Earthly 0.8
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Functions

UDCs have been renamed to Functions
Functions used to be called UDCs (User Defined Commands). Earthly 0.7 uses COMMAND instead of FUNCTION.
Earthly Functions are reusable sets of instructions that can be inserted in targets or other functions. In other words, it is a way to import common build steps which can be reused in multiple contexts.
Unlike targets, functions inherit the (1) build context and (2) the build environment from the caller. Meaning that
  1. 1.
    Any local COPY operation will use the directory where the calling Earthfile exists, as the source.
  2. 2.
    Any files, directories and dependencies created by a previous step of the caller are available to the function to operate on; and any file changes resulting from executing the function's commands are passed back to the caller as part of the build environment.
Thus, when importing and reusing functions across a complex build, it is very much like reusing libraries in a regular programming language.

Usage

Functions are defined similarly to regular targets, with a couple of exceptions: the name is in ALL_UPPERCASE_SNAKE_CASE and the recipe must start with FUNCTION. For example:
MY_COPY:
FUNCTION
ARG src
ARG dest=./
ARG recursive=false
RUN cp $(if $recursive = "true"; then printf -- -r; fi) "$src" "$dest"
This function can be invoked from a target via DO
build:
FROM alpine:3.18
WORKDIR /function-example
RUN echo "hello" >./foo
DO +MY_COPY --src=./foo --dest=./bar
RUN cat ./bar # prints "hello"
A few things to note about this example:
  • The definition of MY_COPY does not contain a FROM so the build environment it operates in is the build environment of the caller.
  • This means that +MY_COPY has access to the file ./foo.
  • Although the copy file operation is performed within +MY_COPY, its effects are seen in the environment of the caller - so the resulting ./bar is available to the caller.

Scope

Functions create their own ARG scope, which is distinct from the caller. Any ARG that needs to be passed from the caller needs to be passed explicitly via DO +MY_FUNCTION --<build-arg-key>=<build-arg-value>, as in the following example.
build:
ARG var=value-in-build
# prints "something-else"
DO +PRINT_VAR
# prints "value-in-build"
DO +PRINT_VAR --var=$var
PRINT_VAR:
FUNCTION
ARG var=something-else
RUN echo "$var"
Global imports and global args are inherited from the base target of the same Earthfile where the command is defined in (this may be distinct from the base target of the caller).
VERSION 0.8
ARG --global a_global_var=value-in-global
build:
# prints "value-in-global"
DO +PRINT_VAR
PRINT_VAR:
FUNCTION
RUN echo "$a_global_var"

Targets vs Functions

Targets and functions are Earthly's core primitives for organizing build recipes. They encapsulate build logic, and from afar they look pretty similar. However, the use-cases for each are vastly different.
In general, targets are used to produce specific build results, while functions are used as a way to reuse build logic, when certain commands are repeated in multiple places. As a real-world analogy, targets are more like factories, while functions are more like components that are used to put together factories.
Here is a comparison of the two primitives:
Text
Targets
Functions
Represents a collection of Earthly commands
Can reference other targets in its body
Can reference other functions in its body
Build context
The directory where the Earthfile resides
Inherited from the caller
Build environment, when no FROM is specified
Inherited from the base of its own Earthfile
Inherited from the caller
IMPORT statements
Inherited from the base of its own Earthfile
Inherited from the base of its own Earthfile
ARG context
Creates its own scope
Creates its own scope
Requires that ARGs be passed in explicitly
Global ARG context
Inherited from the base of its own Earthfile
Inherited from the base of its own Earthfile
Can output artifacts
❌ - can issue SAVE ARTIFACT, but it's the caller that emits the artifacts
Can output images
❌ - can issue SAVE IMAGE, but it's the caller that emits the images
Can be called via earthly CLI
Can be used in conjunction with an IMPORT
✅ - FROM some-import+my-target
✅ - DO some-import+MY_FUNCTION
Commands that can reference it
FROM, BUILD, COPY, WITH DOCKER --load, FROM DOCKERFILE
DO