Bazel BUILD Files
External
Internal
Overview
A BUILD is a short program, an interpretable list of Starlark statements.
The BUILD file specified of how the files of a Bazel package can be used to produce artifacts.
There is a one-to-one relationship between a BUILD file and a Bazel package. The BUILD file resides in the package directory.
The BUILD file defines the package's targets.
The BUILD file can be named BUILD or BUILD.bazel and where both exist, BUILD.bazel takes precedence.
Example
A simple BUILD file that produces an executable whose source is written in Go and that are fist built into a library:
General Syntax
Variables must be defined before they are used.
The relative order in which build rules are declared is unimportant.
The file cannot contain function definitions, for or if control statements. List comprehensions and if expressions are allowed.
Starlark programs can't perform arbitrary I/O, which makes the interpretation of the BUILD files hermetic.
BUILD files need to be updated whenever the dependencies of the underlying code change. IntelliJ does that via the "Sync" functionality.
Rules
A rule specifies the relationship between a set of input and a set of output files.
The inputs to a rule may be source files, but they also may be the outputs of other rules. It this case is said that the input of a rule is other rule.
The output files of rule always belong to the same package as the rule itself. It is not possible to generate files into another package. It is not uncommon for a rule's inputs to come from another package, though.
When a build rule function is executed, it creates a new target in the graph, which can later referred using a label, declared as name in the rule declaration.
Rules are language-specific. The majority of build rules come in families, grouped together by language. For example, go_binary, go_library and cc_test are the build rules for Go binaries, libraries, and tests, respectively.
*_binary rules build executable in a given language. After a build, the executable will reside in the build tool's binary output tree at the corresponding name for the rule's label, so //my:program would appear at $(BINDIR)/my/program.
*_test rules are a specialization of a *_binary rule used for automated testing.
*_library rules specify separately-compiled modules in the given programming language.
Go Rules
go_library
go_test
go_binary
go_repository
Other Go Rules
Functions
Functions can be declared in .bzl files.
Extensions
Bazel extensions are files ending in .bzl. Symbols from an extension are imported with the load() statement:
load("//foo/bar:file.bzl", "some_library")
load("//meta/rules:go.bzl", "go_library")
load("//somewhere/gomock:defs.bzl", "gomock")
some_library(
...
)
go_library(
...
)
gomock(
...
)
This can be used to lad new build rules, functions or constants.
TODO: https://bazel.build/concepts/build-files#load
Dependency Management
The dependencies can be autogenerated with Gazelle. Add them to the Go source file and run gazelle.