How to read files and stdout from a running Docker container

Question

How would I go about starting an application in my host machine in order to read files and stdout from a running docker container    Essentially I want to do this   docker start containerid      myapp    This app will  somehow  have access files and stdout generated by the container I just stared     How would I go about doing that  To be more specific with where I am trying to go with this  I want to read the logs and stdout of a docker container and have those logs processed somewhere else    I am also willing to create another docker container which can read files and stdout from another container  but I don t know if that s possible

User · Accepted Answer

The stdout of the process started by the docker container is available through the docker logs  containerid command  use -f to keep it going forever   Another option would be to stream the logs directly through the docker remote API   For accessing log files  only if you must  consider logging to stdout or other standard solution like syslogd  your only real-time option is to configure a volume  like Marcus Hughes suggests  so the logs are stored outside the container and available for processing from the host or another container   If you do not need real-time access to the logs  you can export the files  in tar format  with docker export

User · Answer

To view the stdout  you can start the docker container with -i  This of course does not enable you to leave the started process and explore the container   docker start -i containerid   Alternatively you can view the filesystem of the container at   var lib docker containers containerid root    However neither of these are ideal  If you want to view logs or any persistent storage  the correct way to do so would be attaching a volume with the -v switch when you use docker run  This would mean you can inspect log files either on the host or attach them to another container and inspect them there

User · Answer

You can view the filesystem of the container at   var lib docker devicemapper mnt  CONTAINER ID rootfs    and you can just   tail -f mylogfile log

User · Answer

A bit late but this is what I m doing with journald  It s pretty powerful    You need to be running your docker containers on an OS with systemd-journald   docker run -d --log-driver journald myapp  This pipes the whole lot into host s journald which takes care of stuff like log pruning  storage format etc and gives you some cool options for viewing them   journalctl CONTAINER NAME myapp -f   which will feed it to your console as it is logged   journalctl CONTAINER NAME myapp  gt  output log   which gives you the whole lot in a file to take away  or   journalctl CONTAINER NAME myapp --since 17 45  Plus you can still see the logs via docker logs      if that s your preference   No more  gt  my log or -v   apps myapp logs  logs  etc

User · Answer

Sharing files between a docker container and the host system  or between separate containers is best accomplished using volumes   Having your app running in another container is probably your best solution since it will ensure that your whole application can be well isolated and easily deployed  What you re trying to do sounds very close to the setup described in this excellent blog post  take a look

[logging] How to read files and stdout from a running Docker container

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