Running Journalctl Inside A Docker Container

Accessing journalctl inside a Docker container is not straightforward because it depends on systemd, and most containers do not run systemd at all. Installing the journalctl binary alone is not enough; it needs a running systemd-journald service to function, which is normally absent in container environments.

If you need journalctl for development or testing, the key requirement is to recreate a working systemd environment inside your container.

The most direct approach is to run systemd inside the container. This involves starting the container in privileged mode and giving it access to cgroups, which systemd relies on. For example, you can run:

docker run -it --privileged \
  --cgroupns=host \
  -v /sys/fs/cgroup:/sys/fs/cgroup:rw \
  ubuntu:22.04 /sbin/init

There are a few important requirements for this setup:

1. Privileged mode (--privileged) – systemd needs direct access to kernel features that are normally restricted in a container. Running the container in privileged mode allows it to manage devices, cgroups, and other low-level system resources, which are required for systemd to operate correctly.

2. Cgroup access (-v /sys/fs/cgroup:/sys/fs/cgroup:rw and --cgroupns=host) – systemd uses cgroups to manage and track services and their resources. Without access to cgroups, systemd cannot enforce limits, monitor services, or properly start journald. Mounting the host’s cgroup filesystem into the container gives systemd the visibility it needs.

3. PID 1 init process (/sbin/init) – systemd must run as the first process in the container (PID 1) because it is designed to be the init system. It handles process supervision, signal management, and service lifecycle. If it is not PID 1, many systemd functions, including journald, will not work properly.

With these requirements met, the container effectively has a fully functional systemd environment. systemd-journald will run normally, and you can use journalctl inside the container just as you would on a normal Linux system. This setup is heavier than a standard container and is intended for development and testing, not production use, but it provides the closest simulation of a real system for applications that rely on journald.