Exec and Logs

Opening a shell in a running container

docker exec runs a new process inside an already-running container. The most common use is opening an interactive shell:

docker exec -it <container> sh

-i keeps stdin open (interactive)
-t allocates a pseudo-TTY (so you get a proper terminal prompt)
sh is the command to run (use bash if the image has it)

Inside the shell you can inspect files, run commands, and debug problems — exactly as if you were on the host machine.

# Example session
docker run -d --name webserver nginx:alpine
docker exec -it webserver sh

# Inside the container:
# / # cat /etc/os-release
# / # ls /usr/share/nginx/html
# / # exit

Viewing container logs

Every container writes its stdout/stderr to a log managed by Docker:

# Print all logs so far
docker logs webserver

# Follow (stream) new log lines in real time — like tail -f
docker logs -f webserver

# Show the last 20 lines
docker logs --tail 20 webserver

# Add timestamps
docker logs -t webserver

Logs are the first place to look when a container misbehaves. If a container keeps restarting, docker logs <name> usually tells you exactly why.

Inspecting processes with `docker top`

docker top lists the processes running inside a container — without entering it:

docker top webserver

UID    PID    PPID   C  STIME  TTY  TIME      CMD
root   12345  12300  0  10:00  ?    00:00:00  nginx: master process nginx -g daemon off;
101    12346  12345  0  10:00  ?    00:00:00  nginx: worker process

Running one-off commands

You do not always need a full shell. docker exec can run any command directly:

# Check nginx config
docker exec webserver nginx -t

# View a file
docker exec webserver cat /etc/nginx/nginx.conf

# 1. Start nginx in the background
docker run -d --name webserver nginx:alpine

# 2. Check its logs
docker logs webserver

# 3. Run a one-off command inside it
docker exec webserver nginx -t

# 4. Open an interactive shell
#    (type 'exit' when done)
docker exec -it webserver sh

# 5. Watch processes
docker top webserver

# 6. Follow logs in real time (Ctrl+C to stop)
docker logs -f webserver