site/en/build/share-variables.md

Project: /_project.yaml
Book: /_book.yaml

# Sharing Variables

{% include "_buttons.html" %}

`BUILD` files are intended to be simple and declarative. They will typically
consist of a series of target declarations. As your code base and your `BUILD`
files get larger, you will probably notice some duplication, such as:

``` python
cc_library(
  name = "foo",
  copts = ["-DVERSION=5"],
  srcs = ["foo.cc"],
)

cc_library(
  name = "bar",
  copts = ["-DVERSION=5"],
  srcs = ["bar.cc"],
  deps = [":foo"],
)
```

Code duplication in `BUILD` files is usually fine. This can make the file more
readable: each declaration can be read and understood without any context. This
is important, not only for humans, but also for external tools. For example, a
tool might be able to read and update `BUILD` files to add missing dependencies.
Code refactoring and code reuse might prevent this kind of automated
modification.

If it is useful to share values (for example, if values must be kept in sync),
you can introduce a variable:

``` python
COPTS = ["-DVERSION=5"]

cc_library(
  name = "foo",
  copts = COPTS,
  srcs = ["foo.cc"],
)

cc_library(
  name = "bar",
  copts = COPTS,
  srcs = ["bar.cc"],
  deps = [":foo"],
)
```

Multiple declarations now use the value `COPTS`. By convention, use uppercase
letters to name global constants.

## Sharing variables across multiple BUILD files

If you need to share a value across multiple `BUILD` files, you have to put it
in a `.bzl` file. `.bzl` files contain definitions (variables and functions)
that can be used in `BUILD` files.

In `path/to/variables.bzl`, write:

``` python
COPTS = ["-DVERSION=5"]
```

Then, you can update your `BUILD` files to access the variable:

``` python
load("//path/to:variables.bzl", "COPTS")

cc_library(
  name = "foo",
  copts = COPTS,
  srcs = ["foo.cc"],
)

cc_library(
  name = "bar",
  copts = COPTS,
  srcs = ["bar.cc"],
  deps = [":foo"],
)
```