Logs, Exec and Copying Files In and Out

A container is a closed box. When something goes wrong inside it, you need three ways to look: read what it printed (logs), run a command inside it (exec), and move files across the wall (cp). This lesson uses one web server and one deliberately failing container to practise all three.

Start the server first, the same way as in the last lesson.

docker run --name hello-web -d -p 8080:80 nginx:1.30-alpine

Status: Downloaded newer image for nginx:1.30-alpine

Read the logs

Whatever a program writes to its standard output or standard error ends up in the container's log. That is why well-behaved containers print to the console instead of to a file. Make one request, then read the last line of the log.

curl.exe -s -o NUL http://localhost:8080

docker logs --tail 1 hello-web

172.17.0.1 - - [15/Oct/2026:07:39:08 +0000] "GET / HTTP/1.1" 200 896 "-" "curl/8.21.0" "-"

That is nginx's access log: who asked (the address, date and time), what they asked for, the status code 200, and the size of the answer. The date and the curl version will be yours, not mine. --tail 1 limits the output to the last line. Without it you get everything since the container started, and a busy container prints a lot. Two more options you will use: --since 5m shows the last five minutes, and -f follows the log live until you press Ctrl+C.

Logs are most useful for a container that stopped. Run one that fails on purpose.

docker run --name boom alpine:3.24 sh -c "echo starting; echo something went wrong; exit 3"

starting
something went wrong

It was attached to your terminal, so you saw both lines at once. Now look at it as if it had run in the background.

docker ps -a --filter name=boom --format "table {{.Names}}\t{{.Status}}"
docker logs boom

NAMES   STATUS
boom    Exited (3) Less than a second ago
starting
something went wrong

"Exited (3)" is the exit code the program chose. Zero means success, anything else means it failed. The logs are still there after the container stops, and they stay until you remove it. That is the first place to look when a container exits straight away. If you skip it and go guessing, you will waste an hour on a problem the log states in one line.

Run a command inside a container

docker exec runs a command inside a container that is already running. To get a shell, add -it, which connects your keyboard to it.

docker exec -it hello-web sh

You get a prompt such as / #, and every command you type now runs inside the container. Type exit to leave. The container keeps running. nginx:1.30-alpine has sh but not bash, because Alpine images are small. If bash is not found, try sh.

You can also run a single command and read its answer, which is easier to copy and paste.

docker exec hello-web ls /usr/share/nginx/html
docker exec hello-web whoami

50x.html
index.html
root

The first command lists the folder nginx serves. The second shows that you are root inside the container. That is root inside a container, not on your Windows machine. It still deserves respect, because a bind mount (lesson 7) can give that root access to your files.

Copy files in and out

docker cp copies between your computer and a container, in either direction. The container name and path go on the container side, separated by a colon. In a terminal Docker may print a line such as "Successfully copied 2.05kB". It is not shown below, because the verified runs did not print it.

docker cp hello-web:/usr/share/nginx/html/index.html .\welcome.html
Get-Content .\welcome.html | Select-Object -First 4

<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>

Now change the page and copy it back in.

Set-Content -Path .\welcome.html -Value "<h1>Hello from my own file</h1>"
docker cp .\welcome.html hello-web:/usr/share/nginx/html/index.html
curl.exe -s http://localhost:8080

<h1>Hello from my own file</h1>

The page changed, and nginx did not need a restart because it reads the file on every request. Remember where this change lives: in the container. Remove the container and the change is gone with it. For a change that survives, build it into an image (lesson 6) or mount a folder (lesson 7).

Clean up before the next lesson.

docker rm -f hello-web boom

If you copy a file into a running container and then run docker rm -f on it, where is your file?