Build Target

A build target is a string that is used to identify a build rule in your project.

Here is an example of a fully-qualified build target:

//java/com/facebook/share:ui
A fully-qualified build target has three components:
  1. The // prefix indicates that this path is relative to the root of your project.
  2. The java/com/facebook/share component between the // prefix and the colon (:) indicates that the build file can be found in the java/com/facebook/share directory.
  3. The ui after the colon (:) indicates the name of the build rule within the build file in which it is defines. Build rule names must be unique within a build file.
A relative build target can be used to reference another build target within the same build file. A relative build target starts with a colon (:) and is followed by only the third component (or "short name") of the fully-qualified build target. For example, within java/com/facebook/share/BUCK, :ui can be used to reference //java/com/facebook/share:ui:
# This is in java/com/facebook/share/BUCK.
java_binary(
  name = 'ui_jar',
  deps = [
    # This would be the same as using:
    # '//java/com/facebook/share:ui'
    ':ui',
  ],
)
Build targets are frequently used as arguments to build rules and to the command-line interface for Buck.

Command-line Protip

When specifying a build target on the command-line, it can be tedious to type:
buck build //java/com/facebook/share:share
Fortunately, Buck is loose with its parsing of build targets on the command-line (but is always strict when parsing them in build files). Specifically, the leading // is optional on the command line, so the above could be:
buck build java/com/facebook/share:share
Also, if there is a forward slash before the colon, it is ignored, so this could also be written as:
buck build java/com/facebook/share/:share
This looks odd until you realize that the red text can be produced by tab-completing the path to the directory, which dramatically reduces how much you need to type:
buck build java/com/facebook/share/:share
Finally, if the final path element matches the value specified after the colon, it can be omitted from the command line:
# This is treated as //java/com/facebook/share:share.
buck build java/com/facebook/share/
Note that this is even easier to tab-complete! For this reason, the name of the build rule for the primary deliverable in a build file is often named the same thing as the parent directory so it can be built from the command-line with less typing.