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Build Target

A build target is a string that identifies a build rule in your project. Build targets are used as arguments to Buck2 commands, such as buck2 build and buck2 run. Build targets are also used as arguments to build rules to enable one build rule to reference another. For example, a build rule might use a build target to reference another rule in order to specify that rule as a dependency.

Fully-qualified build targets​

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 the subsequent path is from the root of your project. You can use the buck2 root command to identify the root of your project.
  2. The java/com/facebook/share between the // prefix and the colon (:) indicates that the build file (usually named BUCK) is located in the directory java/com/facebook/share.
  3. The ui after the colon (:) indicates the name of the build rule within the build file. Build rule names must be unique within a build file. By name we mean, more formally, the value of the name argument to the build rule.

Note that the name of the build file itself—usually BUCK—does not occur in the build target. All build files within a given Buck2 project must have the same name—defined in the [buildfile].name entry of .buckconfig. Therefore, it is unnecessary to include the name in the target. The full regular expression for a fully-qualified build target is as follows:

[A-Za-z0-9._-]*//[A-Za-z0-9/._-]*:[A-Za-z0-9_/.=,@~+-]+
|- cell name -|  | package path | |--- rule name ----|

In Buck2, a cell defines a directory tree of one or more Buck2 packages. For more information about Buck2 cells and their relationship to packages and projects, see the Key Concepts topic. NOTE: All target paths are assumed to start from the root of the Buck2 project. Buck2 does not support specifying a target path that starts from a directory below the root. Although the double forward slash (//) that prefixes target paths can be omitted when specifying a target from the command line (see Pro Tips below), Buck2 still assumes that the path is from the root. Buck2 does support relative build paths, but in Buck2, that concept refers to specifying build targets from within a build file. See Relative build targets below for more details.

Relative build targets​

A relative build target can be used to reference a build rule 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. The following snippet from a build file shows an example of using a relative path.

## Assume this rule is in //java/com/facebook/share/BUCK#
java_binary(
  name = 'ui_jar',
  deps = [## The following target path##   //java/com/facebook/share:ui## is the same as using the following relative path.#':ui',],)

Command-line Pro Tips​

Here are some ways that you can reduce your typing when you specify build targets as command-line arguments to the buck2 build or buck2 run commands. Consider the following example of a fully-qualified build target used with the buck2 build command:

buck2 build //java/com/facebook/share:share

Although Buck2 is always strict when parsing build targets in build files, Buck2 is flexible when parsing build targets on the command-line. Specifically, the leading // is optional on the command line, so the above could be:

buck2 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:

buck2 build java/com/facebook/share/:share

which enables you to produce the red text shown below using tab-completion, which dramatically reduces how much you need to type:

buck2 build java/com/facebook/share/:share

Finally, if the final path element matches the value specified after the colon, it can be omitted:

# This is treated as //java/com/facebook/share:share.
buck2 build java/com/facebook/share/

which makes the build target 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 as the parent directory. That way, it can be built from the command-line with less typing.

See also​

Buck2 supports the ability to define aliases for build targets; using aliases can improve brevity when specifying targets on the Buck2 command line. For more information, see the [alias] section in the documentation for .buckconfig. A build target pattern is a string that describes a set of one or more build targets. For example, the pattern //... is used to build an entire project. For more information, see the Build Target Pattern topic.