Docker Host vs Container processes

NOTE: All configurations were taken from a lab environment.

Linux Process ID in Container is 1 which is the main Process. This same PID in the Docker Host is PID X. This is a fundamental part for the isolation that containers provide.

Here a bit of the proof:

Get PID 1 in the Docker Host

PID 1 is assigned to /sbin/init

ps -eaf

output

ubuntu@ip-XXX-XX-XX-XXX:~$ ps -eaf
UID          PID    PPID  C STIME TTY          TIME CMD
root           1       0  0 22:33 ?        00:00:04 /sbin/init
...

Starting a Container

docker run -d --rm -p 8888:8080 tomcat:8.0

output

ubuntu@ip-XXX-XX-XX-XXX:~$ sudo docker run -d --rm -p 8888:8080 tomcat:8.0
Unable to find image 'tomcat:8.0' locally
8.0: Pulling from library/tomcat
f189db1b88b3: Pull complete 
3d06cf2f1b5e: Pull complete 
edd0da9e3091: Pull complete 
eb7768aae14e: Pull complete 
e2780f585e0f: Pull complete 
e5ed720afeba: Pull complete 
d9e134700cfc: Pull complete 
e4804b33d02a: Pull complete 
b9df0c24315e: Pull complete 
49fdae8eaa20: Pull complete 
1aea3d9a32e6: Pull complete 
Digest: sha256:8ecb10948deb32c34aeadf7bf95d12a93fbd3527911fa629c1a3e7823b89ce6f
Status: Downloaded newer image for tomcat:8.0
6f513d1ae9990846c7516ccd437f86a86e4f48ca1b2126cabd3db0d8a015c30e
ubuntu@ip-XXX-XX-XX-XXX:~$ 

Get PID 1 in the Container

docker exec <container_id> ps -ef

output

ubuntu@ip-XXX-XX-XX-XXX:~$ sudo docker ps
CONTAINER ID   IMAGE        COMMAND             CREATED         STATUS         PORTS                                       NAMES
6f513d1ae999   tomcat:8.0   "catalina.sh run"   2 minutes ago   Up 2 minutes   0.0.0.0:8888->8080/tcp, :::8888->8080/tcp   nostalgic_ganguly
ubuntu@ip-XXX-XX-XX-XXX:~$ 

ubuntu@ip-XXX-XX-XX-XXX:~$ sudo docker exec 6f513d1 ps -ef
UID          PID    PPID  C STIME TTY          TIME CMD
root           1       0  3 23:06 ?        00:00:03 /docker-java-home/jre/bin/java -Djava.util.logging.config.file=/usr/local/tomcat/conf/logging.properties -Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager -Djdk.tls.ephemeralDHKeySize=2048 -Djava.protocol.handler.pkgs=org.apache.catalina.webresources -Dignore.endorsed.dirs= -classpath /usr/local/tomcat/bin/bootstrap.jar:/usr/local/tomcat/bin/tomcat-juli.jar -Dcatalina.base=/usr/local/tomcat -Dcatalina.home=/usr/local/tomcat -Djava.io.tmpdir=/usr/local/tomcat/temp org.apache.catalina.startup.Bootstrap start
root          51       0  0 23:08 ?        00:00:00 ps -ef
ubuntu@ip-XXX-XX-XX-XXX:~$ 

We see from this output that PID 1 in the Tomcat container is running the process /docker-java-home/jre/bin/java

Locate this same process in the Docker Host

If we look for this /docker-java-home/jre/bin/java process in the Docker Host, we will find it with a PID othen than 1 of course.

ps -eaf | grep /docker-java-home/jre/bin/java

output

ubuntu@ip-XXX-XX-XX-XXX:~$ ps -eaf | grep /docker-java-home/jre/bin/java
root        2964    2945  0 23:06 ?        00:00:04 /docker-java-home/jre/bin/java -Djava.util.logging.config.file=/usr/local/tomcat/conf/logging.properties -Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager -Djdk.tls.ephemeralDHKeySize=2048 -Djava.protocol.handler.pkgs=org.apache.catalina.webresources -Dignore.endorsed.dirs= -classpath /usr/local/tomcat/bin/bootstrap.jar:/usr/local/tomcat/bin/tomcat-juli.jar -Dcatalina.base=/usr/local/tomcat -Dcatalina.home=/usr/local/tomcat -Djava.io.tmpdir=/usr/local/tomcat/temp org.apache.catalina.startup.Bootstrap start

As a conclusion, in the Docker Host we see the process /docker-java-home/jre/bin/java as PID 2964 while in the Containter it is the running as PID 1