Pipe Docker log output to STDOUT

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Anything written to append-only files within containers should be piped to standard output (STDOUT). This makes application logs easy to diagnose and prevents append-only files from consuming disk space.

Some widely used Docker images, such as Nginx, already pipe their logs to STDOUT. You may still have an application-specific log file that you want to apply the same approach to. For example, this Node application overrides console.log() and writes every call to a file named debug.log:

var fs = require('fs');
var util = require('util');
var logFile = fs.createWriteStream(__dirname + '/debug.log', {flags : 'w'});

console.log = (d) => {
  logFile.write(util.format(d) + '\n');
};

var i = 0;
var log = () => {
  console.log(`foo bar ${i}`);
  i++;
};

setInterval(log, 2000);

Node’s original console.log() already writes to STDOUT, so this is a deliberately nonsensical example, but it shows how to redirect writes from a log file to STDOUT.

Run the script for a few moments:

node index.js

Then check the contents of debug.log:

cat debug.log

The output shows that the script is running and correctly writing to debug.log.

The Dockerfile starts with a Node base image, copies the application into the image, and runs it with the node command:

FROM mhart/alpine-node
COPY index.js .
CMD node index.js

To redirect those writes to STDOUT, add a RUN instruction before the CMD instruction. Use ln -sf to create a symbolic link from /debug.log to /dev/stdout:

FROM mhart/alpine-node
COPY index.js .
RUN ln -sf /dev/stdout /debug.log
CMD node index.js

Whatever would normally be written to debug.log is now piped straight to the container’s standard output.

Build the image and run a container in daemon mode:

docker build -t foo .
docker run -d --name=foo foo

Use docker logs with -f to follow the container output:

docker logs -f foo

The messages that would normally be written to debug.log now appear in the container logs:

foo bar 0
foo bar 1
foo bar 2
foo bar 3
foo bar 4

You can also confirm that nothing is written to debug.log inside the container:

docker exec -it foo cat /debug.log

The application keeps its existing file path while sending its output to STDOUT, where Docker can collect and display it without the container accumulating disk space over time.