Build arguments and secrets

Introduction

One of the core features of earthly is support for build arguments. Build arguments can be used to dynamically set environment variables inside the context of RUN commands.

Build arguments can be passed between targets or from the command line. They encourage writing generic Earthfiles and ultimately promote greater code-reuse.

Additionally, earthly defines secrets which are similar to build arguments, but are exposed as environment variables when explicitly allowed.

A Quick Example

Arguments are declared with the ARG keyword.

Let's consider a "hello world" example that allows us to change who is being greeted (e.g. hello banana, hello eggplant etc). We will create a hello target that accepts the name argument:

FROM alpine:latest
hello:
ARG name
RUN echo "hello $name"

Then we will specify a value for the name argument on the command line when we invoke earthly:

earthly --build-arg name=world +hello

This will output

buildkitd | Found buildkit daemon as docker container (earthly-buildkitd)
alpine:latest | --> Load metadata linux/amd64
+foo | --> FROM alpine:latest
+foo | [██████████] resolve docker.io/library/alpine:[email protected]:69e70a79f2d41ab5d637de98c1e0b055206ba40a8145e7bddb55ccc04e13cf8f ... 100%
+foo | name=world
+foo | --> RUN echo "hello $name"
+foo | hello world
output | --> exporting outputs

If we re-run earthly --build-arg name=world +hello, we will see that the echo command is cached (and won't re-display the hello world text):

+foo | *cached* --> RUN echo "hello $name"

Setting Argument Values

Argument values can be set multiple ways:

  1. On the command line

    The value can be directly specified on the command line (as shown in the previous example):

    earthly --build-arg name=world +hello
  2. From an environment variable

    If no value is given for name, then earthly will look for the value in the corresponding environment variable on the localhost:

    export name="banana"
    earthly --build-arg name +hello
  3. From a .env file

    It is also possible to create an .env file to contain the build arguments to pass to earthly. First create an .env file with:

    name eggplant

    Then simply run earthly:

    earthly +hello

Passing Argument values to targets

Build arguments can also be set when calling build targets. If multiple build arguments values are defined for the same argument name, earthly will build the target for each value; this makes it easy to configure a "build matrix" within Earthly.

For example, we can create a new greetings target which calls +hello multiple times:

greetings:
BUILD \
--build-arg name=world \
--build-arg name=banana \
--build-arg name=eggplant \
+hello

Then when we call earthly +greetings, earthly will call +hello three times:

buildkitd | Found buildkit daemon as docker container (earthly-buildkitd)
alpine:latest | --> Load metadata linux/amd64
+base | --> FROM alpine:latest
+base | [██████████] resolve docker.io/library/alpine:[email protected]:69e70a79f2d41ab5d637de98c1e0b055206ba40a8145e7bddb55ccc04e13cf8f ... 100%
+hello | name=banana
+hello | --> RUN echo "hello $name"
+hello | name=eggplant
+hello | --> RUN echo "hello $name"
+hello | name=world
+hello | --> RUN echo "hello $name"
+hello | hello banana
+hello | hello eggplant
+hello | hello world
output | --> exporting outputs

In addition to the BUILD command, the --build-arg flag can also be used with FROM, COPY and a number of other commands.

Passing secrets to RUN commands

Secrets are similar to build arguments; however, they are not defined in targets, but instead are explicitly defined for each RUN command that is permitted to access them.

Here's an example Earthfile that accesses a secret stored under +secrets/passwd and exposes it under the environment variable mypassword:

FROM alpine:latest
hush:
RUN --secret mypassword=+secrets/passwd echo "my password is $mypassword"

It's also possible to temporarily mount a secret as a file:

RUN --mount type=secret,target=/root/mypassword,id=+secrets/passwd echo "my password is $(cat /root/mypassword)"

The file will not be saved to the image snapshot.

The value for +secrets/passwd must then be supplied when earthly is invoked. This can be either done directly via:

earthly --secret passwd=itsasecret +hush

or if the value is omitted, then earthly will attempt to lookup the value from an environment variable on the localhost:

passwd=itsasecret \
earthly --secret passwd +hush

Alternatively, earthly offers cloud-based secrets if you need to share secrets between colleagues.

Once earthly is invoked, it will output:

+hush | --> RUN echo "my password is $mypassword"
+hush | my password is itsasecret

How Arguments affect caching

Commands in earthly must be re-evaluated when the command itself changes (e.g. echo "hello $name" is changed to echo "greetings $name"), or when one of it's inputs has changed (e.g. --build-arg name=world is changed to --build-arg name=banana). Earthly creates a hash based on both the contents of the command and the contents of all defined arguments of the target build context.

However, in the case of secrets, the contents of the secret is not included in the hash; therefore, if the contents of a secret changes, earthly is unable to detect such a change, and thus the command will not be re-evaluated.

Storage of local secrets

Earthly stores the contents of command-line-supplied secrets in memory on the localhost. When a RUN command that requires a secret is evaluated by BuildKit, the BuildKit daemon will request the secret from the earthly command-line process and will temporarily mount the secret inside the runc container that is evaluating the RUN command. Once the command finishes the secret is unmounted. It will not persist as an environment variable within the saved container snapshot. Secrets will persist in-memory until the earthly command exits.

Earthly also supports cloud-based shared secrets which can be stored in the cloud. Secrets are never stored in the cloud unless a user creates an earthly account and explicitly calls the earthly secrets set ... command to transmit the secret to the earthly cloud-based secrets server. For more information about cloud-based secrets, check out our cloud-based secrets management guide.