19.2.2.1 Ps Running Containers

Running containers are those in the active execution state: their primary process is alive, consuming CPU and memory, and responding to requests or performing work. docker ps without any flags shows only these containers, making it the default tool for a quick operational snapshot of what is currently active on a Docker host.

What "Running" Means

A container enters the running state when its primary process (PID 1 inside the container) starts successfully and has not yet exited. As long as that process is alive, the container is running. If the process exits — whether successfully or with an error — the container transitions to the exited state and disappears from docker ps output (though it remains visible with docker ps -a).

Listing Running Containers

docker ps

A typical output on a server:

CONTAINER ID   IMAGE              COMMAND                  CREATED        STATUS          PORTS                    NAMES
a1b2c3d4e5f6   nginx:latest       "/docker-entrypoint…"   3 hours ago    Up 3 hours      0.0.0.0:443->443/tcp     proxy
b2c3d4e5f6a7   postgres:15        "docker-entrypoint.s…"  3 hours ago    Up 3 hours      5432/tcp                 db
c3d4e5f6a7b8   myapp:2.1.0        "./server"               2 hours ago    Up 2 hours      0.0.0.0:8080->8080/tcp   app
d4e5f6a7b8c9   redis:7            "docker-entrypoint.s…"  1 hour ago     Up 1 hour       6379/tcp                 cache

Each line represents one actively running container.

Reading the STATUS Column

The STATUS field for a running container shows how long it has been up:

• Up 5 seconds — just started
• Up 3 hours — running for three hours
• Up 4 days — long-running service

This elapsed time is measured from when the container last started, not from when it was created.

A container that was restarted (via a restart policy or manual restart) shows the time since the most recent start, not since its creation.

Health Checks in Running Containers

If the image or container was configured with a health check, the STATUS column includes health status:

Up 3 hours (healthy)
Up 1 hour (unhealthy)
Up 10 seconds (health: starting)

Health checks are custom commands defined in the Dockerfile or at run time that Docker executes periodically inside the container. The container remains in the running state even when unhealthy — health status does not automatically stop a container. It is used by orchestrators (like Docker Swarm or Kubernetes) to route traffic away from unhealthy instances.

Filtering Only Running Containers

Running is the default filter, but it can be specified explicitly:

docker ps -f "status=running"

This is equivalent to docker ps without flags, but the explicit filter is useful when constructing pipelines that need to be unambiguous.

Checking Running Containers by Image

To see which containers are running from a specific image:

docker ps -f "ancestor=nginx:latest"

For any image derived from a base:

docker ps -f "ancestor=ubuntu:22.04"

This shows containers built from ubuntu:22.04 or any image that used it as a base.

Monitoring Running Container Count

docker ps -q | wc -l

On Windows PowerShell:

(docker ps -q).Count

This returns the number of currently running containers. Useful for health dashboards or alert thresholds.

Continuous Monitoring with watch

On Linux and macOS, watch reruns a command at intervals, providing a live-updating view:

watch docker ps

This refreshes every 2 seconds by default. With a custom interval:

watch -n 5 docker ps

Real-Time Resource Usage for Running Containers

docker stats complements docker ps by showing live CPU, memory, and network metrics for running containers:

docker stats

CONTAINER ID   NAME      CPU %     MEM USAGE / LIMIT   MEM %     NET I/O          BLOCK I/O
a1b2c3d4e5f6   proxy     0.12%     24MB / 512MB        4.69%     1.4MB / 890kB    0B / 0B
b2c3d4e5f6a7   db        2.31%     185MB / 2GB         9.25%     320kB / 124kB    12MB / 8MB
c3d4e5f6a7b8   app       1.45%     112MB / 1GB         11.2%     560kB / 210kB    0B / 0B

For a single container:

docker stats db

Checking Processes Inside a Running Container

docker top app

UID    PID     PPID    C    STIME   TTY   TIME       CMD
root   12345   12300   0    10:00   ?     00:00:01   ./server

This lists the processes running inside the container from the host's perspective.

Viewing Logs of a Running Container

For a running container, logs accumulate in real time. To follow them:

docker logs -f app

To see only recent output:

docker logs --tail 50 app

Common Troubleshooting Patterns

Container not appearing in docker ps

If a container you just started does not appear, it likely exited immediately. Check:

docker ps -a -f "status=exited" -l

Inspect the last exited container's logs:

docker logs $(docker ps -a -q -f "status=exited" -l)

Container running but not responding

If the container shows Up X minutes but the application is not responding:

1. Check logs: docker logs app
2. Check health: look for (unhealthy) in the STATUS column
3. Check port bindings: docker port app
4. Exec into the container: docker exec -it app bash