How to Run a Daemon Service in the Foreground

Usually, we don’t do so, but running processes in Daemon mode gives us a lot of benefits. Service can be easily managed. Remember service XXX start and chkconfig XXX on? When the terminal disconnects, this process remains.

However, sometimes services just fail to start. In these cases, we may need to run the process in foreground mode. For example, you may encounter duplicated global ENVs, incorrect CONF files, missing folders, access being denied, etc. (For some related reading, check out How to Check Linux Process Deeply With Common Sense.)

In container ENVs, each container will usually host one and only one process. One useful trick is to lower the operation cost. When containers start, the service automatically starts. When service stops, the container automatically stops. To achieve this, we would like to find a way that we can run the process in foreground, and then change the entry point to the start command.

Take the Redis container, for example.

From...

docker run -t -d --name my-nginx nginx /bin/sh
# Start containers, then start process
docker exec -it my-nginx bash
service nginx start

...to:

docker stop my-nginx; docker rm my-nginx
# When container starts, start process inside
docker run -t -d --name my-nginx nginx \
  /usr/sbin/nginx -g "daemon off;"

docker exec -it my-nginx bash
ps -ef | grep nginx

You must figure out the start command in order to run in the foreground.

(Note: Daemon Service = Binary + Configuration + Initscript.)

To run the process in the foreground, we just need these two ingredients:

1. Binary: bash -x /etc/init.d/XXX start.
   Here, -x instructs shell to display verbose output. Should be easy to figure out binary path. Another trick is _”ps -ef | grep $PROCESS_PATTERN”_.
2. Configuration: Typically speaking, we can dig out from console output of bash -x. Sometimes, it’s just too long to read. Figure out the process ID and the list environment from cat /proc/$pid/environ.

A lot of decent services refuse to run as root because of security concerns. Here is how to impersonate other users with root.

su -l elasticsearch -c "java XXX"

Sample output of bash -x /etc/init.d/jenkins start:

root@denny:/# bash -x /etc/init.d/jenkins start
bash -x /etc/init.d/jenkins start
+ PATH=/bin:/usr/bin:/sbin:/usr/sbin
+ DESC='Jenkins Continuous Integration Server'
+ NAME=jenkins
+ SCRIPTNAME=/etc/init.d/jenkins
+ '[' -r /etc/default/jenkins ']'
+ . /etc/default/jenkins
++ NAME=jenkins
++ JAVA=/usr/bin/java
++ JAVA_ARGS=-Djava.awt.headless=true
...
...
+ '[' no '!=' no ']'
+ ulimit -n 8192
+ '[' -n '' ']'
+ /bin/su -l jenkins --shell=/bin/bash -c
'/usr/bin/daemon --name=jenkins --inherit
--env=JENKINS_HOME=/var/lib/jenkins
--output=/var/log/jenkins/jenkins.log
--pidfile=/var/run/jenkins/jenkins.pid
-- /usr/bin/java  -Dhudson.diyChunking=false
-Djenkins.install.runSetupWizard=false
-jar /usr/share/jenkins/jenkins.war
--webroot=/var/cache/jenkins/war 
--httpPort=18080'
+ START_STATUS=0
+ do_check_started_ok 0
+ sleep 1
+ '[' 0 -ne 0 ']'
+ get_running
++ grep -c .
++ egrep -e '(java|daemon)'
++ ps -U jenkins --no-headers -f
...
...
+ return 0
+ exit 0

How to Run Apache, Jenkins, or MySQL in the Foreground

Using the above technique, we can find start commands running popular services in the foreground.

Run Apache in the foreground:

cd /etc/apache2/ && \
apachectl -d /etc/apache2 -e info -DFOREGROUND

Run Jenkins in the foreground:

/bin/su -l jenkins --shell=/bin/bash \
-c '/usr/bin/daemon --name=jenkins \
--inherit --env=JENKINS_HOME=/var/lib/jenkins \
--output=/var/log/jenkins/jenkins.log \
--pidfile=/var/run/jenkins/jenkins.pid -- \
/usr/bin/java  -Dhudson.diyChunking=false \
-Djenkins.install.runSetupWizard=false -jar \
/usr/share/jenkins/jenkins.war \
--webroot=/var/cache/jenkins/war \
--httpPort=18080'

Run mySQL in the foreground:

/bin/sh /usr/bin/mysqld_safe

And finally, run Nginx in the foreground:

/usr/sbin/nginx -g "daemon off;"