World map

 

BM

World D1: First Containers

Follow the logs

You will learn: to read what a container printed, and to watch new lines live.

Checked with Docker Engine 29 on 31 Oct 2026

The problem

Your laptop~

Lunchboxes (containers)

  • webrunning
    from nginx:1.30-alpine 8080→80
  • timerfailed (exit 127)
    from alpine:3.22

Frozen packs (images)

  • alpine:3.221 layers · 12.9MB · in use
  • nginx:1.30-alpine8 layers · 93.5MB · in use

Your timer box should tick for an hour. It stopped at once: Exited (127). No error on your screen. How do you find out why?

Try it

A box keeps notes of everything it prints: its logs: Everything a container printed while it worked. Read them with docker logs.. Read timer’s notes.

Your laptop~

Lunchboxes (containers)

  • webrunning
    from nginx:1.30-alpine 8080→80
  • timerfailed (exit 127)
    from alpine:3.22

Frozen packs (images)

  • alpine:3.221 layers · 12.9MB · in use
  • nginx:1.30-alpine8 layers · 93.5MB · in use

Check

The logs end with the real error sh: sleeep: not found. What does it mean?

Watch

Your laptop~

Lunchboxes (containers)

  • webrunning
    from nginx:1.30-alpine 8080→80
  • timerfailed (exit 127)
    from alpine:3.22

Frozen packs (images)

  • alpine:3.221 layers · 12.9MB · in use
  • nginx:1.30-alpine8 layers · 93.5MB · in use

Every visit to web adds a line. --tail 1 shows only the last one. Add -f (follow) to watch new lines live. Ctrl+C stops watching.

Try it

Visit web with curl localhost:8080. Then read web’s logs and find your visit.

Your laptop~

Lunchboxes (containers)

  • webrunning
    from nginx:1.30-alpine 8080→80
  • timerfailed (exit 127)
    from alpine:3.22

Frozen packs (images)

  • alpine:3.221 layers · 12.9MB · in use
  • nginx:1.30-alpine8 layers · 93.5MB · in use

Predict

docker logs -f web is running. You press Ctrl+C. What happens to web?

You did it!

You can now read a container’s logs to find the problem.

Go deeper (optional)

Logs are what the app prints to the screen (stdout and stderr). The nginx image sends its log files there too. Handy flags: --tail 20, --since 10m, and -t for times. Logs vanish when you remove the box.

Real rep (optional):

Real mission

  1. docker run -d --name web -p 8080:80 nginx:1.30-alpine
  2. docker logs -f web
  4. docker rm -f web

0 of 4 steps done · Your ticks are saved.