 &

# pid=$( jobs -p )

# echo pid=${pid}
pid=15

# ps -fux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.1  0.1   4624  3760 pts/0    Ss   23:23   0:00 /bin/bash
root        15  0.1  0.0   2788  1028 pts/0    S    23:24   0:00 sleep 100
root        17  0.0  0.0   7060  1580 pts/0    R+   23:25   0:00 ps -fu

# kill ${pid}

# ps -fux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.1  0.1   4624  3760 pts/0    Ss   23:23   0:00 /bin/bash
root        15  0.1  0.0   2788  1028 pts/0    S    23:24   0:00 sleep 100
root        18  0.0  0.0   7060  1576 pts/0    R+   23:25   0:00 ps -fu

Notice that this time the process keeps running. In order to kill the process we have to send it a different signal, e.g. HUP. By default, kill sends a TERM signal ( aka SIGTERM or 15 ). See man kill for more details.

# kill -s HUP ${pid}

# ps -fux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.1  0.1   4624  3760 pts/0    Ss   23:23   0:00 /bin/bash
root        19  0.0  0.0   7060  1576 pts/0    R+   23:25   0:00 ps -fu

# exit

sigkills.sh

# The same ( mostly ) but in script form

<< 'eof' docker run --rm -i ubuntu /bin/bash 2> /dev/null

  # Create a background process then kill it
  sleep 100 &
  echo
  pid=$( jobs -p )
  echo pid=${pid}
  ps -fux
  kill ${pid}
  echo
  ps -fux
  
  # Add the trap and repeat
  trap '' TERM
  sleep 100 &
  echo
  pid=$( jobs -p )
  echo pid=${pid}
  ps -fux
  kill ${pid}
  echo
  ps -fux

  # Kill with another signal
  kill -s HUP ${pid}
  sleep 1
  echo
  ps -fux

eof