User-defined commands (UDCs) are templates (much like functions in regular programming languages), which can be used to define a series of steps to be executed in sequence. In other words, it is a way to import common build steps which can be reused in multiple contexts.

Unlike targets, UDCs inherit the (1) build context and (2) the build environment from the caller. Meaning that

1. Any local COPY opperation will use the directory where the calling Earthfile exists, as the source.

2. Any files, directories and dependencies created by a previous step of the caller are available to the UDC to operate on; and any file changes resulting from executing the UDC commands are passed back to the caller as part of the build environment.

Thus, when importing and reusing UDCs across a complex build, it is very much like reusing libraries in a regular programming language.

Usage

UDCs 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 COMMAND. For example:

MY_COPY:
    COMMAND
    ARG src
    ARG dest=./
    ARG recursive=false
    RUN cp $(if $recursive =  "true"; then printf -- -r; fi) "$src" "$dest"

This UDC can be invoked from a target via DO

build:
    FROM alpine:3.13
    WORKDIR /udc-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

UDCs 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 +COMMAND --<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=$var
​
PRINT_VAR:
    COMMAND
    ARG var=something-else
    RUN echo "$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).

ARG a_global_var=value-in-global
​
build:
    # prints "value-in-global"
    DO +PRINT_VAR
​
PRINT_VAR:
    COMMAND
    RUN echo "$a_global_var"